Test quak backup end to end against a fake Ente server (closes #186)

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

test/ente-server.ts

@@ -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;
+};