document encryption key handling in README.md

README.md

@@ -3,10 +3,10 @@
 A Bash wrapper around Proxmox's `proxmox-backup-client` for Proxmox Backup Server (PBS).
 
 It keeps the repository, credentials and the list of paths to back up in a single config
-file, injects the configured namespace (`--ns`) into every command that needs it, and lets
-you pick a snapshot group, snapshot and archive from an interactive menu instead of typing
-them out. Any command it does not implement itself is passed straight through to
-`proxmox-backup-client`.
+file, injects the configured namespace (`--ns`) and encryption key (`--keyfile`) into every
+command that needs them, and lets you pick a snapshot group, snapshot and archive from an
+interactive menu instead of typing them out. Any command it does not implement itself is
+passed straight through to `proxmox-backup-client`.
 
 ## Requirements
 
@@ -50,24 +50,15 @@ sudo chmod 640 /etc/pbc/config
 sudo editor /etc/pbc/config
 ```
 
-```bash
-PBS_SERVER='pbs.domain.tld:8007'
-PBS_USER='backup@pam:token-name'
-PBS_PASSWORD='XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX'
-PBS_DATASTORE='backup'
-PBS_NAMESPACE='MyBackups'
-
-BACKUP=('root.pxar:/' 'data.pxar:/mnt/data')
-```
-
 | Key | Description |
 |---|---|
 | `PBS_SERVER` | PBS host and port |
 | `PBS_USER` | User and API token name, `user@realm:token-name` |
 | `PBS_PASSWORD` | The API token secret |
 | `PBS_DATASTORE` | Datastore to back up to |
-| `PBS_NAMESPACE` | Namespace inside the datastore |
+| `PBS_NAMESPACE` | Optional, namespace inside the datastore |
 | `BACKUP` | Array of `archive-name.pxar:/path` entries to back up |
+| `ENCRYPTION_KEYFILE` | Optional, path to the client encryption key — see [Encryption](#encryption) |
 | `PBC` | Optional, path to `proxmox-backup-client` (default `/usr/local/bin/proxmox-backup-client`) |
 
 `pbc` never creates the config for you — copy the example and edit it yourself. The file
@@ -93,6 +84,45 @@ Use a different config with `-c`:
 pbc -c ./myconfig list
 ```
 
+## Encryption
+
+Backups can be encrypted client-side, so PBS only ever sees ciphertext. Create a key and
+point `ENCRYPTION_KEYFILE` at it:
+
+```bash
+sudo proxmox-backup-client key create /etc/pbc/backup.key --kdf none
+sudo chmod 600 /etc/pbc/backup.key
+```
+
+`pbc` errors out if the key is missing or unreadable, and warns if it is readable by
+others. As with the config, the file has to be readable for the user that runs `pbc` —
+`sudo chown youruser /etc/pbc/backup.key` if that is not root.
+
+**Back the key up somewhere else.** Without it the backups are unrecoverable, and a key
+stored only on the machine you are backing up is gone exactly when you need it. Print a
+recovery sheet and keep it off-host:
+
+```bash
+sudo proxmox-backup-client key paperkey /etc/pbc/backup.key
+```
+
+`--kdf none` leaves the key unprotected on disk, which is what makes unattended backups
+possible. With a passphrase-protected key (`--kdf scrypt`) the client prompts on every run
+and `backup-cron` hangs; export `PBS_ENCRYPTION_PASSWORD` in that case.
+
+The key is passed to `backup`, `backup-cron`, `mount` and `catalog-shell`. It is *not*
+passed to commands that fall through to `proxmox-backup-client` — `pbc restore` needs both
+flags spelled out:
+
+```bash
+pbc restore --ns MyBackups --keyfile /etc/pbc/backup.key \
+  host/myhost/2026-08-24T01:00:00Z root.pxar /mnt/restore
+```
+
+Setting `ENCRYPTION_KEYFILE` only affects snapshots made from then on. Older unencrypted
+snapshots stay readable, and listing works without the key either way — only reading
+archive contents needs it.
+
 ## Usage
 
 Run `pbc --help` for the synopsis and the list of options, or `pbc -H` to additionally
@@ -150,7 +180,8 @@ pbc mount host/myhost/2026-08-24T01:00:00Z root.pxar /mnt/restore
 ```
 
 The target directory is not created for you — create it beforehand and make sure it is
-writable for the user that runs `pbc`. Unmount when done:
+writable for the user that runs `pbc`. Encrypted archives are decrypted transparently when
+`ENCRYPTION_KEYFILE` is configured. Unmount when done:
 
 ```bash
 umount /mnt/restore
@@ -159,6 +190,7 @@ umount /mnt/restore
 ### catalog-shell
 
 Open an interactive shell to browse an archive and restore selected files — see below.
+Encrypted archives are decrypted transparently when `ENCRYPTION_KEYFILE` is configured.
 
 ```bash
 pbc catalog-shell