Correct remote layout and privacy docs for hashed snapshot keys (closes #67)

The remote layout and threat model in three documents described plaintext
snapshot IDs as directory names and misattributed the observable backup
time to those IDs. In fact `RemoteSnapshotKey` names each metadata
directory (and the manifest `snapshot_id`) with a one-way double SHA-256
hash of the human ID, so hostname and snapshot name are not observable;
the backup time is, via the plaintext manifest timestamp, an accepted
design property (issue 81).

Document the derivation once in `docs/REPOSTRUCTURE.md` with a worked
example; README, ARCHITECTURE and DATAMODEL now show the hashed layout and
link to it. Rewrite the privacy section to state what the unencrypted
manifest really exposes. Fix two code comments that claimed the public
bytes hide the timestamp. Docs and comments only; no behaviour change.

Model: opus-4-8

ARCHITECTURE.md

@@ -366,11 +366,17 @@ bucket/
 │           └── {full-hash}          # Compressed+encrypted blob
 │
 └── metadata/
-    └── {snapshot-id}/
+    └── {remote-key}/
         ├── db.zst.age               # Encrypted binary SQLite database
         └── manifest.json.zst        # Blob list (for pruning/verification)
 ```
 
+The `{remote-key}` directory name is a one-way double SHA-256 hash of the human
+snapshot ID, so the human ID (hostname, snapshot name, timestamp) is never
+written to the store as a directory name. See
+[docs/REPOSTRUCTURE.md](docs/REPOSTRUCTURE.md#remote-key-derivation) for the
+derivation and a worked example.
+
 ## Thread Safety
 
 - `Packer`: Thread-safe via mutex. Multiple goroutines can call `AddChunk()`.

README.md

@@ -344,7 +344,7 @@ both are set.
 ├── blobs/
 │   └── <aa>/<bb>/<full_blob_hash>
 └── metadata/
-    └── <snapshot_id>/
+    └── <remote-key>/
         ├── db.zst.age          # Encrypted binary SQLite database
         └── manifest.json.zst   # Unencrypted blob list (for pruning)
 ```
@@ -355,8 +355,18 @@ both are set.
 * `manifest.json.zst` is an unencrypted compressed JSON blob list, enabling
   pruning without the private key
 
-Snapshot IDs follow the format `<hostname>_<snapshot-name>_<RFC3339-timestamp>`
-(e.g. `server1_home_2025-06-01T12:00:00Z`).
+Snapshot IDs follow the human-readable format
+`<hostname>_<snapshot-name>_<RFC3339-timestamp>` (e.g.
+`server1_home_2025-06-01T12:00:00Z`), but this ID is never written to the
+destination store in plaintext. Each snapshot's metadata directory is named
+with its `<remote-key>`, a one-way double SHA-256 hash of the ID, so a listing
+of the store reveals no hostname or snapshot name. The backup time is not
+hidden: manifest.json.zst carries a plaintext timestamp, and object
+modification times are visible at the storage layer regardless. For example,
+`server1_home_2025-06-01T12:00:00Z` is stored under
+`metadata/17f97bcde958748af076b926af59823943db59e80ce7170b40f124dfa28f64aa/`.
+See [docs/REPOSTRUCTURE.md](docs/REPOSTRUCTURE.md#remote-key-derivation) for the
+derivation.
 
 ### data flow
 
@@ -373,7 +383,7 @@ Snapshot IDs follow the format `<hostname>_<snapshot-name>_<RFC3339-timestamp>`
 
 **restore:**
 
-1. Download and decrypt `metadata/<snapshot_id>/db.zst.age`
+1. Download and decrypt `metadata/<remote-key>/db.zst.age`
 2. Open the binary SQLite database
 3. Query files (optionally filtered by paths)
 4. Download and decrypt required blobs

docs/DATAMODEL.md

@@ -194,8 +194,10 @@ After a snapshot is completed:
 2. Clean temporary database to contain only current snapshot data
 3. Export to SQL dump using sqlite3
 4. Compress with zstd and encrypt with age
-5. Upload to S3 as `metadata/{snapshot-id}/db.zst.age`
-6. Generate blob manifest and upload as `metadata/{snapshot-id}/manifest.json.zst`
+5. Upload to S3 as `metadata/{remote-key}/db.zst.age`
+6. Generate blob manifest and upload as `metadata/{remote-key}/manifest.json.zst`
+
+The `{remote-key}` directory name is a one-way hash of the human snapshot ID, so the ID is never written to the store in plaintext; see [REPOSTRUCTURE.md](REPOSTRUCTURE.md#remote-key-derivation).
 
 ### 4. Restore Process
 

docs/REPOSTRUCTURE.md

@@ -17,11 +17,13 @@ Vaultik stores all backup data in an S3-compatible object store. The repository
 │       └── <hash[2:4]>/
 │           └── <full-hash>
 └── metadata/
-    └── <snapshot-id>/
+    └── <remote-key>/
         ├── db.zst.age
         └── manifest.json.zst
 ```
 
+The metadata subdirectory is named with the **remote key**, a one-way hash of the snapshot ID, not with the human-readable snapshot ID itself. See [Remote Key Derivation](#remote-key-derivation).
+
 ## Blobs Directory (`blobs/`)
 
 ### Structure
@@ -40,9 +42,11 @@ Blobs contain the actual file data from backups and must be encrypted for securi
 
 ## Metadata Directory (`metadata/`)
 
-Each snapshot has its own subdirectory named with the snapshot ID.
+Each snapshot has its own subdirectory. The directory is **not** named with the human-readable snapshot ID; it is named with the remote key — a one-way hash of that ID. The human ID is never written to the destination store as a directory name (see [Remote Key Derivation](#remote-key-derivation)).
 
 ### Snapshot ID Format
+
+The human-readable snapshot ID is used in CLI arguments, log lines, and the local database. It is not written to the destination store.
 - **Format**: `<hostname>_<snapshot-name>_<RFC3339>` (or `<hostname>_<RFC3339>` if no
   name was specified)
 - **Example**: `laptop_home_2024-01-15T14:30:52Z`
@@ -51,6 +55,19 @@ Each snapshot has its own subdirectory named with the snapshot ID.
   - Snapshot name from the configured `snapshots:` map (optional)
   - RFC3339 UTC timestamp
 
+This ID reveals the hostname, the configured snapshot name, and the backup time, so it is never used as the on-disk directory name — the remote key is used instead.
+
+### Remote Key Derivation
+
+The remote key is `hex(SHA256(SHA256("vaultik|" + snapshot-id)))`: a double SHA-256 over the snapshot ID, with a `vaultik|` domain-separation prefix. The result is a 64-character hex string with no structure a remote observer can reverse. Implemented in `internal/snapshot/remotekey.go`.
+
+Worked example:
+- Snapshot ID: `server1_home_2025-06-01T12:00:00Z`
+- Remote key: `17f97bcde958748af076b926af59823943db59e80ce7170b40f124dfa28f64aa`
+- Directory: `metadata/17f97bcde958748af076b926af59823943db59e80ce7170b40f124dfa28f64aa/`
+
+Because the hash is one-way, a listing of the destination store reveals neither the hostname nor the snapshot name of any backup. The same remote key is stored in the manifest's `snapshot_id` field.
+
 ### Files in Each Snapshot Directory
 
 #### `db.zst.age` - Encrypted Database
@@ -68,16 +85,17 @@ Each snapshot has its own subdirectory named with the snapshot ID.
 - **Structure**:
 ```json
 {
-  "snapshot_id": "laptop_home_2024-01-15T14:30:52Z",
-  "timestamp": "2024-01-15T14:30:52Z",
+  "snapshot_id": "17f97bcde958748af076b926af59823943db59e80ce7170b40f124dfa28f64aa",
+  "timestamp": "2025-06-01T12:00:00Z",
   "blob_count": 42,
+  "total_compressed_size": 1048576,
   "blobs": [
-    "cafebabe1234567890abcdef1234567890abcdef1234567890abcdef12345678",
-    "deadbeef1234567890abcdef1234567890abcdef1234567890abcdef12345678",
-    ...
+    { "hash": "cafebabe1234567890abcdef1234567890abcdef1234567890abcdef12345678", "compressed_size": 24576 },
+    { "hash": "deadbeef1234567890abcdef1234567890abcdef1234567890abcdef12345678", "compressed_size": 32768 }
   ]
 }
 ```
+`snapshot_id` is the remote key (a hash), not the human ID; `timestamp` is written in the clear.
 
 ### Why Manifest is Unencrypted
 The manifest must be readable without the private key to enable:
@@ -86,7 +104,7 @@ The manifest must be readable without the private key to enable:
 3. **Verification** - Checking blob existence without decryption
 4. **Cross-snapshot deduplication analysis** - Finding shared blobs between snapshots
 
-The manifest only contains blob hashes, not file names or any other sensitive information.
+The manifest contains the remote key, the backup timestamp, the blob count and total compressed size, and each blob's hash and compressed size. It contains no file names, paths, or other decrypted metadata.
 
 ## Security Considerations
 
@@ -96,19 +114,21 @@ The manifest only contains blob hashes, not file names or any other sensitive in
 - **File-to-chunk mappings** (in db.zst.age)
 
 ### What's Not Encrypted
-- **Blob hashes** (in manifest.json.zst)
-- **Snapshot IDs** (directory names)
-- **Blob count per snapshot** (in manifest.json.zst)
+- **The remote key** — directory names and the manifest `snapshot_id`, a one-way hash of the snapshot ID (see [Remote Key Derivation](#remote-key-derivation))
+- **The backup timestamp** (in manifest.json.zst)
+- **Blob hashes and their compressed sizes** (in manifest.json.zst)
+- **Blob count and total compressed size per snapshot** (in manifest.json.zst)
 
 ### Privacy Implications
-From the unencrypted data, an observer can determine:
-- When backups were taken (from snapshot IDs)
-- Which hostname created backups (from snapshot IDs)
-- How many blobs each snapshot references
-- Which blobs are shared between snapshots (deduplication patterns)
-- The size of each encrypted blob
+From the unencrypted data, an observer of the destination store can determine:
+- **When each backup was taken** — not from the directory name, which is a one-way hash, but from the plaintext `timestamp` field in manifest.json.zst, which is published in the clear
+- How many blobs each snapshot references, and the total compressed size
+- The compressed size of each blob, and which blobs are shared between snapshots (deduplication patterns)
+
+Together these give an observer a timing-and-size profile of every snapshot. This is an accepted, documented property of the format, not a defect: the manifest is unencrypted so that pruning can run without the private key, and the timing channel could not be closed by encrypting it anyway — object creation times and per-object sizes stay visible at the storage layer on both `s3://` and `file://` destinations regardless.
 
 An observer cannot determine:
+- The hostname or snapshot name of any backup (the directory name and the manifest `snapshot_id` are one-way hashes of the human ID)
 - File names or paths
 - File contents
 - File permissions or ownership

internal/snapshot/remotekey.go

@@ -22,8 +22,9 @@ const remoteKeyPrefix = "vaultik|"
 //
 //   - the "metadata/<remote-key>/..." subdirectory on the storage
 //     backend so a directory listing of the bucket / file:// dest
-//     doesn't reveal hostnames, configured snapshot names, or backup
-//     timestamps;
+//     doesn't reveal hostnames or configured snapshot names. (The
+//     backup time is not hidden: the manifest.json.zst inside that
+//     directory carries a plaintext RFC3339 timestamp.)
 //   - the `snapshot_id` field of the unencrypted manifest.json.zst
 //     for the same reason;
 //   - any code path that needs to translate a known local snapshot ID

internal/snapshot/snapshot.go

@@ -840,8 +840,10 @@ func (sm *SnapshotManager) generateBlobManifest(
 	}
 
 	// Create manifest. SnapshotID in the unencrypted manifest is the
-	// double-SHA256 remote key, not the human ID, so the public bytes
-	// don't reveal hostname/snapshot-name/timestamp metadata.
+	// double-SHA256 remote key (see RemoteSnapshotKey), not the human ID,
+	// so neither this field nor the directory name reveals the hostname or
+	// snapshot name. Timestamp below is written in the clear, so the backup
+	// time is observable to anyone who can read the manifest.
 	manifest := &Manifest{
 		SnapshotID:          RemoteSnapshotKey(snapshotID),
 		Timestamp:           time.Now().UTC().Format(time.RFC3339),