New module kiosk/src/at_rest.rs. Derives an AES-256-GCM key via HKDF
from a Pi-bound value:
1. /proc/device-tree/serial-number (Pi 5 firmware exposes it)
2. /proc/cpuinfo Serial line (older kernels)
3. /etc/machine-id (non-Pi dev fallback)
File format: "BFE1" magic || 12-byte random nonce || ciphertext+tag.
Atomic write via tempfile + rename so a crash mid-write can't leave a
half-encrypted file.
Wired into kiosk/src/server.rs at every file I/O touching sensitive
state:
- kiosk.key (bearer token to BF server)
- local.key (LAN-side API auth key)
- bundle.json (cached bundle with RTSP credentials in URL form)
Migration: read paths tolerate legacy plaintext (kiosks upgraded from a
pre-at_rest build) AND re-store as ciphertext on the first read. One-
shot upgrade — subsequent boots skip the migration write.
Threat model defended: SD card extraction. Attacker who pulls the card
can't decrypt without also having the same physical Pi (CPU serial is
hardware-bound). Doesn't defeat an attacker who has both — at that
point they ARE the kiosk. Bar is raised from "trivially extract every
camera password" to "must steal the device intact."
Not defended: TPM-style attestation, remote attestation, sealed boot.
Pi 5 has no TPM and we don't ship a secure-boot config.
Tests in-module: round-trip short bytes, round-trip JSON, legacy
plaintext passthrough.
