Reject unverified events only when they carry a key version

AGENTS.md

@@ -60,10 +60,12 @@ Don't add speculative configurability or one-off helpers.
 
 ## 4. Security invariants — never weaken to make something pass
 
-- **Verification is not optional.** `decryptEvent` rejects unverified events by
-  default; a below-floor key version must never verify; an invalid signature must
-  never yield plaintext. Verify against caller-supplied signing keys — **never** a
-  key carried inside the event. Verification covers every signed event type, not
+- **Verification is not optional.** `decryptEvent` rejects unverified encrypted
+  messages and key changes by default, and any event whose signature is present
+  but invalid; other events without a checkable signature are returned with
+  `verified: false`, never silently dropped. A below-floor key version must never
+  verify; an invalid signature must never yield plaintext. Verify against
+  caller-supplied signing keys — **never** a key carried inside the event. Verification covers every signed event type, not
   just messages. Signature mismatches are bugs to investigate, not checks to silence.
 - **Key downgrade protection is automatic.** Conversation-key versions move
   forward only (monotonic high-water mark held for the `Chat` lifetime); the

crates/core/examples/gen_sdk_vectors.rs

@@ -28,6 +28,8 @@ const EVENT_MESSAGE_TEXT: &str = "fixture event message";
 const EVENT_REPLY_TEXT: &str = "fixture reply message";
 const EVENT_REPLY_FORGED_PREVIEW_TEXT: &str = "forged preview text";
 const EVENT_GARBAGE: &str = "!!!not-an-event!!!";
+const EVENT_UNENCRYPTED_MESSAGE_TEXT: &str = "fixture unencrypted message";
+const EVENT_READ_SEEN_UNTIL_ID: &str = "42";
 
 // Raw Thrift enum values embedded in the failure vector; suites assert the
 // decoded names ("RateLimitUpsell" / "Premium", camelCased in JS).
@@ -174,6 +176,23 @@ fn main() {
     )
     .expect("frame failure event");
 
+    // Unencrypted message and unsigned read receipt: both decode under the
+    // default reject-unverified policy with `verified: false`.
+    let event_unencrypted_message_b64 = internals::frame_unencrypted_message(
+        "plain-msg-1",
+        EVENT_SENDER_ID,
+        EVENT_CONVERSATION_ID,
+        EVENT_UNENCRYPTED_MESSAGE_TEXT,
+    )
+    .expect("frame unencrypted message event");
+    let event_unsigned_read_receipt_b64 = internals::frame_unsigned_read_receipt(
+        "read-msg-1",
+        EVENT_SENDER_ID,
+        EVENT_CONVERSATION_ID,
+        EVENT_READ_SEEN_UNTIL_ID,
+    )
+    .expect("frame unsigned read receipt event");
+
     let obj = json!({
         "identity_private_b64": B64.encode(identity_private),
         "signing_private_b64": B64.encode(signing_private),
@@ -199,6 +218,10 @@ fn main() {
         "event_signing_key_version": EVENT_SIGNING_KEY_VERSION,
         "event_recipient_key_version": EVENT_RECIPIENT_KEY_VERSION,
         "event_message_text": EVENT_MESSAGE_TEXT,
+        "event_unencrypted_message_b64": event_unencrypted_message_b64,
+        "event_unencrypted_message_text": EVENT_UNENCRYPTED_MESSAGE_TEXT,
+        "event_unsigned_read_receipt_b64": event_unsigned_read_receipt_b64,
+        "event_read_seen_until_id": EVENT_READ_SEEN_UNTIL_ID,
     });
 
     println!("{}", serde_json::to_string_pretty(&obj).unwrap());

crates/core/src/chat.rs

@@ -177,10 +177,12 @@ impl Chat {
         }
     }
 
-    /// When enabled — the default — `decrypt_event` returns an error for any
-    /// signed event whose signature cannot be verified (invalid, missing, or
-    /// no matching signing key) instead of returning it with
-    /// `verified: false`.
+    /// When enabled — the default — `decrypt_event` returns an error for an
+    /// encrypted message or key change whose signature cannot be verified
+    /// (invalid, missing, or no matching signing key), and for any other
+    /// signed event whose signature is present but invalid. Unencrypted
+    /// messages and other events without a checkable signature are returned
+    /// with `verified: false`. See `ChatCore::set_reject_unverified`.
     pub fn set_reject_unverified(&mut self, reject: bool) {
         self.inner.set_reject_unverified(reject);
     }