feat: add no-hmac-equality-compare ast-grep rule

src/rust/rules/no-hmac-equality-compare.yml

@@ -0,0 +1,22 @@
+id: no-hmac-equality-compare
+message: "Use constant_time_eq() for HMAC comparison, not == or !=. Non-constant-time comparison leaks timing information."
+severity: error
+language: rust
+note: |
+  HMAC challenge IDs must be compared using constant_time_eq() to prevent
+  timing side-channel attacks. Using == or != leaks information about how
+  many leading bytes match, which can allow an attacker to forge valid HMACs.
+
+  Replace:
+    if credential.challenge.id != expected_id { ... }
+  With:
+    if !constant_time_eq(&credential.challenge.id, &expected_id) { ... }
+
+  To disable this rule:
+  - Line: add `// ast-grep-ignore: no-hmac-equality-compare` with justification
+rule:
+  any:
+    - pattern: $A == expected_id
+    - pattern: $A != expected_id
+    - pattern: expected_id == $A
+    - pattern: expected_id != $A

src/rust/tests/__snapshots__/no-hmac-equality-compare-snapshot.yml

@@ -0,0 +1,45 @@
+id: no-hmac-equality-compare
+snapshots:
+  ? |
+    if credential.challenge.id != expected_id {
+        return Err("mismatch");
+    }
+  : labels:
+    - source: credential.challenge.id != expected_id
+      style: primary
+      start: 3
+      end: 41
+  ? |
+    if credential.challenge.id == expected_id {
+        return Ok(());
+    }
+  : labels:
+    - source: credential.challenge.id == expected_id
+      style: primary
+      start: 3
+      end: 41
+  ? |
+    if expected_id != credential.challenge.id {
+        return Err("mismatch");
+    }
+  : labels:
+    - source: expected_id != credential.challenge.id
+      style: primary
+      start: 3
+      end: 41
+  ? |
+    if expected_id == credential.challenge.id {
+        return Ok(());
+    }
+  : labels:
+    - source: expected_id == credential.challenge.id
+      style: primary
+      start: 3
+      end: 41
+  ? |
+    let matches = self.id == expected_id;
+  : labels:
+    - source: self.id == expected_id
+      style: primary
+      start: 14
+      end: 36

src/rust/tests/no-hmac-equality-compare-test.yml

@@ -0,0 +1,32 @@
+id: no-hmac-equality-compare
+valid:
+  - |
+    if !constant_time_eq(&credential.challenge.id, &expected_id) {
+        return Err("mismatch");
+    }
+  - |
+    constant_time_eq(&self.id, &expected_id)
+  - |
+    let expected_id = compute_challenge_id(secret, realm, method, intent, request, None, None, None);
+  - |
+    let is_valid = hmac_verify(&id, &expected_id);
+
+invalid:
+  - |
+    if credential.challenge.id != expected_id {
+        return Err("mismatch");
+    }
+  - |
+    if credential.challenge.id == expected_id {
+        return Ok(());
+    }
+  - |
+    if expected_id != credential.challenge.id {
+        return Err("mismatch");
+    }
+  - |
+    if expected_id == credential.challenge.id {
+        return Ok(());
+    }
+  - |
+    let matches = self.id == expected_id;