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Document encryption at rest as an accepted risk, with options #46

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21 changes: 19 additions & 2 deletions docs/security.md
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Expand Up @@ -100,8 +100,25 @@ If your age key leaks, generate a new one, update `.sops.yaml`, run

## Not covered

- **Encryption at rest.** The server's disk isn't encrypted (an unattended
server would need the key on the same machine).
- **Encryption at rest** (accepted risk). The disk is not encrypted, so
whoever can read the provider's storage, snapshots or disk images can read
all mail and the server's SSH host key (and with it, all secrets). Full-disk
encryption on an unattended VM doesn't change that much: the unlock key
must be on the machine, or someone must type it after every reboot. The
options, from most to least effort:
- **End-to-end encryption** (OpenPGP or S/MIME) for mail that matters: the
only option that also protects against a compromised server.
- **Dovecot `mail_crypt`** with per-user keys derived from the user's
password: mail is encrypted on disk and only readable while the user is
logged in. Breaks server-side features that need to read mail (search
indexes, Sieve on stored mail) and makes a forgotten password fatal.
- **LUKS with remote unlock**: full-disk encryption, unlocked over SSH in
the initrd (`boot.initrd.network.ssh`) after every reboot. Protects
discarded disks and offline snapshots, not a running server; reboots
(and therefore kernel updates) then need you.

The off-site backups (`docs/backups.md`) are encrypted by restic before
they leave the server.
- **Virus scanning** (ClamAV) is off because of the 1 GB RAM budget; see the
README.
- **DNSSEC** for your own zones is up to your DNS host; turn it on in
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