Validate and apply declarative policy resolution. ## Replay protection When `auth.delta_signature` is non-zero, the following steps execute in this order to prevent replay attacks at minimum cost: 1. **Cheap seq_no check** — `seq_no > last_seen[(user_id, device_id)]`. Fails fast before any HMAC computation. 2. **HMAC verification** — constant-time comparison prevents timing attacks. 3. **Atomic
(
&mut self,
state: &CrdtState,
peer_id: u64,
auth: CrdtAuthContext,
change: &ProposedChange,
delta_bytes: Vec<u8>,
)
| 49 | /// - If AutoResolved: returns Ok(()) |
| 50 | /// - If Deferred/Webhook/Escalate: returns appropriate error |
| 51 | pub fn validate_or_reject( |
| 52 | &mut self, |
| 53 | state: &CrdtState, |
| 54 | peer_id: u64, |
| 55 | auth: CrdtAuthContext, |
| 56 | change: &ProposedChange, |
| 57 | delta_bytes: Vec<u8>, |
| 58 | ) -> Result<()> { |
| 59 | // Check auth expiry: agents that accumulated deltas offline must |
| 60 | // re-authenticate before syncing. |
| 61 | if auth.auth_expires_at > 0 { |
| 62 | let now_ms = std::time::SystemTime::now() |
| 63 | .duration_since(std::time::UNIX_EPOCH) |
| 64 | .unwrap_or_default() |
| 65 | .as_millis() as u64; |
| 66 | if now_ms > auth.auth_expires_at { |
| 67 | return Err(CrdtError::AuthExpired { |
| 68 | user_id: auth.user_id, |
| 69 | expired_at: auth.auth_expires_at, |
| 70 | }); |
| 71 | } |
| 72 | } |
| 73 | |
| 74 | // Replay protection + signature verification (signed path only). |
| 75 | // |
| 76 | // The unsigned path (all-zeros signature) bypasses replay protection. |
| 77 | // Old clients that send device_id=0 / seq_no=0 with a non-zero |
| 78 | // signature will be rejected by the seq_no check (0 is never > 0). |
| 79 | if auth.delta_signature != [0u8; 32] |
| 80 | && let Some(ref verifier) = self.delta_verifier |
| 81 | { |
| 82 | // Step 1: cheap seq_no check before any HMAC computation. |
| 83 | verifier |
| 84 | .registry() |
| 85 | .check_seq(auth.user_id, auth.device_id, auth.seq_no)?; |
| 86 | |
| 87 | // Step 2: constant-time HMAC verification. |
| 88 | verifier.verify( |
| 89 | auth.user_id, |
| 90 | auth.device_id, |
| 91 | auth.seq_no, |
| 92 | &delta_bytes, |
| 93 | &auth.delta_signature, |
| 94 | )?; |
| 95 | |
| 96 | // Step 3: advance last_seen atomically on success. |
| 97 | verifier |
| 98 | .registry() |
| 99 | .commit_seq(auth.user_id, auth.device_id, auth.seq_no)?; |
| 100 | } |
| 101 | |
| 102 | let hlc_timestamp = std::time::SystemTime::now() |
| 103 | .duration_since(std::time::UNIX_EPOCH) |
| 104 | .unwrap_or_default() |
| 105 | .as_millis() as u64; |
| 106 | |
| 107 | match self.validate_with_policy(state, peer_id, auth, change, delta_bytes, hlc_timestamp)? { |
| 108 | PolicyResolution::AutoResolved(_) => Ok(()), |