README: say which records quak could not decrypt lib.backup() reports

It reports each such album, and each such file in its scope that has no copy that decrypted, not every record the library left out. Model: opus-5-5
Back up from a copy that decrypted even outside a run's albums; README wording
2026-10-14 01:41:38 +00:00 · 2026-10-14 01:41:38 +00:00 · 2026-10-14 01:41:38 +00:00 · 2026-10-14 01:41:38 +00:00 · 2026-10-14 03:38:38 +02:00 · 2026-10-14 03:06:35 +02:00
16 changed files with 2375 additions and 288 deletions
@@ -10,9 +10,12 @@ account into a deduplicated local directory tree, skipping files that already
 exist on disk and continuing past individual download failures instead of
 crashing. For each file it persists the basic metadata fields quak keeps (title,
 file type, creation and modification time, latitude, longitude, content hash),
-and the private and public magic metadata in full. A helper subcommand can
-detect and regenerate missing thumbnails, encrypting and uploading them back to
-the server.
+its update time, the private and public magic metadata in full, Ente's ML data
+for it when there is any, and, for an image (for a live photo, its image), its
+original's EXIF and XMP, with its dimensions for a JPEG only; for a video it
+keeps none of these three. It runs unattended from cron (see "Running the backup
+from cron"). A helper subcommand can detect and regenerate missing thumbnails,
+encrypting and uploading them back to the server.

 ## Getting Started

@@ -80,6 +83,66 @@ await lib.close();
 The lower-level `Client` (login, session serialization, and the raw
 enumeration/download calls) is exported too and documented under Design below.

+## Running the backup from cron
+
+`quak backup` never prompts, so cron can run it. `make install` builds quak as a
+single binary and copies it to `~/bin/quak`. Log in once with it, as the user
+the cron job will run as; the session is saved in that user's data directory
+(see "Session handling"):
+
+```bash
+~/bin/quak login
+```
+
+Then add the backup to that user's crontab with `crontab -e`. These two lines
+back up the account to `~/photos-backup` at 03:30 every night, with `--verify`
+on Sundays, and append all output to `~/quak-backup.log`:
+
+```
+30 3 * * 1-6 $HOME/bin/quak backup $HOME/photos-backup >> $HOME/quak-backup.log 2>&1
+30 3 * * 0   $HOME/bin/quak backup --verify $HOME/photos-backup >> $HOME/quak-backup.log 2>&1
+```
+
+A run with `--verify` does all a plain run does, and also checks each original
+already in the backup against the content hash Ente records for it, and replaces
+any that do not match (see "Backup layout"). Sunday's run is the `--verify` one,
+not a second job that night, because a backup that starts while another backup
+of the same directory is running exits 2 without backing anything up.
+
+Cron runs the job with a short `PATH`, usually `/usr/bin:/bin`, so the crontab
+names quak by its full path. To find the saved session, quak needs the same
+`HOME` as when you logged in, which cron sets from the password file, and on
+Linux the same `XDG_DATA_HOME`: quak looks for the session in
+`$XDG_DATA_HOME/quak`, or in `~/.local/share/quak` when that is not set. Cron
+does not set `XDG_DATA_HOME`, so if your login shell does, set it at the top of
+the crontab too. Cron does not expand variables in such a line, so give the full
+path:
+
+```
+XDG_DATA_HOME=/home/you/.data
+```
+
+On macOS the session is in `~/Library/Application Support/quak`, and only `HOME`
+matters.
+
+### Exit codes
+
+| Code | Meaning                                                                                                                                                                           |
+| ---- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `0`  | The backup is complete: every original is at its save path, and no file failed.                                                                                                   |
+| `1`  | The run finished with files in `failures.json` or an album it could not decrypt, or it stopped on another error, printed as one line `quak: <message>`. The next run tries again. |
+| `2`  | Another backup of the same directory is running. This run sent no request and changed nothing.                                                                                    |
+| `3`  | There is no usable session: none is saved, the saved one is corrupt, or the server no longer accepts it. Run `quak login` as the user the cron job runs as.                       |
+
+After a `1`, the next run fetches each missing original again, fetches the ML
+data that is not cached, and rebuilds the album links. A file or album quak
+could not decrypt keeps the exit code at `1` until the server sends a copy that
+decrypts, or deletes it (see "Backup layout"). An original that a `--verify` run
+could not read, or put back with bytes that still do not match, stays at its
+save path. Only a later `--verify` run checks it again: a run without `--verify`
+leaves it as it is and takes the file out of `failures.json`, so that run can
+exit 0.
+
 ## Examples

 `examples/download-albums.ts` downloads every album's photos and their metadata
@@ -549,7 +612,7 @@ quak collections [--json]                                 list all collections
 quak files --collection <id> [--json]                     list files in a collection
 quak get <fileID> [--out path] [--collection]             download and decrypt a file
 quak get-thumb <fileID> [--out] [--collection]            download and decrypt a thumbnail
-quak backup <dir> [--json]                                full incremental backup
+quak backup <dir> [--json] [--verify]                     full incremental backup
 quak backup-metadata <dir> [--exif]                       dump the metadata quak keeps as JSON
 quak helper list-missing-thumbnails [--json]              find files with missing thumbnails
 quak helper fix-missing-thumbnails [--file ids] [--json]  generate + upload missing thumbnails
@@ -562,8 +625,17 @@ library. The read commands — `collections`, `files`, `get`, `get-thumb`,
 answer, so they report current account state rather than whatever the cache last
 held. If that round-trip fails, the command prints the error on one line and
 exits 1, or 3 when the server no longer accepts the saved session (see "Session
-handling"). `--cache-dir` overrides where the cache lives; without it each
-account gets its own directory under the per-user cache path.
+handling"). An album or file quak cannot decrypt does not fail it; the command
+answers without it (see "Default reads vs. fresh reads"). `--cache-dir`
+overrides where the cache lives; without it each account gets its own directory
+under the per-user cache path.
+
+An ID given to `files --collection`, `get`, `get-thumb` or
+`helper fix-missing-thumbnails --file` is written in decimal digits, no larger
+than 9007199254740991. Anything else, such as `0x10`, `1e3` or `-1`, is refused
+before any request with one line, `Invalid file ID: <arg>` or
+`Invalid collection ID: <arg>`, and exit status 1. One bad `--file` ID refuses
+the whole command, so nothing is fixed.

 `get` and `get-thumb` resolve the file by ID directly, so `--collection` is
 accepted for backward compatibility but ignored. For a live photo, `get` writes
@@ -582,6 +654,10 @@ reads no tag from is recorded, base64, as `exifRaw`, with the reason in
 `exifError`. `collections`, `files`, `backup`, `helper list-missing-thumbnails`
 and `helper fix-missing-thumbnails` take `--json` for machine-readable output.

+`backup --verify` also hashes the originals already in the backup and replaces
+any that do not match the content hash Ente records; one it cannot replace goes
+into `failures.json` (see "Backup layout").
+
 `backup-metadata` fetches ML data in requests of up to 200 files. When a request
 fails, the error is logged, each of its files is written with the reason in an
 `mlDataError` field instead of `mlData`, and the dump goes on. The exit code is
@@ -611,8 +687,11 @@ the smallest does not.
                                    below)
        YYYY-MM-DD.<fileID>.json    the file's basic metadata fields quak
                                    keeps, its update time, its private and
-                                    public magic metadata, its ML data, and
-                                    its original's EXIF, XMP and dimensions
+                                    public magic metadata, its ML data, and,
+                                    for an image (for a live photo, its
+                                    image), its original's EXIF and XMP, with
+                                    dimensions for a JPEG only; none of these
+                                    three for a video
        YYYY-MM-DD.<fileID>.livephoto.json
                                    which of a live photo's two files is which
    collections/
@@ -666,6 +745,17 @@ succeeds, or once it is no longer in the library or in the backup's scope. The
 library's `lib.backup({ includeThumbnails: true })` also writes
 `thumbnails/<fileID>.jpg` beside `collections/`; `quak backup` does not.

+A file the library could not decrypt (see "Default reads vs. fresh reads") goes
+into `failures.json` with an empty title, since its title is encrypted too, and
+an error that starts `could not be decrypted:` and gives the reason. A file in
+two albums is backed up as usual if its copy in the other album decrypted. An
+album the library could not decrypt has no name quak can read and no files it
+can list. A run without `--json` prints one line for it,
+`FAILED album <id>: could not be decrypted: <reason>`, and `--json` and the
+`BackupResult` list its ID in `undecryptableAlbums`. Every other file is backed
+up. Either kind keeps the exit code at 1, run after run, until the server sends
+a copy that decrypts or deletes it; the next run then drops it.
+
 `backup.lock` keeps two backups of the same directory from running at once, such
 as a cron run that starts while the previous one is still going. A backup
 creates `<dir>` if it is missing and takes the lock before its refresh, and
@@ -715,6 +805,25 @@ if any files failed. `quak backup` opens its library with the thumbnail and
 originals precache off, so the only file content it fetches is the originals the
 backup stores.

+With `--verify`, or `lib.backup({ verify: true })`, a run also hashes each
+original already at its save path the way a download is checked (see "On-disk
+cache layout" below): its bytes, read in chunks, or a live photo's image and
+video, joined as `<imageHash>:<videoHash>`. An original that matches the content
+hash its metadata records is left as it is. One that does not is logged on one
+line naming the file, deleted (a live photo's image and video both), and put
+back in the same run like a missing one: downloaded, or copied from the cache if
+the cache holds it. What is put back is hashed too, because a copy from the
+cache is not checked as a download is. If it still does not match, it stays at
+its save path and the file goes into `failures.json`, as it does when the
+download fails. A file whose metadata records no hash is left as it is and
+counted as unchecked. A stored original that cannot be read is left as it is and
+counts as failed. The summary and `--json` add the counts `verified`,
+`mismatched` and `unchecked`, all of originals that were already stored; one
+first downloaded in this run is in none of them. A mismatch that was put back
+with matching bytes does not make the exit code non-zero. Without `--verify`
+nothing is hashed, the summary is unchanged, and the three counts are 0 in
+`--json`.
+
 Each original is written to a temporary file in the same directory, synced to
 disk, and renamed into place, so an original is either complete or absent, even
 after a power cut. A downloaded original's temporary file is named
@@ -756,7 +865,8 @@ The library's primary surface is the cache-backed `Library`; the lower-level
 `Client` sits underneath it and is covered by the Design sections above. The
 test suite is the canonical, executable documentation — `test/library/` and
 `test/client/usage.test.ts` walk most operations, `test/cli/backup.test.ts`
-walks `lib.backup()`, and `yarn test` verifies them.
+walks `lib.backup()`, `test/cli/backup-end-to-end.test.ts` runs `quak backup`
+against a fake of the Ente HTTP API, and `yarn test` verifies them.

 ### Opening a library

@@ -819,6 +929,16 @@ rejects the caller (default reads stay silent and keep serving the last good
 copy). The CLI's read commands use fresh reads (issue
 https://git.eeqj.de/sneak/quak/issues/75).

+An album or file in the server's answer that quak cannot decrypt, or whose
+metadata is not JSON, does not fail the refresh. The refresh leaves it out, an
+album with all its files, and drops any older copy the cache held, so reads
+answer from what did decrypt. The library keeps a list of what it left out, with
+the reason, in `metadata.json`: the next refresh asks the server only for what
+changed, so it would not see the record again. A record stays on that list until
+a later refresh gets a copy that decrypts or learns that the server deleted it.
+`lib.backup()` reports each such album, and each such file in its scope that has
+no copy that decrypted (see "Backup layout").
+
 ### Read surface

 - `lib.albums.list()` → `Album[]`, newest-updated first.
@@ -939,14 +1059,16 @@ photos newest first). `lib.subscribe({ onChange })` delivers a `LibraryChange`
  `fresh()` does, puts every in-scope original not already at its save path
  there as `photo.download()` does (and, with `includeThumbnails`, fetches
  thumbnails) through the content cache, waits for an ML data fetch, and
-  rebuilds the on-disk backup tree, each file's JSON with its ML data and its
-  original's EXIF, XMP and dimensions, with a durable failure ledger. A fetched
-  original is written straight to its save path and not into the cache, which
-  then counts it as present; one the cache already held is copied from there.
-  `BackupOptions`: `downloadDirectory` (falls back to the library's),
-  `includeOriginals` (default `true`), `includeThumbnails` (default `false`),
-  `onlyAlbumNames`, `onProgress`, and `lockHeld` (default `false`). See Backup
-  layout above for the tree it writes.
+  rebuilds the on-disk backup tree with a durable failure ledger. Each file's
+  JSON holds its ML data and, for an image (for a live photo, its image), its
+  original's EXIF and XMP, with its dimensions for a JPEG only; a video's JSON
+  holds none of these three. A fetched original is written straight to its save
+  path and not into the cache, which then counts it as present; one the cache
+  already held is copied from there. `BackupOptions`: `downloadDirectory` (falls
+  back to the library's), `includeOriginals` (default `true`),
+  `includeThumbnails` (default `false`), `onlyAlbumNames`, `verify` (default
+  `false`), `onProgress`, and `lockHeld` (default `false`). See Backup layout
+  above for the tree it writes.

 ### Request pools

@@ -25,6 +25,56 @@ declares one.

 # Completed Steps

+- 2026-10-13: An album or file quak cannot decrypt, or whose metadata is not
+  JSON, no longer fails the refresh, so one bad record no longer stops
+  `quak backup` from storing anything (issue 181). The refresh leaves it out, an
+  album with its files, and the library keeps it with the reason in
+  `metadata.json` until the server sends a copy that decrypts or deletes it. The
+  backup stores every other file, records each such file in `failures.json` with
+  an error starting `could not be decrypted:`, prints one line naming each such
+  album's ID, lists those IDs in `undecryptableAlbums`, and exits 1 while any
+  remain. A file whose copy in another album decrypts is backed up as usual. The
+  read commands answer from what decrypted.
+
+- 2026-10-14: `test/cli/backup-end-to-end.test.ts` runs `quak backup` as cron
+  does (issue 186): `backupCommand` and `run`, the real `loadSession` reading a
+  session saved after logging in, and every request answered by a fake of the
+  Ente HTTP API serving data encrypted with libsodium. It covers the backup tree
+  of an account with two albums of its own and one shared with it, a second run
+  that downloads nothing, a download that succeeds after two 503s, a 404 that
+  lands in `failures.json` with exit 1, a refresh answered 401 with exit 3, and
+  `--verify` putting back an original changed on disk. The fake server and the
+  account it logs in moved from the backup-metadata test into
+  `test/ente-server.ts`, unchanged.
+
+- 2026-10-14: `files --collection`, `get`, `get-thumb` and
+  `helper fix-missing-thumbnails --file` take an ID only when it is written in
+  decimal digits and is a safe integer (issue 185). Before, `Number()` read
+  `0x10` as 16, `1e3` as 1000 and an empty string as 0, and `--file` dropped an
+  ID it could not read without a word. Anything else now prints
+  `Invalid file ID: <arg>` or `Invalid collection ID: <arg>` and exits 1 before
+  any request; one bad `--file` ID fails the whole command. `parseID` in
+  `src/cli-commands.ts` does the check for all four.
+
+- 2026-10-06: The README says how to run `quak backup` from cron (issue 170):
+  log in once as the job's user, a crontab with a backup every night and
+  `--verify` on Sundays appending to a log file, and the `HOME` and
+  `XDG_DATA_HOME` the job needs to find the saved session. A table gives each
+  exit code: 0, the backup is complete; 1, files are in `failures.json` or
+  another error stopped the run; 2, another backup of the directory is running;
+  3, there is no usable session. The introduction lists everything the backup
+  keeps for each file.
+
+- 2026-10-06: `quak backup --verify` and `lib.backup({ verify: true })` hash
+  each original already at its save path as the download check does, streamed, a
+  live photo as `<imageHash>:<videoHash>` (issue 168). One that does not match
+  the content hash its metadata records is logged, removed (both files of a live
+  photo) and put back in the same run, downloaded or copied from the cache, and
+  what is put back is hashed too. One that still does not match, or whose
+  download fails, goes into `failures.json`. One with no recorded hash is left
+  alone. The result, `--json` and the summary gain `verified`, `mismatched` and
+  `unchecked`. Without `--verify` nothing is hashed.
+
 - 2026-10-06: Two backups of the same directory never run at once (issue 169).
  `lib.backup()` takes a lock, `backup.lock` in its download directory, made
  with `proper-lockfile`, before its refresh, and removes it when it ends,
@@ -130,9 +130,13 @@ program
    )
    .argument("<dir>", "Output directory")
    .option("--json", "Print result as JSON instead of human-readable summary")
+    .option(
+        "--verify",
+        "Re-hash stored originals and download again any that do not match",
+    )
    // A backup usually runs from cron with nobody watching, so every request
    // it makes retries for longer than the other commands' requests do.
-    .action((dir: string, opts: { json?: boolean }) =>
+    .action((dir: string, opts: { json?: boolean; verify?: boolean }) =>
        run(
            backupCommand(
                {
@@ -38,10 +38,18 @@
 // why they could not be read), so that a run does not read every stored
 // original again; a sidecar without them gets them read from the original.
 //
+// With `verify`, each original already at its save path is hashed as the
+// download check hashes it, and one that does not match the content hash its
+// metadata records is removed and fetched again in the same run. What is put
+// back is hashed too, and recorded as failed if it still does not match.
+//
 // Resilience (issue #8): no per-file condition aborts the run. A failed
 // download, a failed symlink, or ML data missing because the ML data fetch
 // failed is caught, recorded in `failures.json` with a classification, a
 // running attempt count, and the last-tried time, and the run continues.
+// A file the library could not decrypt is recorded there too, unless a copy of
+// it in another album decrypted; an album it could not decrypt is logged by ID
+// and listed in `result.undecryptableAlbums`, as its name is not known.
 // `result.failed` — and thus the CLI's exit code — stays non-zero while any
 // failure remains unresolved and clears once every one succeeds. Each run
 // reconciles the ledger against the files it attempted, so an entry for a file
@@ -50,6 +58,7 @@
 // code.

 import {
+    createReadStream,
    lstatSync,
    mkdirSync,
    readdirSync,
@@ -65,6 +74,12 @@ import { readFile } from "node:fs/promises";
 import { dirname, extname, join, relative, resolve } from "node:path";
 import lockfile from "proper-lockfile";

+import {
+    chunkHashFinal,
+    chunkHashInit,
+    chunkHashUpdate,
+    init,
+} from "./crypto/index.js";
 import { removeLeftoverTempFiles } from "./download/index.js";
 import { sanitizeFileName, withExtension } from "./filename.js";
 import {
@@ -74,6 +89,7 @@ import {
    storedAtSavePath,
 } from "./library/content.js";
 import { representative } from "./library/records.js";
+import type { Undecryptable } from "./library/store.js";
 import { extractImageMetadata } from "./metadata-backup.js";
 import type { MLData } from "./mldata-fetch.js";
 import type { Collection, EnteFile, FileMetadata } from "./model/types.js";
@@ -92,6 +108,9 @@ export interface BackupOptions {
    includeThumbnails?: boolean;
    // Restrict the backup to albums with these names; others are left untouched.
    onlyAlbumNames?: string[];
+    // Hash each original already at its save path, and fetch again any whose
+    // bytes do not match the content hash its metadata records. Default false.
+    verify?: boolean;
    onProgress?: ProgressCallback;
    // The caller already holds the lock in `downloadDirectory`, taken with
    // `lockBackupDirectory`, and releases it itself, so the backup does not
@@ -113,12 +132,22 @@ export interface BackupResult {
    downloaded: number;
    // Originals already at their save path and left untouched.
    skipped: number;
+    // With `verify`, the originals already at their save path whose hash
+    // matched, those whose hash did not (each removed and fetched again), and
+    // those whose metadata records no hash (left as they are). All three are
+    // zero without `verify`.
+    verified: number;
+    mismatched: number;
+    unchecked: number;
    // Files with an unresolved failure after this run (the ledger size); the
    // CLI exits non-zero while this is above zero. A file can be both
    // downloaded and failed if its bytes landed but its symlink did not.
    failed: number;
    // This run's per-file errors, in encounter order.
    errors: BackupError[];
+    // The IDs of the albums the library could not decrypt, whose files it
+    // could not list; the CLI exits non-zero while there are any.
+    undecryptableAlbums: number[];
 }

 // The slice of the library that backup drives. `Library` implements it; a test
@@ -129,6 +158,9 @@ export interface BackupLibrary {
    refresh(): Promise<void>;
    listCollections(): Collection[];
    listFiles(collectionID: number): EnteFile[];
+    // The albums, and files in albums, that the library could not decrypt,
+    // which `listCollections` and `listFiles` leave out.
+    undecryptable(): Undecryptable[];
    // Get an original's bytes onto disk through the content cache/pools,
    // returning where they landed: `destination` when they were fetched now,
    // otherwise wherever they already were (the cache, or the library's save
@@ -369,6 +401,26 @@ const saveLedger = (path: string, ledger: Map<number, FailureEntry>): void => {
    );
 };

+// The content hash of the original stored at `stored`, computed as the download
+// check computes it: over each file's bytes, read in chunks, and for a live
+// photo `<imageHash>:<videoHash>`.
+const storedHash = async (stored: {
+    path: string;
+    videoPath?: string;
+}): Promise<string> => {
+    await init();
+    const hashFile = async (path: string): Promise<string> => {
+        const state = chunkHashInit();
+        for await (const chunk of createReadStream(path)) {
+            chunkHashUpdate(state, chunk as Buffer);
+        }
+        return chunkHashFinal(state);
+    };
+    const hash = await hashFile(stored.path);
+    if (stored.videoPath === undefined) return hash;
+    return `${hash}:${await hashFile(stored.videoPath)}`;
+};
+
 // A file's EXIF, XMP and dimensions as its JSON holds them: what
 // `extractImageMetadata` found in its original, or why the original could not
 // be read.
@@ -468,6 +520,7 @@ const runLockedBackup = async (
    const includeThumbnails = opts.includeThumbnails ?? false;
    const log = opts.onProgress ?? (() => {});
    const only = opts.onlyAlbumNames ? new Set(opts.onlyAlbumNames) : undefined;
+    const verify = opts.verify ?? false;

    log("Refreshing library...");
    await lib.refresh();
@@ -526,41 +579,105 @@ const runLockedBackup = async (
    const storedThisRun = new Set<number>();
    let downloaded = 0;
    let skipped = 0;
+    let verified = 0;
+    let mismatched = 0;
+    let unchecked = 0;

    const recordFailure = (
-        file: EnteFile,
+        fileID: number,
+        title: string,
        collection: string,
        err: unknown,
    ): void => {
        // Count at most one attempt per file per run: a file whose original
        // and thumbnail both fail this run must not double its attempt count
        // or appear twice in errors.
-        if (failedThisRun.has(file.id)) return;
+        if (failedThisRun.has(fileID)) return;
        const error = errorMessage(err);
-        errors.push({
-            fileID: file.id,
-            title: file.metadata.title,
-            collection,
-            error,
-        });
-        const prior = ledger.get(file.id);
-        ledger.set(file.id, {
-            fileID: file.id,
-            title: file.metadata.title,
+        errors.push({ fileID, title, collection, error });
+        const prior = ledger.get(fileID);
+        ledger.set(fileID, {
+            fileID,
+            title,
            classification: classify(err),
            attempts: (prior?.attempts ?? 0) + 1,
            lastTriedAt: now,
            error,
        });
-        failedThisRun.add(file.id);
+        failedThisRun.add(fileID);
    };

+    // What the library could not decrypt. An album is logged by its ID, as
+    // its name is not known, so a run limited to some albums logs it too. A
+    // file in an album in scope is backed up from a copy of it in another
+    // album that decrypted, even an album out of scope. With no such copy it
+    // is recorded as failed, with an empty title as its metadata is not known.
+    const scope = new Set(collections.map((c) => c.id));
+    const undecryptableAlbums: number[] = [];
+    for (const record of lib.undecryptable()) {
+        const error = `could not be decrypted: ${record.error}`;
+        if (record.fileID === undefined) {
+            log(`FAILED album ${record.collectionID}: ${error}`);
+            undecryptableAlbums.push(record.collectionID);
+        } else if (scope.has(record.collectionID)) {
+            const copies = memberships.get(record.fileID);
+            if (copies) {
+                distinct.set(record.fileID, representative(copies));
+            } else {
+                recordFailure(
+                    record.fileID,
+                    "",
+                    collectionName.get(record.collectionID) ?? "",
+                    new Error(error),
+                );
+            }
+        }
+    }
+
    // Phase 1: get the bytes. Put each pending original at its save path
    // through the content cache/pools, as `Photo.download()` does, and fetch
-    // the optional thumbnails; a present file is left as is.
+    // the optional thumbnails; a present file is left as is. With `verify`, a
+    // present original is hashed first, and one that does not match the hash
+    // its metadata records is removed and fetched like a missing one. One
+    // that cannot be read is recorded as failed and left where it is.
    if (includeOriginals) {
        for (const [fileID, file] of distinct) {
-            if (storedAtSavePath(downloadDirectory, file) !== undefined) {
+            let stored = storedAtSavePath(downloadDirectory, file);
+            let mismatch = false;
+            if (stored !== undefined && verify) {
+                try {
+                    if (file.metadata.hash === undefined) {
+                        unchecked++;
+                    } else if (
+                        (await storedHash(stored)) === file.metadata.hash
+                    ) {
+                        verified++;
+                    } else {
+                        log(
+                            `MISMATCH original ${file.metadata.title} (${fileID}): its bytes do not match its content hash`,
+                        );
+                        mismatched++;
+                        mismatch = true;
+                        rmSync(stored.path);
+                        if (stored.videoPath !== undefined) {
+                            rmSync(stored.videoPath);
+                        }
+                        stored = undefined;
+                    }
+                } catch (err) {
+                    log(
+                        `FAILED verifying original ${file.metadata.title}: ${errorMessage(err)}`,
+                    );
+                    recordFailure(
+                        file.id,
+                        file.metadata.title,
+                        collectionName.get(file.collectionID) ?? "",
+                        err,
+                    );
+                    continue;
+                }
+            }
+            if (stored !== undefined) {
                skipped++;
                continue;
            }
@@ -569,17 +686,32 @@ const runLockedBackup = async (
                // A fetched original is written straight to its save path (a
                // live photo beside it); only one that was already cached
                // elsewhere is copied.
-                await placeOriginal(downloadDirectory, file, (dest) =>
-                    lib.original(fileID, dest),
+                const placed = await placeOriginal(
+                    downloadDirectory,
+                    file,
+                    (dest) => lib.original(fileID, dest),
                );
                storedThisRun.add(fileID);
+                // A copy from the cache is not checked as a download is and can
+                // hold the same bad bytes, so what is put back after a
+                // mismatch is hashed too. A bad copy stays where it is and the
+                // file fails.
+                if (
+                    mismatch &&
+                    (await storedHash(placed)) !== file.metadata.hash
+                ) {
+                    throw new Error(
+                        "the original put back does not match its content hash either",
+                    );
+                }
                downloaded++;
            } catch (err) {
                log(
                    `FAILED original ${file.metadata.title}: ${errorMessage(err)}`,
                );
                recordFailure(
-                    file,
+                    file.id,
+                    file.metadata.title,
                    collectionName.get(file.collectionID) ?? "",
                    err,
                );
@@ -596,7 +728,8 @@ const runLockedBackup = async (
                await copyAtomic(path, dest);
            } catch (err) {
                recordFailure(
-                    file,
+                    file.id,
+                    file.metadata.title,
                    collectionName.get(file.collectionID) ?? "",
                    err,
                );
@@ -636,7 +769,8 @@ const runLockedBackup = async (
            if (mlData === undefined && mlDataError !== undefined) {
                writeSidecar(path, file, { mlDataError }, image);
                recordFailure(
-                    file,
+                    file.id,
+                    file.metadata.title,
                    collectionName.get(file.collectionID) ?? "",
                    new Error(`ML data: ${mlDataError}`),
                );
@@ -709,7 +843,12 @@ const runLockedBackup = async (
                log(
                    `FAILED symlink ${c.name}/${linkName}: ${errorMessage(err)}`,
                );
-                recordFailure(link.file, c.name, err);
+                recordFailure(
+                    link.file.id,
+                    link.file.metadata.title,
+                    c.name,
+                    err,
+                );
            }
        }

@@ -735,8 +874,12 @@ const runLockedBackup = async (
        totalFiles: distinct.size,
        downloaded,
        skipped,
+        verified,
+        mismatched,
+        unchecked,
        failed: ledger.size,
        errors,
+        undecryptableAlbums,
    };
 };

@@ -97,6 +97,20 @@ const requireSession = (ctx: CliContext): Client | undefined => {
    return client;
 };

+// `text` as an ID, or undefined after telling the user it is not one. An ID is
+// written in decimal digits and is a safe integer; `Number()` alone would read
+// `0x10` as 16, `1e3` as 1000 and "" as 0.
+const parseID = (
+    ctx: CliContext,
+    kind: "file" | "collection",
+    text: string,
+): number | undefined => {
+    const id = Number(text);
+    if (/^[0-9]+$/.test(text) && Number.isSafeInteger(id)) return id;
+    ctx.stderr.write(`Invalid ${kind} ID: ${text}\n`);
+    return undefined;
+};
+
 // A library client that omits `fetchMLData`, so the point commands below do not
 // kick the library's background ML backfill: they read metadata, or fetch one
 // file's content, and exit. `backup` and `backup-metadata` handle ML on their
@@ -247,11 +261,8 @@ export const filesCommand = async (
    await init();
    const client = requireSession(ctx);
    if (!client) return 3;
-    const collectionID = Number(opts.collection);
-    if (!Number.isFinite(collectionID)) {
-        ctx.stderr.write("Invalid collection ID\n");
-        return 1;
-    }
+    const collectionID = parseID(ctx, "collection", opts.collection);
+    if (collectionID === undefined) return 1;

    const lib = await openReadLibrary(ctx, client);
    try {
@@ -289,11 +300,8 @@ export const getCommand = async (
    await init();
    const client = requireSession(ctx);
    if (!client) return 3;
-    const fileID = Number(fileIDStr);
-    if (!Number.isFinite(fileID)) {
-        ctx.stderr.write("Invalid file ID\n");
-        return 1;
-    }
+    const fileID = parseID(ctx, "file", fileIDStr);
+    if (fileID === undefined) return 1;

    const lib = await openReadLibrary(ctx, client);
    try {
@@ -347,11 +355,8 @@ export const getThumbCommand = async (
    await init();
    const client = requireSession(ctx);
    if (!client) return 3;
-    const fileID = Number(fileIDStr);
-    if (!Number.isFinite(fileID)) {
-        ctx.stderr.write("Invalid file ID\n");
-        return 1;
-    }
+    const fileID = parseID(ctx, "file", fileIDStr);
+    if (fileID === undefined) return 1;

    const lib = await openReadLibrary(ctx, client);
    try {
@@ -402,7 +407,7 @@ export const backupMetadataCommand = async (
 export const backupCommand = async (
    ctx: CliContext,
    dir: string,
-    opts: { json?: boolean },
+    opts: { json?: boolean; verify?: boolean },
 ): Promise<number> => {
    await init();
    const client = requireSession(ctx);
@@ -433,6 +438,7 @@ export const backupCommand = async (
            const result = await lib.backup({
                downloadDirectory: dir,
                lockHeld: true,
+                verify: opts.verify,
                onProgress: (msg) => {
                    if (!opts.json) ctx.stderr.write(msg + "\n");
                },
@@ -445,6 +451,11 @@ export const backupCommand = async (
                ctx.stderr.write(`  Total files: ${result.totalFiles}\n`);
                ctx.stderr.write(`  Downloaded:  ${result.downloaded}\n`);
                ctx.stderr.write(`  Skipped:     ${result.skipped}\n`);
+                if (opts.verify) {
+                    ctx.stderr.write(`  Verified:    ${result.verified}\n`);
+                    ctx.stderr.write(`  Mismatched:  ${result.mismatched}\n`);
+                    ctx.stderr.write(`  Unchecked:   ${result.unchecked}\n`);
+                }
                ctx.stderr.write(`  Failed:      ${result.failed}\n`);
                if (result.errors.length > 0) {
                    ctx.stderr.write("\nFailed files:\n");
@@ -456,7 +467,9 @@ export const backupCommand = async (
                }
            }

-            return result.failed > 0 ? 1 : 0;
+            return result.failed > 0 || result.undecryptableAlbums.length > 0
+                ? 1
+                : 0;
        } finally {
            await lib.close();
        }
@@ -510,15 +523,19 @@ export const fixMissingThumbnailsCommand = async (
    await init();
    const client = requireSession(ctx);
    if (!client) return 3;
+    // One bad --file ID fails the whole command, before anything is fetched.
+    let fileIDs: number[] = [];
+    for (const text of opts.file ?? []) {
+        const fileID = parseID(ctx, "file", text);
+        if (fileID === undefined) return 1;
+        fileIDs.push(fileID);
+    }
    const lib = await openReadLibrary(ctx, client);
    try {
        // Refresh first so files added since the cache was written are found;
        // a failed refresh throws.
        await lib.fresh();
-        let fileIDs: number[];
-        if (opts.file && opts.file.length > 0) {
-            fileIDs = opts.file.map(Number).filter(Number.isFinite);
-        } else {
+        if (fileIDs.length === 0) {
            ctx.stderr.write("Scanning for missing thumbnails...\n");
            const missing = await listMissingThumbnails(lib, client, (msg) => {
                if (!opts.json) ctx.stderr.write(msg + "\n");
@@ -44,20 +44,28 @@ export interface ClientSnapshot {

 // The result of a resumable enumeration. Live decrypted records and deleted
 // ids are kept apart on purpose: a tombstone carries no key or metadata to
-// decrypt, so it is a bare id rather than a hollowed-out record. `cursor` is
-// the max `updationTime` seen, to pass back into the next call.
+// decrypt, so it is a bare id rather than a hollowed-out record. A live row
+// that cannot be decrypted, or whose metadata cannot be parsed, is in
+// `undecryptable` with the reason instead, so one bad row does not fail the
+// call; absent means there was none. `cursor` is the max `updationTime` seen,
+// those rows included, to pass back into the next call.
 export interface CollectionsPage {
    collections: Collection[];
    deleted: number[];
+    undecryptable?: { id: number; error: string }[];
    cursor: number;
 }

 export interface FilesPage {
    files: EnteFile[];
    deleted: number[];
+    undecryptable?: { id: number; error: string }[];
    cursor: number;
 }

+const errorMessage = (err: unknown): string =>
+    err instanceof Error ? err.message : String(err);
+
 export class Client {
    private readonly api: ApiClient;
    private readonly email: string;
@@ -237,11 +245,12 @@ export class Client {
    }

    // Enumerate collections changed since `sinceTime`. Live collections are
-    // decrypted; tombstoned ones (isDeleted) are surfaced as bare ids. The
-    // returned cursor is the max `updationTime` seen — including tombstones, so
-    // the next sync resumes past them — falling back to `sinceTime` when the
-    // response is empty. `/collections/v2` returns the whole changed set in one
-    // response, so there is no pagination here.
+    // decrypted; tombstoned ones (isDeleted) are surfaced as bare ids, and
+    // ones that do not decrypt as ids with the reason. The returned cursor is
+    // the max `updationTime` seen — including tombstones, so the next sync
+    // resumes past them — falling back to `sinceTime` when the response is
+    // empty. `/collections/v2` returns the whole changed set in one response,
+    // so there is no pagination here.
    async collectionsSince(args: {
        sinceTime: number;
    }): Promise<CollectionsPage> {
@@ -254,31 +263,40 @@ export class Client {

        const collections: Collection[] = [];
        const deleted: number[] = [];
+        const undecryptable: { id: number; error: string }[] = [];
        let cursor = args.sinceTime;
        for (const raw of raws) {
            if (raw.isDeleted) {
                deleted.push(raw.id);
            } else {
-                collections.push(
-                    decryptCollection(
-                        raw,
-                        {
-                            masterKey: this.masterKey,
-                            publicKey: this.publicKey,
-                            secretKey: this.secretKey,
-                        },
-                        this.userID,
-                    ),
-                );
+                try {
+                    collections.push(
+                        decryptCollection(
+                            raw,
+                            {
+                                masterKey: this.masterKey,
+                                publicKey: this.publicKey,
+                                secretKey: this.secretKey,
+                            },
+                            this.userID,
+                        ),
+                    );
+                } catch (err) {
+                    undecryptable.push({
+                        id: raw.id,
+                        error: errorMessage(err),
+                    });
+                }
            }
            if (raw.updationTime > cursor) cursor = raw.updationTime;
        }
-        return { collections, deleted, cursor };
+        return { collections, deleted, undecryptable, cursor };
    }

    // Enumerate a collection's files changed since `sinceTime`, paginating the
    // diff from that cursor. Live rows are decrypted; tombstoned ones are
-    // surfaced as bare ids. Returns the final cursor to resume from.
+    // surfaced as bare ids, and ones that do not decrypt or parse as ids with
+    // the reason. Returns the final cursor to resume from.
    async filesSince(args: {
        collectionID: number;
        collectionKey: Uint8Array;
@@ -288,6 +306,7 @@ export class Client {
        const { collectionID, collectionKey } = args;
        const files: EnteFile[] = [];
        const deleted: number[] = [];
+        const undecryptable: { id: number; error: string }[] = [];
        let cursor = args.sinceTime;
        for (;;) {
            const { diff, hasMore } = await this.api.getJSON<{
@@ -300,7 +319,14 @@ export class Client {
                if (raw.isDeleted) {
                    deleted.push(raw.id);
                } else {
-                    files.push(decryptFile(raw, collectionKey));
+                    try {
+                        files.push(decryptFile(raw, collectionKey));
+                    } catch (err) {
+                        undecryptable.push({
+                            id: raw.id,
+                            error: errorMessage(err),
+                        });
+                    }
                }
                if (raw.updationTime > pageMax) pageMax = raw.updationTime;
            }
@@ -321,18 +347,19 @@ export class Client {
            }
            cursor = pageMax;
        }
-        return { files, deleted, cursor };
+        return { files, deleted, undecryptable, cursor };
    }

-    // Whole-account listing: every live collection, deletions hidden. A thin
-    // wrapper over `collectionsSince` from the beginning of time.
+    // Whole-account listing: every live collection that decrypts, deletions
+    // hidden. A thin wrapper over `collectionsSince` from the beginning of
+    // time.
    async listCollections(): Promise<Collection[]> {
        const { collections } = await this.collectionsSince({ sinceTime: 0 });
        return collections;
    }

-    // Every live file in a collection, deletions hidden. A thin wrapper over
-    // `filesSince` from the beginning of time.
+    // Every live file in a collection that decrypts, deletions hidden. A thin
+    // wrapper over `filesSince` from the beginning of time.
    async listFiles(
        collectionID: number,
        collectionKey: Uint8Array,
@@ -598,6 +598,7 @@ export class Library {
                refresh: () => this.refreshNow(),
                listCollections: () => this.store.listCollections(),
                listFiles: (id) => this.store.listFiles(id),
+                undecryptable: () => this.store.listUndecryptable(),
                original: (fileID, destination) =>
                    cache!.backupOriginal(fileID, destination),
                thumbnail: (fileID) => cache!.thumbnail(fileID),
@@ -699,7 +700,10 @@ export class Library {

    // Fetch every change since the stored cursor, then commit. All network
    // reads happen before any store mutation, so a fetch that throws leaves the
-    // store untouched and the previous snapshot intact.
+    // store untouched and the previous snapshot intact. A collection or file
+    // that does not decrypt is left out instead, as if deleted, with the reason
+    // kept in the store: the cursor moves past it, so no later refresh sees it
+    // again until the server's copy changes.
    private async refreshOnce(): Promise<void> {
        const page = await this.client.collectionsSince({
            sinceTime: this.store.collectionsSinceTime,
@@ -736,9 +740,17 @@ export class Library {
                this.store.deleteCollection(id);
                changed = true;
            }
+            if (this.store.deleteUndecryptable(id)) changed = true;
+        }
+        // A collection left out takes its files with it.
+        for (const { id, error } of page.undecryptable ?? []) {
+            this.store.deleteCollection(id);
+            this.store.putUndecryptable({ collectionID: id, error });
+            changed = true;
        }
        for (const collection of page.collections) {
            this.store.putCollection(collection);
+            this.store.deleteUndecryptable(collection.id);
            changed = true;
        }

@@ -748,9 +760,22 @@ export class Library {
                    this.store.deleteFile(collectionID, id);
                    changed = true;
                }
+                if (this.store.deleteUndecryptable(collectionID, id)) {
+                    changed = true;
+                }
+            }
+            for (const { id, error } of filePage.undecryptable ?? []) {
+                this.store.deleteFile(collectionID, id);
+                this.store.putUndecryptable({
+                    collectionID,
+                    fileID: id,
+                    error,
+                });
+                changed = true;
            }
            for (const f of filePage.files) {
                this.store.putFile(f);
+                this.store.deleteUndecryptable(collectionID, f.id);
                changed = true;
            }
        }
@@ -2,10 +2,11 @@
 //
 // The store keeps one `metadata.json` file holding the account's server
 // state: the user id, a schema version, the cursor for the incremental
-// collections listing, and the decrypted collection and file records. The
-// whole file is read into RAM on load and rewritten as a whole on save; there
-// is no partial update and no lock file. A separate refresh unit populates the
-// store from the server — this module only stores what it is given.
+// collections listing, the decrypted collection and file records, and the
+// collections and files the refresh could not decrypt. The whole file is read
+// into RAM on load and rewritten as a whole on save; there is no partial
+// update and no lock file. A separate refresh unit populates the store from
+// the server — this module only stores what it is given.
 //
 // The file is a cache, so it is never trusted to exist or to be intact: a
 // missing or unreadable file loads as an empty store rather than an error, and
@@ -33,12 +34,22 @@ const FILE_MODE = 0o600;
 type StoredCollection = Omit<Collection, "key"> & { key: string };
 type StoredFile = Omit<EnteFile, "key"> & { key: string };

+// A collection, or with `fileID` a file in it, that the last refresh to fetch
+// it could not decrypt or parse, and why. It is kept until a later refresh
+// fetches it again, because a refresh fetches only what changed.
+export interface Undecryptable {
+    collectionID: number;
+    fileID?: number;
+    error: string;
+}
+
 interface StoredMetadata {
    schemaVersion: number;
    userID: number;
    collectionsSinceTime: Microseconds;
    collections: StoredCollection[];
    files: StoredFile[];
+    undecryptable: Undecryptable[];
 }

 const encodeKey = (key: Uint8Array): string =>
@@ -67,6 +78,7 @@ export class MetadataStore {

    private readonly collections = new Map<number, Collection>();
    private readonly files = new Map<string, EnteFile>();
+    private undecryptable: Undecryptable[] = [];

    private constructor(path: string) {
        this.path = path;
@@ -107,6 +119,7 @@ export class MetadataStore {
                };
                store.files.set(fileKey(file.collectionID, file.id), file);
            }
+            store.undecryptable = parsed.undecryptable ?? [];
        } catch {
            // Any corruption discards the partial result: a half-read cache is
            // worse than an empty one, since the refresh unit will rebuild it.
@@ -134,6 +147,7 @@ export class MetadataStore {
                ...f,
                key: encodeKey(f.key),
            })),
+            undecryptable: this.undecryptable,
        };

        const dir = dirname(this.path);
@@ -165,7 +179,8 @@ export class MetadataStore {
        this.collections.set(collection.id, collection);
    }

-    // Removing a collection also drops its file memberships: a file record is
+    // Removing a collection also drops its file memberships, and any record
+    // that it or one of its files could not be decrypted: a file record is
    // only meaningful as part of a collection the cache still knows about.
    deleteCollection(id: number): void {
        this.collections.delete(id);
@@ -174,6 +189,9 @@ export class MetadataStore {
                this.files.delete(key);
            }
        }
+        this.undecryptable = this.undecryptable.filter(
+            (record) => record.collectionID !== id,
+        );
    }

    getFile(collectionID: number, fileID: number): EnteFile | undefined {
@@ -205,4 +223,26 @@ export class MetadataStore {
    deleteFile(collectionID: number, fileID: number): void {
        this.files.delete(fileKey(collectionID, fileID));
    }
+
+    listUndecryptable(): Undecryptable[] {
+        return [...this.undecryptable];
+    }
+
+    // Replaces any earlier record of the same collection or file.
+    putUndecryptable(record: Undecryptable): void {
+        this.deleteUndecryptable(record.collectionID, record.fileID);
+        this.undecryptable.push(record);
+    }
+
+    // Forgets the record of the collection, or with `fileID` of that file in
+    // it. Returns whether there was one.
+    deleteUndecryptable(collectionID: number, fileID?: number): boolean {
+        const before = this.undecryptable.length;
+        this.undecryptable = this.undecryptable.filter(
+            (record) =>
+                record.collectionID !== collectionID ||
+                record.fileID !== fileID,
+        );
+        return this.undecryptable.length < before;
+    }
 }
@@ -0,0 +1,635 @@
+/**
+ * `quak backup`, end to end.
+ *
+ * Each test runs the command as cron runs it: `backupCommand` reads the
+ * session file that logging in saved, opens the library on the real `Client`
+ * and backs the whole account up into a directory, and `run` turns the outcome
+ * into the exit code, as `bin/quak.ts` does. Only the server is a stand-in:
+ * `buildMetaFetch` from `test/ente-server.ts` answers the Ente HTTP API from
+ * data this file encrypts with libsodium, as Ente's clients encrypt it. So
+ * quak's own code fetches each original, decrypts it, checks it against the
+ * hash its uploader recorded and unpacks a live photo, and quak's own retries
+ * repeat a failed request.
+ *
+ * The account, user 42, has three albums:
+ *
+ *     Trips              its own: beach.jpg
+ *     Family             its own: beach.jpg and the live photo IMG_0002.HEIC
+ *     Friend's Wedding   shared with it by user 99: toast.mp4
+ *
+ * beach.jpg is one file in two albums. It holds EXIF (a Canon EOS R5), and
+ * Ente has ML data for it. All three files were taken at noon on 1 March 2024.
+ */
+
+import {
+    existsSync,
+    lstatSync,
+    mkdtempSync,
+    readdirSync,
+    readFileSync,
+    readlinkSync,
+    rmSync,
+    writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { Writable } from "node:stream";
+import { gzipSync } from "node:zlib";
+import sodium from "libsodium-wrappers-sumo";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
+
+import {
+    backupCommand,
+    saveSession,
+    type CliContext,
+} from "../../src/cli-commands.js";
+import { run } from "../../src/cli-run.js";
+import { loadSession } from "../../src/cli-session.js";
+import { Client } from "../../src/client.js";
+import { init, toBase64 } from "../../src/crypto/index.js";
+import { UNATTENDED_RETRY_OPTIONS } from "../../src/retry.js";
+import {
+    buildAccount,
+    buildMetaFetch,
+    encryptSecretbox,
+    encryptStreamBlob,
+} from "../ente-server.js";
+import { HEIC_WITH_EXIF } from "../exif-heic.js";
+import { JPEG_WITH_EXIF } from "../exif-jpeg.js";
+import { blake2b, livePhotoHash, livePhotoZip, VIDEO } from "../live-photo.js";
+
+const EMAIL = "backup@example.com";
+const PASSWORD = "backup-password";
+// The fake server logs every account in as user 42.
+const USER_ID = 42;
+const FRIEND_ID = 99;
+
+// Noon on 1 March 2024 in the time zone of the machine running the test, the
+// one quak dates save paths in, in microseconds, as Ente records times.
+const TAKEN = new Date(2024, 2, 1, 12).getTime() * 1000;
+// The date folder every original is saved in.
+const DAY = "2024/2024-03/2024-03-01";
+
+const TOAST = new TextEncoder().encode("a few seconds of the wedding toast");
+
+// Ente's ML data for beach.jpg: a face it found, and the photo's CLIP
+// embedding.
+const ML_DATA = {
+    face: { faces: [{ faceID: "100_1", score: 0.98 }] },
+    clip: { embedding: [0.5, 0.6, 0.7] },
+};
+
+// Each file's ID, its owner, its original as uploaded, and the metadata its
+// uploader recorded. `fileType` is Ente's number for the kind: 0 an image, 1 a
+// video, 2 a live photo, whose original is a ZIP of its image and its video.
+// `hash` is the content hash a download must match.
+const BEACH = {
+    id: 100,
+    ownerID: USER_ID,
+    original: JPEG_WITH_EXIF,
+    metadata: {
+        title: "beach.jpg",
+        fileType: 0,
+        hash: blake2b(JPEG_WITH_EXIF),
+    },
+};
+const TOAST_VIDEO = {
+    id: 200,
+    ownerID: FRIEND_ID,
+    original: TOAST,
+    metadata: { title: "toast.mp4", fileType: 1, hash: blake2b(TOAST) },
+};
+const LIVE_PHOTO = {
+    id: 300,
+    ownerID: USER_ID,
+    original: livePhotoZip({
+        "image.heic": HEIC_WITH_EXIF,
+        "video.mov": VIDEO,
+    }),
+    metadata: {
+        title: "IMG_0002.HEIC",
+        fileType: 2,
+        hash: livePhotoHash(HEIC_WITH_EXIF, VIDEO),
+    },
+};
+
+// The fake Ente server, as the `fetch` quak sends its requests through.
+let server: typeof globalThis.fetch;
+let root: string;
+let sessionDir: string;
+
+// The fake server, holding the account encrypted as Ente's clients upload it.
+const buildServer = async () => {
+    const account = await buildAccount(PASSWORD);
+
+    // An album as `/collections/v2` lists it, its name encrypted with its own
+    // key. An album of the account's own carries that key encrypted with the
+    // master key; an album shared with it carries the key sealed to its public
+    // key, and no nonce.
+    const album = (id: number, name: string, ownerID: number) => {
+        const key = sodium.crypto_secretbox_keygen();
+        const encryptedName = encryptSecretbox(
+            new TextEncoder().encode(name),
+            key,
+        );
+        let keyFields: { encryptedKey: string; keyDecryptionNonce?: string };
+        if (ownerID === USER_ID) {
+            const encryptedKey = encryptSecretbox(key, account.masterKey);
+            keyFields = {
+                encryptedKey: toBase64(encryptedKey.ciphertext),
+                keyDecryptionNonce: toBase64(encryptedKey.nonce),
+            };
+        } else {
+            const sealedKey = sodium.crypto_box_seal(key, account.publicKey);
+            keyFields = { encryptedKey: toBase64(sealedKey) };
+        }
+        const row = {
+            id,
+            owner: { id: ownerID },
+            ...keyFields,
+            encryptedName: toBase64(encryptedName.ciphertext),
+            nameDecryptionNonce: toBase64(encryptedName.nonce),
+            type: "album",
+            updationTime: TAKEN,
+        };
+        return { key, row };
+    };
+    const trips = album(10, "Trips", USER_ID);
+    const family = album(20, "Family", USER_ID);
+    const wedding = album(30, "Friend's Wedding", FRIEND_ID);
+
+    // Each file's original is encrypted with the file's own key, as one
+    // secretstream, which the server serves whole.
+    const encrypted = (file: typeof BEACH) => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        return { file, key, content: encryptStreamBlob(file.original, key) };
+    };
+    const beach = encrypted(BEACH);
+    const toast = encrypted(TOAST_VIDEO);
+    const livePhoto = encrypted(LIVE_PHOTO);
+
+    // A file as an album's `/collections/v2/diff` lists it: its key encrypted
+    // with the album's key, its metadata encrypted with its own key, and the
+    // header that starts the decryption of its original.
+    const inAlbum = (
+        { file, key, content }: ReturnType<typeof encrypted>,
+        collectionID: number,
+        albumKey: Uint8Array,
+    ) => {
+        const encryptedKey = encryptSecretbox(key, albumKey);
+        const metadata = encryptStreamBlob(
+            new TextEncoder().encode(
+                JSON.stringify({
+                    ...file.metadata,
+                    creationTime: TAKEN,
+                    modificationTime: TAKEN,
+                }),
+            ),
+            key,
+        );
+        return {
+            id: file.id,
+            collectionID,
+            ownerID: file.ownerID,
+            encryptedKey: toBase64(encryptedKey.ciphertext),
+            keyDecryptionNonce: toBase64(encryptedKey.nonce),
+            metadata: {
+                encryptedData: toBase64(metadata.ciphertext),
+                decryptionHeader: toBase64(metadata.header),
+            },
+            file: { decryptionHeader: toBase64(content.header) },
+            thumbnail: {
+                decryptionHeader: toBase64(sodium.randombytes_buf(24)),
+            },
+            updationTime: TAKEN,
+        };
+    };
+
+    // ML data is gzipped JSON, encrypted with the file's key.
+    const mlData = encryptStreamBlob(
+        gzipSync(JSON.stringify(ML_DATA)),
+        beach.key,
+    );
+
+    return buildMetaFetch({
+        verifier: account.verifier,
+        srpAttributes: account.srpAttributes,
+        keyAttributes: account.keyAttributes,
+        encryptedToken: account.encryptedToken,
+        collections: [trips.row, family.row, wedding.row],
+        filesByCollection: {
+            10: [inAlbum(beach, 10, trips.key)],
+            20: [
+                inAlbum(beach, 20, family.key),
+                inAlbum(livePhoto, 20, family.key),
+            ],
+            30: [inAlbum(toast, 30, wedding.key)],
+        },
+        encryptedMLData: {
+            100: {
+                encryptedData: toBase64(mlData.ciphertext),
+                decryptionHeader: toBase64(mlData.header),
+            },
+        },
+        fileCiphertexts: {
+            100: beach.content.ciphertext,
+            200: toast.content.ciphertext,
+            300: livePhoto.content.ciphertext,
+        },
+    });
+};
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+    root = mkdtempSync(join(tmpdir(), "quak-backup-end-to-end-"));
+    server = await buildServer();
+
+    // Log in once, as `quak login` does: the session file it saves is what
+    // every backup below reads.
+    sessionDir = join(root, "session");
+    const client = await Client.login({
+        email: EMAIL,
+        password: PASSWORD,
+        apiOptions: { fetch: server },
+    });
+    saveSession(sessionDir, client.toJSON());
+});
+
+afterAll(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+// Run `quak backup <dir>` as `bin/quak.ts` runs it, sending its requests
+// through `fetch`, and return its exit code and everything it printed on
+// stderr. Its requests are retried as those of `quak backup` are, except that
+// a retry does not wait. Its cache is beside `dir`, so each run of one
+// directory finds the cache the run before it left, as cron's runs find the
+// user's.
+const backup = async (
+    dir: string,
+    opts: { verify?: boolean } = {},
+    fetch = server,
+): Promise<{ code: number; stderr: string }> => {
+    let printed = "";
+    const stderr = new Writable({
+        write(chunk: Buffer, _encoding, done) {
+            printed += chunk.toString();
+            done();
+        },
+    });
+    const stdout = new Writable({
+        write(_chunk, _encoding, done) {
+            done();
+        },
+    });
+    // A backup never prompts: any of these ends the run with exit 1.
+    const prompted = async (): Promise<never> => {
+        throw new Error("quak backup prompted");
+    };
+    const ctx: CliContext = {
+        stdout,
+        stderr,
+        sessionDir,
+        cacheDir: `${dir}-cache`,
+        loadSession: (path) =>
+            loadSession(path, {
+                fetch,
+                retry: { ...UNATTENDED_RETRY_OPTIONS, sleep: async () => {} },
+            }),
+        login: prompted,
+        prompt: prompted,
+        promptSecret: prompted,
+    };
+    const code = await new Promise<number>((resolve) => {
+        void run(backupCommand(ctx, dir, opts), stdout, stderr, resolve);
+    });
+    return { code, stderr: printed };
+};
+
+// The ID of the file whose original a request downloads, or undefined for any
+// other request. quak downloads an original from
+// `https://files.ente.io/?fileID=<id>`.
+const downloadOf = (input: RequestInfo | URL): number | undefined => {
+    const url = new URL(String(input));
+    return url.hostname === "files.ente.io"
+        ? Number(url.searchParams.get("fileID"))
+        : undefined;
+};
+
+// The fake server, pushing onto `downloads` the ID of each file whose original
+// is downloaded.
+const recordingDownloads =
+    (downloads: number[]): typeof globalThis.fetch =>
+    async (input, init) => {
+        const fileID = downloadOf(input);
+        if (fileID !== undefined) downloads.push(fileID);
+        return server(input, init);
+    };
+
+// Every entry under `dir`, relative to it, sorted, a symlink as
+// `name -> target`.
+const listTree = (dir: string): string[] =>
+    readdirSync(dir, { recursive: true })
+        .sort()
+        .map((name) => {
+            const path = join(dir, name);
+            return lstatSync(path).isSymbolicLink()
+                ? `${name} -> ${readlinkSync(path)}`
+                : name;
+        });
+
+const readJSON = (path: string) => JSON.parse(readFileSync(path, "utf-8"));
+
+describe("the first backup of the account", () => {
+    let dir: string;
+    let code: number;
+    const downloads: number[] = [];
+
+    beforeAll(async () => {
+        dir = mkdtempSync(join(root, "backup-"));
+        ({ code } = await backup(dir, {}, recordingDownloads(downloads)));
+    });
+
+    it("exits 0, having downloaded each file once", () => {
+        expect(code).toBe(0);
+        // beach.jpg is in two albums but is downloaded, and stored, once.
+        expect(downloads.sort()).toEqual([100, 200, 300]);
+    });
+
+    // The tree "Backup layout" in the README describes. Each original is at
+    // its save path, `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>`, with
+    // the extension of its title; the live photo is its image and its video,
+    // each with the extension it had inside the ZIP, and a `.livephoto.json`
+    // naming the two. Beside each original is its JSON. Each album is a
+    // folder of links named after the files' titles, with a JSON file beside
+    // it. No file failed, so there is no `failures.json`, and the run removed
+    // its lock, `backup.lock`, as it ended.
+    it("lays the backup out as the README describes", () => {
+        expect(listTree(dir)).toEqual([
+            "2024",
+            "2024/2024-03",
+            "2024/2024-03/2024-03-01",
+            "2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+            "2024/2024-03/2024-03-01/2024-03-01.100.json",
+            "2024/2024-03/2024-03-01/2024-03-01.200.json",
+            "2024/2024-03/2024-03-01/2024-03-01.200.mp4",
+            "2024/2024-03/2024-03-01/2024-03-01.300.heic",
+            "2024/2024-03/2024-03-01/2024-03-01.300.json",
+            "2024/2024-03/2024-03-01/2024-03-01.300.livephoto.json",
+            "2024/2024-03/2024-03-01/2024-03-01.300.mov",
+            "account.json",
+            "collections",
+            "collections/Family",
+            "collections/Family.json",
+            "collections/Family/IMG_0002.heic -> ../../2024/2024-03/2024-03-01/2024-03-01.300.heic",
+            "collections/Family/IMG_0002.mov -> ../../2024/2024-03/2024-03-01/2024-03-01.300.mov",
+            "collections/Family/beach.jpg -> ../../2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+            "collections/Friend's Wedding",
+            "collections/Friend's Wedding.json",
+            "collections/Friend's Wedding/toast.mp4 -> ../../2024/2024-03/2024-03-01/2024-03-01.200.mp4",
+            "collections/Trips",
+            "collections/Trips.json",
+            "collections/Trips/beach.jpg -> ../../2024/2024-03/2024-03-01/2024-03-01.100.jpg",
+        ]);
+    });
+
+    it("stores each original decrypted, a live photo as its image and its video", () => {
+        const day = join(dir, DAY);
+        expect(readFileSync(join(day, "2024-03-01.100.jpg"))).toEqual(
+            Buffer.from(JPEG_WITH_EXIF),
+        );
+        expect(readFileSync(join(day, "2024-03-01.200.mp4"))).toEqual(
+            Buffer.from(TOAST),
+        );
+        expect(readFileSync(join(day, "2024-03-01.300.heic"))).toEqual(
+            Buffer.from(HEIC_WITH_EXIF),
+        );
+        expect(readFileSync(join(day, "2024-03-01.300.mov"))).toEqual(
+            Buffer.from(VIDEO),
+        );
+        expect(readJSON(join(day, "2024-03-01.300.livephoto.json"))).toEqual({
+            image: "2024-03-01.300.heic",
+            video: "2024-03-01.300.mov",
+        });
+    });
+
+    // A file's JSON holds what Ente records for it, decrypted, and what quak
+    // read from its original: its ML data when Ente has any, and, for an
+    // image, its EXIF. A video gets no image metadata.
+    it("writes each file's JSON with its ML data and image metadata", () => {
+        const day = join(dir, DAY);
+
+        const beach = readJSON(join(day, "2024-03-01.100.json"));
+        expect(beach).toMatchObject({
+            id: 100,
+            ownerID: USER_ID,
+            metadata: {
+                title: "beach.jpg",
+                fileType: "image",
+                creationTime: TAKEN,
+                hash: BEACH.metadata.hash,
+            },
+            updationTime: TAKEN,
+        });
+        expect(beach.mlData).toEqual(ML_DATA);
+        expect(beach.imageMetadata.exif.Make.description).toBe("Canon");
+        expect(beach.imageMetadata.exif.Model.description).toBe("EOS R5");
+
+        const toast = readJSON(join(day, "2024-03-01.200.json"));
+        expect(toast).toMatchObject({
+            id: 200,
+            ownerID: FRIEND_ID,
+            metadata: { title: "toast.mp4", fileType: "video" },
+        });
+        expect(toast).not.toHaveProperty("mlData");
+        expect(toast).not.toHaveProperty("imageMetadata");
+        expect(toast).not.toHaveProperty("imageMetadataError");
+
+        // A live photo's image metadata is read from its image.
+        const livePhoto = readJSON(join(day, "2024-03-01.300.json"));
+        expect(livePhoto).toMatchObject({
+            id: 300,
+            ownerID: USER_ID,
+            metadata: {
+                title: "IMG_0002.HEIC",
+                fileType: "livePhoto",
+                hash: LIVE_PHOTO.metadata.hash,
+            },
+        });
+        expect(livePhoto.imageMetadata.exif.Make.description).toBe("Canon");
+    });
+
+    it("writes each album's JSON and the account's", () => {
+        const albumJSON = (name: string) =>
+            readJSON(join(dir, "collections", `${name}.json`));
+        const fileIDs = (album: { files: { id: number }[] }) =>
+            album.files.map((f) => f.id);
+
+        const trips = albumJSON("Trips");
+        expect(trips).toMatchObject({
+            id: 10,
+            name: "Trips",
+            type: "album",
+            ownerID: USER_ID,
+            isShared: false,
+            updationTime: TAKEN,
+        });
+        expect(fileIDs(trips)).toEqual([100]);
+
+        const family = albumJSON("Family");
+        expect(family).toMatchObject({ id: 20, isShared: false });
+        expect(fileIDs(family)).toEqual([100, 300]);
+
+        const wedding = albumJSON("Friend's Wedding");
+        expect(wedding).toMatchObject({
+            id: 30,
+            ownerID: FRIEND_ID,
+            isShared: true,
+        });
+        expect(fileIDs(wedding)).toEqual([200]);
+
+        expect(readJSON(join(dir, "account.json"))).toEqual({
+            email: EMAIL,
+            userID: USER_ID,
+        });
+    });
+});
+
+describe("quak backup", () => {
+    // An original already at its save path is complete, so a later run, such
+    // as the next night's from cron, only rewrites the JSON files and the
+    // album links.
+    it("downloads nothing on a second run, and exits 0", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        expect((await backup(dir)).code).toBe(0);
+        const firstTree = listTree(dir);
+
+        const downloads: number[] = [];
+        const { code } = await backup(dir, {}, recordingDownloads(downloads));
+        expect(code).toBe(0);
+        expect(downloads).toEqual([]);
+        expect(listTree(dir)).toEqual(firstTree);
+    });
+
+    // The files CDN is briefly unavailable for toast.mp4: its first two
+    // downloads answer 503. A 5xx is worth repeating, so quak tries again,
+    // here without waiting, and the third download succeeds.
+    it("stores a file whose download fails twice with 503 and then succeeds, and exits 0", async () => {
+        let attempts = 0;
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            if (downloadOf(input) === 200 && ++attempts <= 2) {
+                return new Response("unavailable", { status: 503 });
+            }
+            return server(input, init);
+        };
+        const dir = mkdtempSync(join(root, "backup-"));
+
+        const { code } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(0);
+        expect(attempts).toBe(3);
+        expect(readFileSync(join(dir, DAY, "2024-03-01.200.mp4"))).toEqual(
+            Buffer.from(TOAST),
+        );
+        expect(existsSync(join(dir, "failures.json"))).toBe(false);
+    });
+
+    // The server answers 404 for toast.mp4 on every try. A 404 is an answer,
+    // not a hiccup, so quak does not repeat it: the file goes into
+    // `failures.json` with the kind of failure, how many times it has been
+    // tried and why, the backup goes on with the other files, and the exit
+    // code is 1, so the next run tries again.
+    it("records a file whose download always answers 404 in failures.json, stores the rest, and exits 1", async () => {
+        let attempts = 0;
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            if (downloadOf(input) === 200) {
+                attempts++;
+                return new Response("not found", { status: 404 });
+            }
+            return server(input, init);
+        };
+        const dir = mkdtempSync(join(root, "backup-"));
+
+        const { code, stderr } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(1);
+        expect(attempts).toBe(1);
+        const failures = readJSON(join(dir, "failures.json"));
+        expect(Object.keys(failures.files)).toEqual(["200"]);
+        expect(failures.files["200"]).toMatchObject({
+            fileID: 200,
+            title: "toast.mp4",
+            classification: "permanent",
+            attempts: 1,
+        });
+        expect(failures.files["200"].error).toContain("404");
+        expect(stderr).toContain("[Friend's Wedding] toast.mp4 (id 200): ");
+
+        // The rest of the account is backed up; the video is not, and its
+        // album folder has no link to it.
+        expect(readFileSync(join(dir, DAY, "2024-03-01.100.jpg"))).toEqual(
+            Buffer.from(JPEG_WITH_EXIF),
+        );
+        expect(readFileSync(join(dir, DAY, "2024-03-01.300.heic"))).toEqual(
+            Buffer.from(HEIC_WITH_EXIF),
+        );
+        expect(existsSync(join(dir, DAY, "2024-03-01.200.mp4"))).toBe(false);
+        expect(
+            readdirSync(join(dir, "collections", "Friend's Wedding")),
+        ).toEqual([]);
+    });
+
+    // The server no longer accepts the saved session's token, as after
+    // `quak logout` elsewhere, and answers every request with 401. The run
+    // stops at the refresh it starts with, says to log in again, and exits 3.
+    // It took its lock first, so every request was sent while the lock was
+    // there, and it removed the lock as it ended: the directory is left as it
+    // was, empty.
+    it("exits 3 when the refresh is answered with 401, writing nothing but the lock", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const lockAtEachRequest: boolean[] = [];
+        const fetch: typeof globalThis.fetch = async () => {
+            lockAtEachRequest.push(existsSync(join(dir, "backup.lock")));
+            return new Response(null, { status: 401 });
+        };
+
+        const { code, stderr } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(3);
+        expect(stderr).toBe(
+            "Starting backup...\n" +
+                "Refreshing library...\n" +
+                `quak: the saved session is no longer valid; run "quak login"\n`,
+        );
+        expect(lockAtEachRequest).not.toHaveLength(0);
+        expect(lockAtEachRequest).not.toContain(false);
+        expect(readdirSync(dir)).toEqual([]);
+    });
+
+    // Something changed beach.jpg on disk after it was backed up. A run with
+    // --verify hashes every stored original, finds that one no longer matches
+    // the hash Ente recorded, says so, and downloads it again; it put back the
+    // right bytes, so the exit code is 0.
+    it("--verify puts back an original that changed on disk, and exits 0", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        expect((await backup(dir)).code).toBe(0);
+        const beach = join(dir, DAY, "2024-03-01.100.jpg");
+        writeFileSync(beach, "not the photo any more");
+
+        const downloads: number[] = [];
+        const { code, stderr } = await backup(
+            dir,
+            { verify: true },
+            recordingDownloads(downloads),
+        );
+
+        expect(code).toBe(0);
+        expect(downloads).toEqual([100]);
+        expect(readFileSync(beach)).toEqual(Buffer.from(JPEG_WITH_EXIF));
+        expect(stderr).toContain(
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash",
+        );
+        expect(stderr).toContain("  Verified:    2\n  Mismatched:  1\n");
+    });
+});
@@ -0,0 +1,462 @@
+/**
+ * `quak backup` over an account holding an album or a file quak cannot decrypt
+ * (issue 181).
+ *
+ * The server sends every album and file encrypted, and a refresh decrypts each
+ * one it fetches. A record whose key no key of the account opens, or whose
+ * metadata is not JSON, must not fail the refresh: a refresh that fails commits
+ * nothing, so `quak backup` would store nothing, on every run, until the
+ * server's copy changed. Instead the refresh leaves that record out, and the
+ * library keeps a list of what it left out in its cache. It has to: the next
+ * refresh asks the server only for what changed since the last one, so it will
+ * not see the record again.
+ *
+ * The backup stores every other file. It records each file it could not
+ * decrypt in `failures.json`, prints one line for each album it could not
+ * decrypt, and exits 1 while any remain. Once the server sends a copy that
+ * decrypts, or deletes the record, the next run drops it and can exit 0. The
+ * read commands answer from what did decrypt.
+ *
+ * These tests run the commands with a real saved session over a stand-in Ente
+ * server, which serves albums, files and originals encrypted with libsodium as
+ * the real server does. A record is broken the way a corrupt one arrives: its
+ * key replaced by bytes no key opens, or its metadata text that is not JSON.
+ */
+
+import { existsSync, mkdtempSync, readFileSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import sodium from "libsodium-wrappers-sumo";
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
+
+import {
+    backupCommand,
+    collectionsCommand,
+    filesCommand,
+    saveSession,
+    type CliContext,
+} from "../../src/cli-commands.js";
+import { loadSession } from "../../src/cli-session.js";
+import { Client, type ClientSnapshot } from "../../src/client.js";
+import { encryptBlob, init, toBase64 } from "../../src/crypto/index.js";
+import { Library } from "../../src/library/index.js";
+
+const USER_ID = 42;
+
+// Every file is taken at noon local time on 2026-03-01, in microseconds as Ente
+// stores times, so its original is saved in the folder `DAY` whatever the time
+// zone of the machine running the tests.
+const TAKEN = new Date(2026, 2, 1, 12).getTime() * 1000;
+const DAY = join("2026", "2026-03", "2026-03-01");
+
+type Row = { id: number; updationTime: number } & Record<string, unknown>;
+
+// `plaintext` sealed under `key` with a fresh nonce, both base64, as Ente
+// sends a key or an album name.
+const secretbox = (
+    plaintext: Uint8Array,
+    key: Uint8Array,
+): { ciphertext: string; nonce: string } => {
+    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    return {
+        ciphertext: toBase64(
+            sodium.crypto_secretbox_easy(plaintext, nonce, key),
+        ),
+        nonce: toBase64(nonce),
+    };
+};
+
+// What a corrupt row carries in place of its sealed key: random bytes of the
+// same length, which no key opens.
+const corruptKey = (): string =>
+    toBase64(
+        sodium.randombytes_buf(
+            sodium.crypto_secretbox_KEYBYTES + sodium.crypto_secretbox_MACBYTES,
+        ),
+    );
+
+const json = (body: unknown): Response =>
+    new Response(JSON.stringify(body), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+    });
+
+// A stand-in Ente server for one account. It holds each album, and each file in
+// an album, as the row `/collections/v2` and `/collections/v2/diff` serve, and
+// answers a request with the rows whose `updationTime` is past the request's
+// `sinceTime`, as the real server does. A change to a file also moves its
+// album's `updationTime`, which is what tells a client to list the album's
+// files again. Originals are served by file ID, as the files CDN serves them.
+class FakeEnte {
+    private readonly masterKey = sodium.crypto_secretbox_keygen();
+    private readonly keyPair = sodium.crypto_box_keypair();
+    private readonly albums = new Map<number, Row>();
+    private readonly albumKeys = new Map<number, Uint8Array>();
+    // Each album's file rows, by album ID and then file ID.
+    private readonly files = new Map<number, Map<number, Row>>();
+    // Each file's key and its original as served, by file ID. A file in two
+    // albums is one original under one key, sealed under each album's key.
+    private readonly originals = new Map<
+        number,
+        { key: Uint8Array; header: string; body: Uint8Array }
+    >();
+
+    // The session `quak login` would have saved for the account.
+    snapshot(): ClientSnapshot {
+        return {
+            email: "user@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: toBase64(this.masterKey),
+            secretKey: toBase64(this.keyPair.privateKey),
+            publicKey: toBase64(this.keyPair.publicKey),
+        };
+    }
+
+    // Album `id`, named `name`, as of time `t`. With `corrupt`, its key cannot
+    // be decrypted. An album keeps its key from one call to the next, so its
+    // files' rows still decrypt once it is put back without `corrupt`.
+    putAlbum(id: number, name: string, t: number, corrupt = false): void {
+        const key = this.albumKeys.get(id) ?? sodium.crypto_secretbox_keygen();
+        this.albumKeys.set(id, key);
+        const sealedKey = secretbox(key, this.masterKey);
+        const sealedName = secretbox(new TextEncoder().encode(name), key);
+        this.albums.set(id, {
+            id,
+            owner: { id: USER_ID },
+            encryptedKey: corrupt ? corruptKey() : sealedKey.ciphertext,
+            keyDecryptionNonce: sealedKey.nonce,
+            encryptedName: sealedName.ciphertext,
+            nameDecryptionNonce: sealedName.nonce,
+            type: "album",
+            updationTime: t,
+        });
+        if (!this.files.has(id)) this.files.set(id, new Map());
+    }
+
+    // File `id`, titled `photo-<id>.jpg`, in album `albumID` as of time `t`.
+    // With `corruptKey`, its key cannot be decrypted. With `metadata`, its
+    // metadata decrypts to that text instead of the JSON a client uploads.
+    putFile(
+        albumID: number,
+        id: number,
+        t: number,
+        opts: { corruptKey?: boolean; metadata?: string } = {},
+    ): void {
+        const original = this.originalOf(id);
+        const sealedKey = secretbox(original.key, this.albumKeys.get(albumID)!);
+        const metadata =
+            opts.metadata ??
+            JSON.stringify({
+                title: `photo-${id}.jpg`,
+                fileType: 0,
+                creationTime: TAKEN,
+                modificationTime: TAKEN,
+            });
+        const sealedMetadata = encryptBlob(
+            new TextEncoder().encode(metadata),
+            original.key,
+        );
+        this.files.get(albumID)!.set(id, {
+            id,
+            collectionID: albumID,
+            ownerID: USER_ID,
+            encryptedKey: opts.corruptKey ? corruptKey() : sealedKey.ciphertext,
+            keyDecryptionNonce: sealedKey.nonce,
+            metadata: {
+                encryptedData: toBase64(sealedMetadata.ciphertext),
+                decryptionHeader: toBase64(sealedMetadata.header),
+            },
+            file: { decryptionHeader: original.header },
+            thumbnail: { decryptionHeader: original.header },
+            updationTime: t,
+        });
+        this.moveAlbum(albumID, t);
+    }
+
+    // A deleted record stays on the server as a tombstone holding only its
+    // ID, its time and `isDeleted`.
+    deleteFile(albumID: number, id: number, t: number): void {
+        this.files
+            .get(albumID)!
+            .set(id, { id, updationTime: t, isDeleted: true });
+        this.moveAlbum(albumID, t);
+    }
+
+    deleteAlbum(id: number, t: number): void {
+        this.albums.set(id, { id, updationTime: t, isDeleted: true });
+    }
+
+    // The `fetch` the saved session's client is given.
+    readonly fetch = (async (input: string | URL | Request) => {
+        const url = new URL(input instanceof Request ? input.url : input);
+        const since = Number(url.searchParams.get("sinceTime"));
+        const changed = (rows: Iterable<Row>): Row[] =>
+            [...rows].filter((row) => row.updationTime > since);
+        switch (url.pathname) {
+            case "/collections/v2":
+                return json({ collections: changed(this.albums.values()) });
+            case "/collections/v2/diff": {
+                const albumID = Number(url.searchParams.get("collectionID"));
+                const rows = this.files.get(albumID)?.values() ?? [];
+                return json({ diff: changed(rows), hasMore: false });
+            }
+            case "/files/data/fetch":
+                // No file has ML data.
+                return json({ data: [] });
+            default: {
+                const fileID = Number(url.searchParams.get("fileID"));
+                const original = this.originals.get(fileID);
+                return original
+                    ? new Response(original.body)
+                    : new Response("not found", { status: 404 });
+            }
+        }
+    }) as typeof globalThis.fetch;
+
+    private originalOf(id: number): {
+        key: Uint8Array;
+        header: string;
+        body: Uint8Array;
+    } {
+        const known = this.originals.get(id);
+        if (known) return known;
+        const key = sodium.crypto_secretbox_keygen();
+        const { header, ciphertext } = encryptBlob(
+            new TextEncoder().encode(`original of ${id}`),
+            key,
+        );
+        const original = { key, header: toBase64(header), body: ciphertext };
+        this.originals.set(id, original);
+        return original;
+    }
+
+    private moveAlbum(albumID: number, t: number): void {
+        this.albums.set(albumID, {
+            ...this.albums.get(albumID)!,
+            updationTime: t,
+        });
+    }
+}
+
+// Album 1, "Trip", holds file 10, which decrypts, file 11, whose key does not,
+// and file 12, whose metadata is not JSON. Album 2, "Family", holds file 20,
+// but the album's own key does not decrypt. All of it as of time 1.
+const brokenAccount = (): FakeEnte => {
+    const ente = new FakeEnte();
+    ente.putAlbum(1, "Trip", 1);
+    ente.putFile(1, 10, 1);
+    ente.putFile(1, 11, 1, { corruptKey: true });
+    ente.putFile(1, 12, 1, { metadata: "not json" });
+    ente.putAlbum(2, "Family", 1, true);
+    ente.putFile(2, 20, 1);
+    return ente;
+};
+
+// Collects everything written to it.
+class Output {
+    text = "";
+    write(text: string): void {
+        this.text += text;
+    }
+}
+
+let root: string;
+let stdout: Output;
+let stderr: Output;
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+});
+
+beforeEach(() => {
+    root = mkdtempSync(join(tmpdir(), "quak-undecryptable-"));
+});
+
+afterEach(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+// A context for one command, with fresh output, whose saved session talks to
+// `ente`. Every run shares one cache directory, as runs of `quak` do.
+const context = (ente: FakeEnte): CliContext => {
+    stdout = new Output();
+    stderr = new Output();
+    saveSession(join(root, "session"), ente.snapshot());
+    return {
+        stdout,
+        stderr,
+        sessionDir: join(root, "session"),
+        cacheDir: join(root, "cache"),
+        loadSession: (path) =>
+            loadSession(path, { fetch: ente.fetch, retry: { attempts: 1 } }),
+        login: async () => {
+            throw new Error("login not expected");
+        },
+        prompt: async () => {
+            throw new Error("prompt not expected");
+        },
+        promptSecret: async () => {
+            throw new Error("prompt not expected");
+        },
+    };
+};
+
+const backupDir = (): string => join(root, "backup");
+
+// Runs `quak backup` into the backup directory and returns its exit code.
+const backup = (ente: FakeEnte, opts: { json?: boolean } = {}) =>
+    backupCommand(context(ente), backupDir(), opts);
+
+// The backup's `failures.json` entries by file ID, or {} when it has none.
+const failures = (): Record<string, { error: string; attempts: number }> => {
+    const path = join(backupDir(), "failures.json");
+    return existsSync(path)
+        ? JSON.parse(readFileSync(path, "utf-8")).files
+        : {};
+};
+
+// Whether file `id`'s original is at its save path in the backup.
+const stored = (id: number): boolean =>
+    existsSync(join(backupDir(), DAY, `2026-03-01.${id}.jpg`));
+
+describe("quak backup with records it cannot decrypt", () => {
+    it("backs up every other file, records each file it cannot decrypt in failures.json, names each such album, and exits 1", async () => {
+        expect(await backup(brokenAccount())).toBe(1);
+
+        expect(stored(10)).toBe(true);
+        // Without its album's key, file 20 cannot even be listed.
+        expect(stored(20)).toBe(false);
+        expect(failures()).toEqual({
+            "11": expect.objectContaining({
+                fileID: 11,
+                error: expect.stringMatching(
+                    /^could not be decrypted: .*wrong secret key/,
+                ),
+            }),
+            "12": expect.objectContaining({
+                fileID: 12,
+                error: expect.stringMatching(/^could not be decrypted: .*JSON/),
+            }),
+        });
+        expect(stderr.text).toMatch(
+            /^FAILED album 2: could not be decrypted: .*wrong secret key/m,
+        );
+        expect(stderr.text).toContain("  Failed:      2\n");
+    });
+
+    it("names each album it cannot decrypt in the --json result", async () => {
+        expect(await backup(brokenAccount(), { json: true })).toBe(1);
+
+        expect(JSON.parse(stdout.text)).toMatchObject({
+            downloaded: 1,
+            failed: 2,
+            undecryptableAlbums: [2],
+        });
+    });
+
+    it("still reports them on the next run, though the server does not send them again", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        // Nothing changed on the server, so this run's refresh gets no rows at
+        // all. What it reports, it remembered from the first run.
+        expect(await backup(ente)).toBe(1);
+
+        expect(Object.keys(failures())).toEqual(["11", "12"]);
+        expect(failures()["11"]!.attempts).toBe(2);
+        expect(stderr.text).toMatch(/^FAILED album 2: could not be decrypted/m);
+    });
+
+    it("drops each once the server sends a copy that decrypts, and then exits 0", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        // At time 2 files 11 and 12 and album 2 change on the server, and now
+        // each decrypts. Album 2 brings file 20 with it.
+        ente.putFile(1, 11, 2);
+        ente.putFile(1, 12, 2);
+        ente.putAlbum(2, "Family", 2);
+        expect(await backup(ente)).toBe(0);
+
+        expect([11, 12, 20].map(stored)).toEqual([true, true, true]);
+        expect(existsSync(join(backupDir(), "failures.json"))).toBe(false);
+        expect(stderr.text).not.toContain("FAILED");
+    });
+
+    it("drops each once the server deletes it, and then exits 0", async () => {
+        const ente = brokenAccount();
+        await backup(ente);
+
+        ente.deleteFile(1, 11, 2);
+        ente.deleteFile(1, 12, 2);
+        ente.deleteAlbum(2, 2);
+        expect(await backup(ente)).toBe(0);
+
+        expect(existsSync(join(backupDir(), "failures.json"))).toBe(false);
+        expect(stderr.text).not.toContain("FAILED");
+    });
+
+    it("backs up a file that does not decrypt in one album but does in another, and does not report it", async () => {
+        // File 30 is in albums 1 and 3, each holding its key sealed under the
+        // album's own key. In album 1 that is corrupt; in album 3 it is not,
+        // and one copy that decrypts is all the backup needs.
+        const ente = new FakeEnte();
+        ente.putAlbum(1, "Trip", 1);
+        ente.putFile(1, 30, 1, { corruptKey: true });
+        ente.putAlbum(3, "Favorites", 1);
+        ente.putFile(3, 30, 1);
+
+        expect(await backup(ente)).toBe(0);
+
+        expect(stored(30)).toBe(true);
+        expect(failures()).toEqual({});
+    });
+
+    it("backs up that file in a run limited to the album where it does not decrypt", async () => {
+        // The same account, backed up by `lib.backup()` limited to album 1,
+        // "Trip", as `quak backup` cannot be. File 30 is in Trip, so the run
+        // must store it, from its copy in album 3 that the run otherwise
+        // skips.
+        const ente = new FakeEnte();
+        ente.putAlbum(1, "Trip", 1);
+        ente.putFile(1, 30, 1, { corruptKey: true });
+        ente.putAlbum(3, "Favorites", 1);
+        ente.putFile(3, 30, 1);
+        const lib = await Library.open({
+            client: Client.fromJSON(ente.snapshot(), {
+                fetch: ente.fetch,
+                retry: { attempts: 1 },
+            }),
+            downloadDirectory: backupDir(),
+            cacheDirectory: join(root, "cache"),
+            precacheThumbnails: false,
+            precacheOriginals: false,
+        });
+        try {
+            const result = await lib.backup({
+                downloadDirectory: backupDir(),
+                onlyAlbumNames: ["Trip"],
+            });
+            expect(result).toMatchObject({ downloaded: 1, failed: 0 });
+        } finally {
+            await lib.close();
+        }
+
+        expect(stored(30)).toBe(true);
+        expect(failures()).toEqual({});
+    });
+});
+
+describe("the read commands with records quak cannot decrypt", () => {
+    it("quak collections and quak files answer from what decrypts", async () => {
+        const ente = brokenAccount();
+
+        expect(await collectionsCommand(context(ente), {})).toBe(0);
+        expect(stdout.text).toBe("1\talbum\tTrip\n");
+
+        expect(await filesCommand(context(ente), { collection: "1" })).toBe(0);
+        expect(stdout.text).toBe("10\timage\tphoto-10.jpg\n");
+    });
+});
@@ -62,6 +62,7 @@ import { HEIC_WITH_EXIF } from "../exif-heic.js";
 import { JPEG_WITH_EXIF } from "../exif-jpeg.js";
 import {
    asLivePhoto,
+    blake2b,
    cdnSource,
    IMAGE,
    livePhotoHash,
@@ -1271,6 +1272,181 @@ describe("image metadata in each file's JSON", () => {
    });
 });

+// With `verify`, each original already stored is hashed, and one that does not
+// match the content hash its metadata records is downloaded again.
+describe("backup with verify", () => {
+    // MockClient's files, each recording the hash of what `stubSource` writes
+    // for it, except diagram.png (200), which records none.
+    class HashedClient extends MockClient {
+        override async filesSince(args: {
+            collectionID: number;
+        }): Promise<FilesPage> {
+            const page = await super.filesSince(args);
+            const files = page.files.map((f) =>
+                f.id === 200
+                    ? f
+                    : {
+                          ...f,
+                          metadata: {
+                              ...f.metadata,
+                              hash: blake2b(Buffer.alloc(SIZE_BY_ID[f.id]!)),
+                          },
+                      },
+            );
+            return { ...page, files };
+        }
+    }
+
+    // A backup of the account in `root/backup`, the library that made it, and
+    // the source it fetched from.
+    const backedUp = async (): Promise<{
+        lib: Library;
+        source: StubSource;
+        outDir: string;
+    }> => {
+        const source = stubSource();
+        const lib = await openLibrary(source, new HashedClient());
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+        return { lib, source, outDir };
+    };
+
+    // What `stubSource` writes for beach.jpg (100), and other bytes of the
+    // same length.
+    const good = Buffer.alloc(SIZE_BY_ID[100]!);
+    const corrupt = Buffer.alloc(SIZE_BY_ID[100]!, 1);
+
+    it("leaves an original that matches its hash, and one with no hash, as they are", async () => {
+        const { lib, source, outDir } = await backedUp();
+        const calls = source.originalCalls;
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            skipped: 3,
+            verified: 2,
+            mismatched: 0,
+            unchecked: 1,
+            failed: 0,
+        });
+        expect(source.originalCalls).toBe(calls);
+        await lib.close();
+    });
+
+    it("downloads again an original that does not match its hash", async () => {
+        const { lib, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+        const log: string[] = [];
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+            onProgress: (msg) => log.push(msg),
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 1,
+            skipped: 2,
+            verified: 1,
+            mismatched: 1,
+            unchecked: 1,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(good);
+        expect(log.filter((msg) => msg.startsWith("MISMATCH"))).toEqual([
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash",
+        ]);
+        await lib.close();
+    });
+
+    it("records a failed download in failures.json, with the original removed", async () => {
+        const { lib, source, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+        source.failID = 100;
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            mismatched: 1,
+            failed: 1,
+        });
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["100"]);
+        expect(existsSync(saved(outDir, "100.jpg"))).toBe(false);
+        await lib.close();
+    });
+
+    it("records in failures.json an original the cache puts back with the same bad bytes", async () => {
+        const lib = await openLibrary(stubSource(), new HashedClient());
+        const outDir = join(root, "backup");
+        // The cache holds a bad copy, and a backup copies an original the
+        // cache holds to its save path.
+        const cached = await lib.photos.byID({ fileID: 100 })!.original();
+        writeFileSync(cached.path, corrupt);
+        await lib.backup({ downloadDirectory: outDir });
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            mismatched: 1,
+            failed: 1,
+        });
+        expect(result.errors.map((e) => e.error)).toEqual([
+            "the original put back does not match its content hash either",
+        ]);
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["100"]);
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(corrupt);
+        await lib.close();
+    });
+
+    it("hashes nothing without verify", async () => {
+        const { lib, outDir } = await backedUp();
+        writeFileSync(saved(outDir, "100.jpg"), corrupt);
+
+        const result = await lib.backup({ downloadDirectory: outDir });
+
+        expect(result).toMatchObject({
+            downloaded: 0,
+            skipped: 3,
+            verified: 0,
+            mismatched: 0,
+            unchecked: 0,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "100.jpg"))).toEqual(corrupt);
+        await lib.close();
+    });
+
+    // Root ignores file permissions, so this fails when run as root. The
+    // `test` phase of the `Dockerfile` runs as the `node` user.
+    it("records an original it cannot read as failed, and leaves it", async () => {
+        const { lib, outDir } = await backedUp();
+        const original = saved(outDir, "100.jpg");
+        chmodSync(original, 0o000);
+
+        const result = await lib
+            .backup({ downloadDirectory: outDir, verify: true })
+            .finally(() => chmodSync(original, 0o600));
+
+        expect(result).toMatchObject({ downloaded: 0, verified: 1, failed: 1 });
+        expect(result.errors.map((e) => e.fileID)).toEqual([100]);
+        expect(result.errors[0]!.error).toMatch(/EACCES/);
+        expect(readFileSync(original)).toEqual(good);
+        await lib.close();
+    });
+});
+
 // Every entry under collections/, one level of directories deep, with each
 // symlink's target.
 const tree = (outDir: string): string[] => {
@@ -1306,6 +1482,7 @@ describe("backup album folders", () => {
        listCollections: () => albums.map((a) => a.collection),
        listFiles: (id) =>
            albums.find((a) => a.collection.id === id)?.files ?? [],
+        undecryptable: () => [],
        original: async (fileID) => {
            const path = join(root, `source-${fileID}`);
            writeFileSync(path, `original ${fileID}`);
@@ -1766,6 +1943,64 @@ describe("backup of live photos", () => {
        },
    );

+    it("verifies a live photo's image and video together, and downloads it again when one does not match", async () => {
+        const { file: live, body } = await asLivePhoto(
+            file(500, 10, "IMG_0500.HEIC"),
+        );
+        const lib = await open([live], new Map([[500, body]]));
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+
+        const good = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+        expect(good).toMatchObject({ skipped: 1, verified: 1, mismatched: 0 });
+
+        const video = saved(outDir, "500.mov");
+        writeFileSync(video, "another few seconds of video");
+        const bad = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(bad).toMatchObject({
+            downloaded: 1,
+            verified: 0,
+            mismatched: 1,
+            failed: 0,
+        });
+        expect(readFileSync(saved(outDir, "500.heic"))).toEqual(
+            Buffer.from(IMAGE),
+        );
+        expect(readFileSync(video)).toEqual(Buffer.from(VIDEO));
+        expect(tree(outDir)).toEqual(linked);
+        await lib.close();
+    });
+
+    it("removes both files of a live photo that does not match its hash when downloading it again fails", async () => {
+        const { file: live, body } = await asLivePhoto(
+            file(500, 10, "IMG_0500.HEIC"),
+        );
+        const bodies = new Map([[500, body]]);
+        const lib = await open([live], bodies);
+        const outDir = join(root, "backup");
+        await lib.backup({ downloadDirectory: outDir });
+        writeFileSync(saved(outDir, "500.mov"), "another few seconds of video");
+        bodies.delete(500);
+
+        const result = await lib.backup({
+            downloadDirectory: outDir,
+            verify: true,
+        });
+
+        expect(result).toMatchObject({ mismatched: 1, failed: 1 });
+        expect(Object.keys(readLedger(outDir).files)).toEqual(["500"]);
+        expect(existsSync(saved(outDir, "500.heic"))).toBe(false);
+        expect(existsSync(saved(outDir, "500.mov"))).toBe(false);
+        await lib.close();
+    });
+
    it("serves a live photo the backup stored to a library reading the backup", async () => {
        const { file: live, body } = await asLivePhoto(
            file(500, 10, "IMG_0500.HEIC"),
@@ -53,13 +53,14 @@ import {
 import { run } from "../../src/cli-run.js";
 import { loadSession } from "../../src/cli-session.js";
 import type { Client, ClientSnapshot, LoginOptions } from "../../src/client.js";
-import type { ContentSource } from "../../src/library/content.js";
+import { savePath, type ContentSource } from "../../src/library/content.js";
 import type { Collection, EnteFile } from "../../src/model/types.js";
 import { init, toBase64 } from "../../src/crypto/index.js";
 import { defaultCacheDirectory } from "../../src/library/index.js";
 import { HEIC_WITH_EXIF } from "../exif-heic.js";
 import {
    asLivePhoto,
+    blake2b,
    cdnSource,
    IMAGE,
    livePhotoHash,
@@ -510,11 +511,6 @@ describe("files", () => {
        expect(await filesCommand(context(), { collection: "9" })).toBe(1);
        expect(stderr.text).toBe("Collection 9 not found\n");
    });
-
-    it("exits 1 for a collection ID that is not a number", async () => {
-        expect(await filesCommand(context(), { collection: "abc" })).toBe(1);
-        expect(stderr.text).toBe("Invalid collection ID\n");
-    });
 });

 describe("get and get-thumb", () => {
@@ -545,12 +541,6 @@ describe("get and get-thumb", () => {
        expect(stderr.text).toBe("File 999 not found\n");
        expect(existsSync(out)).toBe(false);
    });
-
-    it("both exit 1 for a file ID that is not a number", async () => {
-        expect(await getCommand(context(), "abc", {})).toBe(1);
-        expect(await getThumbCommand(context(), "abc", {})).toBe(1);
-        expect(stderr.text).toBe("Invalid file ID\nInvalid file ID\n");
-    });
 });

 // A live photo, which Ente stores as one ZIP, is written as its image and its
@@ -744,6 +734,61 @@ describe("backup", () => {
        expect(stderr.text).toBe("Starting backup...\n");
    });

+    it("--verify downloads again an original that does not match its hash, prints the counts, and exits 0", async () => {
+        // Each file records the hash of the original the fake writes for it.
+        const client = {
+            ...fakeClient(),
+            filesSince: async (args: { collectionID: number }) => ({
+                files: (FILES[args.collectionID] ?? []).map((f) => ({
+                    ...f,
+                    metadata: {
+                        ...f.metadata,
+                        hash: blake2b(Buffer.alloc(7, f.id & 0xff)),
+                    },
+                })),
+                deleted: [],
+                cursor: 1,
+            }),
+        } as unknown as Client;
+        const ctx = context(client);
+        const dir = join(root, "backup");
+        expect(await backupCommand(ctx, dir, {})).toBe(0);
+        writeFileSync(savePath(dir, FILES[1]![0]!), "corrupt");
+
+        expect(await backupCommand(ctx, dir, { verify: true })).toBe(0);
+
+        expect(stderr.text).toContain(
+            "MISMATCH original beach.jpg (100): its bytes do not match its content hash\n",
+        );
+        expect(stderr.text).toContain(
+            "  Downloaded:  1\n" +
+                "  Skipped:     2\n" +
+                "  Verified:    2\n" +
+                "  Mismatched:  1\n" +
+                "  Unchecked:   0\n" +
+                "  Failed:      0\n",
+        );
+    });
+
+    it("--verify --json adds the verified, mismatched and unchecked counts", async () => {
+        const dir = join(root, "backup");
+        expect(await backupCommand(context(), dir, {})).toBe(0);
+
+        const code = await backupCommand(context(), dir, {
+            verify: true,
+            json: true,
+        });
+
+        expect(code).toBe(0);
+        expect(JSON.parse(stdout.text)).toMatchObject({
+            skipped: 3,
+            verified: 0,
+            mismatched: 0,
+            unchecked: 3,
+            failed: 0,
+        });
+    });
+
    it("exits 1 and lists each file when the ML data fetch fails", async () => {
        const client = {
            ...fakeClient(),
@@ -979,3 +1024,130 @@ describe("backup-metadata and the thumbnail helpers refresh first", () => {
        expect(stdout.text).toBe("");
    });
 });
+
+// `files --collection`, `get`, `get-thumb` and `helper fix-missing-thumbnails
+// --file` take an ID only when it is written in decimal digits and is a safe
+// integer. `Number()` alone reads `0x10` as 16, `1e3` as 1000 and an empty
+// string as 0, so the command would act on a file the user never named.
+// Anything else is refused with one line naming it, and exit 1, before any
+// request.
+//
+// These run on a real client, read back from a saved session, over a fake Ente
+// server holding an empty account, which records each request it receives. With
+// a good ID the command goes on to ask the server, and finds nothing there.
+describe("an ID given on the command line", () => {
+    const NOT_IDS = [
+        "0x10",
+        "1e3",
+        "1.5",
+        "-1",
+        "",
+        "abc",
+        // One past Number.MAX_SAFE_INTEGER: from here on a number cannot
+        // hold every integer, so two IDs could read as one.
+        "9007199254740992",
+    ];
+
+    const requests: Request[] = [];
+
+    const emptyAccount = (): CliContext => {
+        const ctx = context();
+        const key = toBase64(new Uint8Array(32));
+        saveSession(ctx.sessionDir, {
+            email: "cli@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: key,
+            secretKey: key,
+            publicKey: key,
+        });
+        return {
+            ...ctx,
+            loadSession: (path) =>
+                loadSession(path, {
+                    fetch: async (url, init) => {
+                        requests.push(new Request(url, init));
+                        return new Response(
+                            JSON.stringify({ collections: [] }),
+                            { headers: { "content-type": "application/json" } },
+                        );
+                    },
+                }),
+        };
+    };
+
+    beforeEach(() => {
+        requests.length = 0;
+    });
+
+    it("files looks up a decimal collection ID", async () => {
+        const ctx = emptyAccount();
+        expect(await filesCommand(ctx, { collection: "1" })).toBe(1);
+        expect(stderr.text).toBe("Collection 1 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)(
+        "files refuses --collection %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            expect(await filesCommand(ctx, { collection: arg })).toBe(1);
+            expect(stderr.text).toBe(`Invalid collection ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+
+    it("get looks up a decimal file ID", async () => {
+        expect(await getCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get refuses %j before any request", async (arg) => {
+        expect(await getCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    it("get-thumb looks up a decimal file ID", async () => {
+        expect(await getThumbCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get-thumb refuses %j before any request", async (arg) => {
+        expect(await getThumbCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    // The fixer reports each file it was given; on this account neither is
+    // found, which is a failure, so it exits 1.
+    it("fix-missing-thumbnails looks up each decimal --file ID", async () => {
+        const ctx = emptyAccount();
+        const opts = { file: ["100", "200"], json: true };
+        expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+        expect(JSON.parse(stdout.text)).toMatchObject([
+            { fileID: 100, reason: "file not found in any collection" },
+            { fileID: 200, reason: "file not found in any collection" },
+        ]);
+        expect(requests).not.toHaveLength(0);
+    });
+
+    // One bad ID fails the whole command, so the good one before it is not
+    // fixed either.
+    it.each(NOT_IDS)(
+        "fix-missing-thumbnails refuses --file 100 %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            const opts = { file: ["100", arg] };
+            expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+            expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+});
@@ -37,15 +37,8 @@ import {
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import sodium from "libsodium-wrappers-sumo";
-import { SRP, SrpServer } from "fast-srp-hap";
 import { beforeAll, afterAll, describe, expect, it, vi } from "vitest";
-import {
-    init,
-    toBase64,
-    deriveKEK,
-    deriveLoginSubkey,
-    encryptBlob,
-} from "../../src/crypto/index.js";
+import { init, toBase64, encryptBlob } from "../../src/crypto/index.js";
 import * as jpegJs from "jpeg-js";
 import { Client } from "../../src/client.js";
 import { Library } from "../../src/library/index.js";
@@ -54,7 +47,13 @@ import {
    type MetadataBackupOptions,
 } from "../../src/metadata-backup.js";
 import { backupMetadataCommand } from "../../src/cli-commands.js";
-import type { KeyAttributes } from "../../src/auth/types.js";
+import {
+    buildAccount,
+    buildMetaFetch,
+    encryptSecretbox,
+    encryptStreamBlob,
+    type MetaMockState,
+} from "../ente-server.js";

 // One file per ML data request, so the two files of the mock account are
 // fetched in two requests and one of them can fail on its own.
@@ -65,80 +64,13 @@ vi.mock("../../src/mldata-fetch.js", async (importOriginal) => ({

 const TEST_EMAIL = "metabackup@example.com";
 const TEST_PASSWORD = "metapass";
-const TEST_OPS = 2;
-const TEST_MEM = 64 * 1024 * 1024;
-
-interface MetaMockState {
-    verifier: Buffer;
-    srpAttributes: Record<string, unknown>;
-    keyAttributes: KeyAttributes;
-    encryptedToken: string;
-    collections: Record<string, unknown>[];
-    filesByCollection: Record<number, Record<string, unknown>[]>;
-    // For ML data and EXIF tests
-    encryptedMLData: Record<
-        number,
-        { encryptedData: string; decryptionHeader: string }
-    >;
-    fileCiphertexts: Record<number, Uint8Array>;
-}

 let mock: MetaMockState;
 let testDir: string;

-const encryptSecretbox = (plaintext: Uint8Array, key: Uint8Array) => {
-    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
-    const ciphertext = sodium.crypto_secretbox_easy(plaintext, nonce, key);
-    return { ciphertext, nonce };
-};
-
-const encryptStreamBlob = (plaintext: Uint8Array, key: Uint8Array) => {
-    const push = sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
-    const ciphertext = sodium.crypto_secretstream_xchacha20poly1305_push(
-        push.state,
-        plaintext,
-        null,
-        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
-    );
-    return { ciphertext, header: push.header };
-};
-
 const buildMetaMock = async (): Promise<MetaMockState> => {
-    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
-    const kek = await deriveKEK(TEST_PASSWORD, kekSalt, TEST_OPS, TEST_MEM);
-    const loginSubKeyBytes = deriveLoginSubkey(kek);
-    const srpUserID = "meta-srp";
-    const srpSalt = sodium.randombytes_buf(16);
-    const verifier = SRP.computeVerifier(
-        SRP.params["4096"],
-        Buffer.from(srpSalt),
-        Buffer.from(srpUserID),
-        Buffer.from(loginSubKeyBytes),
-    );
-
-    const masterKey = sodium.randombytes_buf(32);
-    const { ciphertext: encMK, nonce: mkNonce } = encryptSecretbox(
-        masterKey,
-        kek,
-    );
-    const kp = sodium.crypto_box_keypair();
-    const { ciphertext: encSK, nonce: skNonce } = encryptSecretbox(
-        kp.privateKey,
-        masterKey,
-    );
-    const tokenBytes = sodium.randombytes_buf(32);
-    const encToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
-
-    const keyAttributes: KeyAttributes = {
-        kekSalt: toBase64(kekSalt),
-        encryptedKey: toBase64(encMK),
-        keyDecryptionNonce: toBase64(mkNonce),
-        publicKey: toBase64(kp.publicKey),
-        encryptedSecretKey: toBase64(encSK),
-        secretKeyDecryptionNonce: toBase64(skNonce),
-        memLimit: TEST_MEM,
-        opsLimit: TEST_OPS,
-    };
+    const account = await buildAccount(TEST_PASSWORD);
+    const masterKey = account.masterKey;

    // Collection 1: "Vacation" with collection-level pubMagicMetadata
    const ck1 = sodium.crypto_secretbox_keygen();
@@ -332,17 +264,10 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
    rawFile1.file.decryptionHeader = toBase64(filePush1.header);

    return {
-        verifier,
-        srpAttributes: {
-            srpUserID,
-            srpSalt: toBase64(srpSalt),
-            memLimit: TEST_MEM,
-            opsLimit: TEST_OPS,
-            kekSalt: toBase64(kekSalt),
-            isEmailMFAEnabled: false,
-        },
-        keyAttributes,
-        encryptedToken: toBase64(encToken),
+        verifier: account.verifier,
+        srpAttributes: account.srpAttributes,
+        keyAttributes: account.keyAttributes,
+        encryptedToken: account.encryptedToken,
        collections: [rawColl1, rawColl2],
        filesByCollection: { 10: [rawFile1], 20: [rawFile2] },
        encryptedMLData: {
@@ -355,90 +280,6 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
    };
 };

-// `failMLDataFor`: answer 500 to every ML data request that asks for this file.
-const buildMetaFetch = (m: MetaMockState, failMLDataFor?: number) => {
-    let srpServer: SrpServer;
-    return (async (
-        input: RequestInfo | URL,
-        init?: RequestInit,
-    ): Promise<Response> => {
-        const url =
-            typeof input === "string"
-                ? input
-                : input instanceof URL
-                  ? input.href
-                  : input.url;
-        const path = new URL(url).pathname;
-        const json = (body: unknown) =>
-            new Response(JSON.stringify(body), {
-                status: 200,
-                headers: { "content-type": "application/json" },
-            });
-
-        if (path === "/users/srp/attributes")
-            return json({ attributes: m.srpAttributes });
-        if (path === "/users/srp/create-session") {
-            const body = JSON.parse(init?.body as string);
-            const serverKey = await SRP.genKey();
-            srpServer = new SrpServer(
-                SRP.params["4096"],
-                m.verifier,
-                serverKey,
-            );
-            const B = srpServer.computeB();
-            srpServer.setA(Buffer.from(body.srpA, "base64"));
-            return json({ sessionID: "s1", srpB: B.toString("base64") });
-        }
-        if (path === "/users/srp/verify-session") {
-            const body = JSON.parse(init?.body as string);
-            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
-            return json({
-                srpM2: srpServer.computeM2().toString("base64"),
-                id: 42,
-                keyAttributes: m.keyAttributes,
-                encryptedToken: m.encryptedToken,
-            });
-        }
-        if (path === "/collections/v2")
-            return json({ collections: m.collections });
-        if (path === "/collections/v2/diff") {
-            const collID = Number(
-                new URL(url).searchParams.get("collectionID"),
-            );
-            return json({
-                diff: m.filesByCollection[collID] ?? [],
-                hasMore: false,
-            });
-        }
-        if (path === "/files/data/fetch") {
-            const body = JSON.parse(init?.body as string);
-            if ((body.fileIDs as number[]).includes(failMLDataFor!))
-                return new Response("server error", { status: 500 });
-            const data = (body.fileIDs as number[])
-                .filter((id: number) => m.encryptedMLData[id])
-                .map((id: number) => ({
-                    fileID: id,
-                    ...m.encryptedMLData[id],
-                    updatedAt: 1700000000000000,
-                }));
-            return json({ data });
-        }
-        if (
-            url.includes("files.ente.io") ||
-            path.startsWith("/files/download/")
-        ) {
-            const parsed = new URL(url);
-            const fileID = Number(
-                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
-            );
-            const ct = m.fileCiphertexts[fileID];
-            if (ct) return new Response(ct, { status: 200 });
-            return new Response("not found", { status: 404 });
-        }
-        return new Response("not found", { status: 404 });
-    }) as typeof globalThis.fetch;
-};
-
 beforeAll(async () => {
    await init();
    await sodium.ready;
@@ -10,7 +10,9 @@
 * The return shape keeps live records and tombstones apart — `collections` /
 * `files` are decrypted live records, `deleted` is a plain list of the ids the
 * server tombstoned. A tombstone carries no decryptable key or metadata, so it
- * is a bare id rather than a hollowed-out `Collection` / `EnteFile`.
+ * is a bare id rather than a hollowed-out `Collection` / `EnteFile`. A live row
+ * that does not decrypt, or whose metadata is not JSON, is listed under
+ * `undecryptable` with the reason, so one bad row does not fail the call.
 *
 * All tests inject a fake `fetch` and drive a real `Client` (built with
 * `Client.fromJSON`) so the decryption path runs for real. Live rows are built
@@ -91,27 +93,38 @@ const ownedCollectionRow = (
    };
 };

-/** A live file row inside a collection, keyed under that collection's key. */
+/**
+ * A live file row inside a collection, keyed under that collection's key. Its
+ * metadata decrypts to `metadata` when given, otherwise to the JSON a client
+ * uploads for `title`.
+ */
 const fileRow = (
    collectionKey: Uint8Array,
-    opts: { id: number; title: string; updationTime: number },
+    opts: {
+        id: number;
+        title: string;
+        updationTime: number;
+        metadata?: string;
+    },
 ): Record<string, unknown> => {
    const fileKey = sodium.crypto_secretbox_keygen();
    const { ciphertext: encFileKey, nonce: fileKeyNonce } = secretboxEncrypt(
        fileKey,
        collectionKey,
    );
-    const metadata = {
-        title: opts.title,
-        fileType: 0,
-        creationTime: opts.updationTime,
-        modificationTime: opts.updationTime,
-    };
+    const metadata =
+        opts.metadata ??
+        JSON.stringify({
+            title: opts.title,
+            fileType: 0,
+            creationTime: opts.updationTime,
+            modificationTime: opts.updationTime,
+        });
    const push =
        sodium.crypto_secretstream_xchacha20poly1305_init_push(fileKey);
    const encMeta = sodium.crypto_secretstream_xchacha20poly1305_push(
        push.state,
-        new TextEncoder().encode(JSON.stringify(metadata)),
+        new TextEncoder().encode(metadata),
        null,
        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
    );
@@ -131,6 +144,21 @@ const fileRow = (
    };
 };

+/**
+ * `row` with its sealed key replaced by random bytes of the same length, as a
+ * corrupt row arrives: no key opens it, so the row cannot be decrypted.
+ */
+const withCorruptKey = (
+    row: Record<string, unknown>,
+): Record<string, unknown> => ({
+    ...row,
+    encryptedKey: toBase64(
+        sodium.randombytes_buf(
+            sodium.crypto_secretbox_KEYBYTES + sodium.crypto_secretbox_MACBYTES,
+        ),
+    ),
+});
+
 /** A tombstone row. Never decrypted, so only these fields are ever read. */
 const tombstoneRow = (
    id: number,
@@ -278,6 +306,58 @@ describe("Client.filesSince", () => {
        // It gave up after the first page rather than looping.
        expect(sinceTimes).toEqual(["50"]);
    });
+
+    it("leaves out each file it cannot decrypt or parse, with the reason, and decrypts the rest", async () => {
+        const keys = buildKeys();
+        const collectionKey = sodium.crypto_secretbox_keygen();
+
+        // File 1002's key is sealed under no key quak holds, and file 1003's
+        // metadata decrypts to text that is not JSON. Neither may cost the
+        // caller file 1001: each is left out of `files` and reported under
+        // `undecryptable` with the reason instead of failing the call.
+        const { fetch } = recordingFetch(
+            jsonResponse({
+                diff: [
+                    fileRow(collectionKey, {
+                        id: 1001,
+                        title: "fine.jpg",
+                        updationTime: 100,
+                    }),
+                    withCorruptKey(
+                        fileRow(collectionKey, {
+                            id: 1002,
+                            title: "bad-key.jpg",
+                            updationTime: 200,
+                        }),
+                    ),
+                    fileRow(collectionKey, {
+                        id: 1003,
+                        title: "bad-metadata.jpg",
+                        updationTime: 300,
+                        metadata: "not json",
+                    }),
+                ],
+                hasMore: false,
+            }),
+        );
+        const client = Client.fromJSON(snapshotFor(keys), { fetch });
+
+        const { files, undecryptable, cursor } = await client.filesSince({
+            collectionID: 1,
+            collectionKey,
+            sinceTime: 0,
+        });
+
+        expect(files.map((f) => f.id)).toEqual([1001]);
+        expect(undecryptable).toEqual([
+            { id: 1002, error: expect.stringContaining("wrong secret key") },
+            { id: 1003, error: expect.stringContaining("JSON") },
+        ]);
+        // The cursor moves past the two left out, as past any other row, so
+        // the server does not send them again until its copy changes. The
+        // caller has to keep what it was told here.
+        expect(cursor).toBe(300);
+    });
 });

 describe("Client.collectionsSince", () => {
@@ -331,6 +411,42 @@ describe("Client.collectionsSince", () => {
        expect(result.cursor).toBe(150);
        expect(sinceTimes).toEqual(["150"]);
    });
+
+    it("leaves out an album it cannot decrypt, with the reason, and decrypts the rest", async () => {
+        const keys = buildKeys();
+
+        // Album 2's key is sealed under no key quak holds. Without its key
+        // neither its name nor its files can be read, so it is left out and
+        // reported under `undecryptable`; album 1 is unaffected.
+        const { fetch } = recordingFetch(
+            jsonResponse({
+                collections: [
+                    ownedCollectionRow(keys.masterKey, {
+                        id: 1,
+                        name: "Vacation",
+                        updationTime: 100,
+                    }),
+                    withCorruptKey(
+                        ownedCollectionRow(keys.masterKey, {
+                            id: 2,
+                            name: "Broken",
+                            updationTime: 200,
+                        }),
+                    ),
+                ],
+            }),
+        );
+        const client = Client.fromJSON(snapshotFor(keys), { fetch });
+
+        const { collections, undecryptable, cursor } =
+            await client.collectionsSince({ sinceTime: 0 });
+
+        expect(collections.map((c) => c.name)).toEqual(["Vacation"]);
+        expect(undecryptable).toEqual([
+            { id: 2, error: expect.stringContaining("wrong secret key") },
+        ]);
+        expect(cursor).toBe(200);
+    });
 });

 describe("Client list wrappers still hide deletions", () => {
@@ -0,0 +1,197 @@
+/**
+ * A fake of the Ente HTTP API, for tests that run quak's real `Client` against
+ * a server.
+ *
+ * `buildMetaFetch` is a `fetch` that answers the requests quak makes, from a
+ * `MetaMockState` the test builds: the SRP login, `/collections/v2`, each
+ * collection's `/collections/v2/diff`, the ML data of `/files/data/fetch`, and
+ * each file's content from the files CDN, a 404 for a file it holds none for.
+ * The test encrypts everything in that state with libsodium, as Ente's clients
+ * do, so quak decrypts it with its own code. Nothing hits the network.
+ *
+ * `buildAccount` builds what the server holds to log an account in, and the
+ * keys a test encrypts that account's collections with. Call `init()` from
+ * `src/crypto` and await `sodium.ready` before using either.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import { SRP, SrpServer } from "fast-srp-hap";
+import { toBase64, deriveKEK, deriveLoginSubkey } from "../src/crypto/index.js";
+import type { KeyAttributes } from "../src/auth/types.js";
+
+const TEST_OPS = 2;
+const TEST_MEM = 64 * 1024 * 1024;
+
+export interface MetaMockState {
+    verifier: Buffer;
+    srpAttributes: Record<string, unknown>;
+    keyAttributes: KeyAttributes;
+    encryptedToken: string;
+    collections: Record<string, unknown>[];
+    filesByCollection: Record<number, Record<string, unknown>[]>;
+    // For ML data and EXIF tests
+    encryptedMLData: Record<
+        number,
+        { encryptedData: string; decryptionHeader: string }
+    >;
+    fileCiphertexts: Record<number, Uint8Array>;
+}
+
+export const encryptSecretbox = (plaintext: Uint8Array, key: Uint8Array) => {
+    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const ciphertext = sodium.crypto_secretbox_easy(plaintext, nonce, key);
+    return { ciphertext, nonce };
+};
+
+export const encryptStreamBlob = (plaintext: Uint8Array, key: Uint8Array) => {
+    const push = sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
+    const ciphertext = sodium.crypto_secretstream_xchacha20poly1305_push(
+        push.state,
+        plaintext,
+        null,
+        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
+    );
+    return { ciphertext, header: push.header };
+};
+
+// An account whose password is `password`. The first four fields are those of
+// `MetaMockState`; with the master key the test encrypts the keys of the
+// account's own collections, and to the public key it seals the keys of the
+// collections other accounts share with it.
+export const buildAccount = async (password: string) => {
+    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
+    const kek = await deriveKEK(password, kekSalt, TEST_OPS, TEST_MEM);
+    const loginSubKeyBytes = deriveLoginSubkey(kek);
+    const srpUserID = "meta-srp";
+    const srpSalt = sodium.randombytes_buf(16);
+    const verifier = SRP.computeVerifier(
+        SRP.params["4096"],
+        Buffer.from(srpSalt),
+        Buffer.from(srpUserID),
+        Buffer.from(loginSubKeyBytes),
+    );
+
+    const masterKey = sodium.randombytes_buf(32);
+    const { ciphertext: encMK, nonce: mkNonce } = encryptSecretbox(
+        masterKey,
+        kek,
+    );
+    const kp = sodium.crypto_box_keypair();
+    const { ciphertext: encSK, nonce: skNonce } = encryptSecretbox(
+        kp.privateKey,
+        masterKey,
+    );
+    const tokenBytes = sodium.randombytes_buf(32);
+    const encToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
+
+    const keyAttributes: KeyAttributes = {
+        kekSalt: toBase64(kekSalt),
+        encryptedKey: toBase64(encMK),
+        keyDecryptionNonce: toBase64(mkNonce),
+        publicKey: toBase64(kp.publicKey),
+        encryptedSecretKey: toBase64(encSK),
+        secretKeyDecryptionNonce: toBase64(skNonce),
+        memLimit: TEST_MEM,
+        opsLimit: TEST_OPS,
+    };
+
+    return {
+        verifier,
+        srpAttributes: {
+            srpUserID,
+            srpSalt: toBase64(srpSalt),
+            memLimit: TEST_MEM,
+            opsLimit: TEST_OPS,
+            kekSalt: toBase64(kekSalt),
+            isEmailMFAEnabled: false,
+        },
+        keyAttributes,
+        encryptedToken: toBase64(encToken),
+        masterKey,
+        publicKey: kp.publicKey,
+    };
+};
+
+// `failMLDataFor`: answer 500 to every ML data request that asks for this file.
+export const buildMetaFetch = (m: MetaMockState, failMLDataFor?: number) => {
+    let srpServer: SrpServer;
+    return (async (
+        input: RequestInfo | URL,
+        init?: RequestInit,
+    ): Promise<Response> => {
+        const url =
+            typeof input === "string"
+                ? input
+                : input instanceof URL
+                  ? input.href
+                  : input.url;
+        const path = new URL(url).pathname;
+        const json = (body: unknown) =>
+            new Response(JSON.stringify(body), {
+                status: 200,
+                headers: { "content-type": "application/json" },
+            });
+
+        if (path === "/users/srp/attributes")
+            return json({ attributes: m.srpAttributes });
+        if (path === "/users/srp/create-session") {
+            const body = JSON.parse(init?.body as string);
+            const serverKey = await SRP.genKey();
+            srpServer = new SrpServer(
+                SRP.params["4096"],
+                m.verifier,
+                serverKey,
+            );
+            const B = srpServer.computeB();
+            srpServer.setA(Buffer.from(body.srpA, "base64"));
+            return json({ sessionID: "s1", srpB: B.toString("base64") });
+        }
+        if (path === "/users/srp/verify-session") {
+            const body = JSON.parse(init?.body as string);
+            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
+            return json({
+                srpM2: srpServer.computeM2().toString("base64"),
+                id: 42,
+                keyAttributes: m.keyAttributes,
+                encryptedToken: m.encryptedToken,
+            });
+        }
+        if (path === "/collections/v2")
+            return json({ collections: m.collections });
+        if (path === "/collections/v2/diff") {
+            const collID = Number(
+                new URL(url).searchParams.get("collectionID"),
+            );
+            return json({
+                diff: m.filesByCollection[collID] ?? [],
+                hasMore: false,
+            });
+        }
+        if (path === "/files/data/fetch") {
+            const body = JSON.parse(init?.body as string);
+            if ((body.fileIDs as number[]).includes(failMLDataFor!))
+                return new Response("server error", { status: 500 });
+            const data = (body.fileIDs as number[])
+                .filter((id: number) => m.encryptedMLData[id])
+                .map((id: number) => ({
+                    fileID: id,
+                    ...m.encryptedMLData[id],
+                    updatedAt: 1700000000000000,
+                }));
+            return json({ data });
+        }
+        if (
+            url.includes("files.ente.io") ||
+            path.startsWith("/files/download/")
+        ) {
+            const parsed = new URL(url);
+            const fileID = Number(
+                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
+            );
+            const ct = m.fileCiphertexts[fileID];
+            if (ct) return new Response(ct, { status: 200 });
+            return new Response("not found", { status: 404 });
+        }
+        return new Response("not found", { status: 404 });
+    }) as typeof globalThis.fetch;
+};
@@ -30,7 +30,8 @@ export const livePhotoZip = (
    },
 ): Uint8Array => zipSync(entries);

-const blake2b = (bytes: Uint8Array): string =>
+// The content hash Ente's clients record for an original's bytes.
+export const blake2b = (bytes: Uint8Array): string =>
    createHash("blake2b512").update(bytes).digest("base64");

 // The hash Ente's clients record for a live photo: the unkeyed BLAKE2b-512 of
Author	SHA1	Message	Date
sneak	8bc542f1e5	README: say which records quak could not decrypt lib.backup() reports check / check (push) Waiting to run Details It reports each such album, and each such file in its scope that has no copy that decrypted, not every record the library left out. Model: opus-5-5	2026-10-14 01:41:38 +00:00
sneak	c465496682	Back up from a copy that decrypted even outside a run's albums; README wording A file whose copy in an album in scope does not decrypt is now backed up from a copy that decrypted in any album, not only one in scope; with no such copy it is still recorded as failed. The README now says the exit code stays at 1 until the server sends a copy that decrypts or deletes it, and that only a run without --json prints the FAILED album line. StoredMetadata.undecryptable is required, like the other fields. Model: opus-5-5	2026-10-14 01:41:38 +00:00
sneak	0a756be5de	Test: a backup limited to one album stores a file whose copy there does not decrypt A run of lib.backup() limited to "Trip" must store a file whose copy in Trip does not decrypt when its copy in an album the run skips does. Today the run neither stores it nor records it as failed. Model: opus-5-5	2026-10-14 01:41:38 +00:00
sneak	a0a97f1019	One album or file quak cannot decrypt no longer stops a refresh or a backup (closes #181 ) collectionsSince and filesSince put a row that does not decrypt or parse in an undecryptable list with the reason instead of throwing. The refresh leaves it out, an album with its files, and the store keeps the list in metadata.json until the server sends a copy that decrypts or deletes it, since the next refresh asks only for changes. lib.backup() records each such file in failures.json as "could not be decrypted: <reason>", unless a copy in another album decrypted, logs one line per such album and lists their IDs in undecryptableAlbums; quak backup exits 1 while any remain. Tests use a stand-in Ente server with real encryption. Model: opus-5-5	2026-10-14 01:41:38 +00:00
clawbot	54133cfa5f	Test quak backup end to end against a fake Ente server (closes #186 ) check / check (push) Waiting to run Details test/cli/backup-end-to-end.test.ts runs backupCommand through run, as bin/quak.ts does, with the real loadSession reading a session saved after logging in, and every request answered by a fake of the Ente HTTP API serving data encrypted with libsodium. It covers the backup tree of an account with two albums of its own and one shared with it, a second run that downloads nothing, a download that succeeds after two 503s, a 404 recorded in failures.json with exit 1, a refresh answered 401 with exit 3, and --verify putting back an original changed on disk. The fake server and the account it logs in moved from the backup-metadata test into test/ente-server.ts, unchanged, so both tests use them. Model: opus-5-5	2026-10-14 03:38:38 +02:00
clawbot	ab28d86aaa	An ID on the command line is decimal digits only (closes #185 ) check / check (push) Waiting to run Details files --collection, get, get-thumb and helper fix-missing-thumbnails --file read an ID with Number(), so 0x10 meant 16, 1e3 meant 1000 and an empty string meant 0, and --file dropped an ID it could not read. parseID in src/cli-commands.ts now takes an ID only when it is decimal digits and a safe integer; anything else prints "Invalid file ID: ARG" or "Invalid collection ID: ARG" and the command exits 1. The fixer checks every --file ID before it opens the library, so one bad ID fails the whole command before any request. Model: opus-5-5	2026-10-14 03:06:35 +02:00
clawbot	2402ec97f7	README: running quak backup from cron, and its exit codes (closes #170 ) check / check (push) Successful in 3m59s Details A new README section shows how to run quak backup unattended: log in once as the job's user, a crontab with a backup every night and --verify on Sundays, output appended to a log file, and the HOME and XDG_DATA_HOME the job needs to find the saved session. A table gives each exit code as the code returns it, and says which failures the next run retries. The introduction now lists everything the backup keeps for each file. It, the backup layout tree and the lib.backup() entry say EXIF and XMP are kept for an image only, and dimensions for a JPEG only. Judgement call: the --verify run takes Sunday's slot rather than a second job that night, since an overlapping run would exit 2. Model: opus-5-5	2026-10-07 00:47:37 +02:00
clawbot	ddf58af3cc	quak backup --verify re-hashes stored originals and downloads again any that do not match (closes #168 ) check / check (push) Failing after 1m22s Details `--verify`, or `lib.backup({ verify: true })`, hashes each original already at its save path as the download check does, streamed, a live photo as `<imageHash>:<videoHash>`. A mismatch is logged, removed and put back in the same run, and what is put back is hashed too, since a copy from the content cache is not checked; one that still does not match, or a failed fetch, goes into `failures.json`. A file with no recorded hash counts as unchecked. The result, the summary and `--json` gain `verified`, `mismatched` and `unchecked`. Judgement call: a stored original that cannot be read for hashing is recorded as failed and left in place. Judgement call: a copy put back that still does not match stays at its save path and is not counted as downloaded. Judgement call: the summary prints the three counts only with `--verify`. Model: opus-5-5	2026-10-06 22:47:26 +02:00