The previous secret store used a repeating-key XOR cipher which is
cryptographically broken:
- Deterministic (no nonce) — identical plaintexts produce identical
ciphertexts
- No authentication — tampered ciphertext decrypts silently
- Vulnerable to known-plaintext attacks (e.g., "sk-" prefix reveals
key bytes)
Replace with ChaCha20-Poly1305 authenticated encryption:
- Random 12-byte nonce per encryption (non-deterministic)
- Poly1305 authentication tag detects tampering
- Uses the same 32-byte key file (no migration needed for keys)
New ciphertext format is `enc2:<hex(nonce || ciphertext || tag)>`.
Legacy `enc:` values (XOR) are still decryptable for backward
compatibility during migration.
Adds chacha20poly1305 0.10 crate (pure Rust, no C dependencies).
New tests: tamper detection, wrong-key rejection, nonce uniqueness,
truncation handling, legacy XOR backward compatibility.
CWE-327 / CRIT-1
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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