feat: Add naive state key verification to OlmMachine

Modifies `OlmMachine::decrypt_room_event_inner` to call a new method
`OlmMachine::verify_packed_state_key` which, if the event is a state
event, verifies that the original event's state key, when unpacked,
matches the state key and event type in the decrypted event content.

Introduces MegolmError::StateKeyVerificationFailed and
UnableToDecryptReason::StateKeyVerificationFailed which are thrown when
the verification fails.

Signed-off-by: Skye Elliot <[email protected]>

Author: kaylendog

crates/matrix-sdk-common/Cargo.toml

@@ -19,6 +19,7 @@ rustdoc-args = ["--generate-link-to-definition"]
 [features]
 js = ["wasm-bindgen-futures"]
 uniffi = ["dep:uniffi"]
+experimental-encrypted-state-events = []
 # Private feature, see
 # https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823 for the gory
 # details.

crates/matrix-sdk-common/src/deserialized_responses.rs

@@ -998,6 +998,11 @@ pub enum UnableToDecryptReason {
     /// cross-signing identity did not satisfy the requested
     /// `TrustRequirement`.
     SenderIdentityNotTrusted(VerificationLevel),
+
+    /// The outer state key could not be verified against the inner encrypted
+    /// state key and type.
+    #[cfg(feature = "experimental-encrypted-state-events")]
+    StateKeyVerificationFailed,
 }
 
 impl UnableToDecryptReason {

crates/matrix-sdk-crypto/Cargo.toml

@@ -18,7 +18,7 @@ rustdoc-args = ["--cfg", "docsrs", "--generate-link-to-definition"]
 default = []
 automatic-room-key-forwarding = []
 experimental-send-custom-to-device = []
-experimental-encrypted-state-events = []
+experimental-encrypted-state-events = ["matrix-sdk-common/experimental-encrypted-state-events"]
 js = ["ruma/js", "vodozemac/js", "matrix-sdk-common/js"]
 qrcode = ["dep:matrix-sdk-qrcode"]
 experimental-algorithms = []

crates/matrix-sdk-crypto/src/error.rs

@@ -133,6 +133,12 @@ pub enum MegolmError {
     /// The nested value is the sender's current verification level.
     #[error("decryption failed because trust requirement not satisfied: {0}")]
     SenderIdentityNotTrusted(VerificationLevel),
+
+    /// The outer state key could not be verified against the inner encrypted
+    /// state key and type.
+    #[cfg(feature = "experimental-encrypted-state-events")]
+    #[error("decryption failed because the state key failed to validate")]
+    StateKeyVerificationFailed,
 }
 
 /// Decryption failed because of a mismatch between the identity keys of the

crates/matrix-sdk-crypto/src/machine/mod.rs

@@ -34,7 +34,7 @@ use matrix_sdk_common::{
     BoxFuture,
 };
 #[cfg(feature = "experimental-encrypted-state-events")]
-use ruma::events::{AnyStateEventContent, StateEventContent};
+use ruma::events::{AnyStateEvent, AnyStateEventContent, StateEventContent};
 use ruma::{
     api::client::{
         dehydrated_device::DehydratedDeviceData,
@@ -2259,9 +2259,62 @@ impl OlmMachine {
                 .await;
         }
 
-        let event = serde_json::from_value::<Raw<AnyTimelineEvent>>(decrypted_event.into())?;
+        let decrypted_event =
+            serde_json::from_value::<Raw<AnyTimelineEvent>>(decrypted_event.into())?;
 
-        Ok(DecryptedRoomEvent { event, encryption_info, unsigned_encryption_info })
+        #[cfg(feature = "experimental-encrypted-state-events")]
+        self.verify_packed_state_key(&event, &decrypted_event)?;
+
+        Ok(DecryptedRoomEvent { event: decrypted_event, encryption_info, unsigned_encryption_info })
+    }
+
+    /// If the passed event is a state event, verify its outer packed state key
+    /// matches the inner state key once unpacked.
+    ///
+    /// * `original` - The original encrypted event received over the wire.
+    /// * `decrypted` - The decrypted event.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if
+    ///
+    /// * The original event's state key failed to unpack;
+    /// * The decrypted event could not be deserialised;
+    /// * The unpacked event type does not match the type of the decrypted
+    ///   event;
+    /// * The unpacked event state key does not match the state key of the
+    ///   decrypted event.
+    #[cfg(feature = "experimental-encrypted-state-events")]
+    pub fn verify_packed_state_key(
+        &self,
+        original: &EncryptedEvent,
+        decrypted: &Raw<AnyTimelineEvent>,
+    ) -> MegolmResult<()> {
+        // We only need to verify state events.
+        let Some(raw_state_key) = &original.state_key else { return Ok(()) };
+
+        // Unpack event type and state key from the raw state key.
+        let (outer_event_type, outer_state_key) =
+            raw_state_key.split_once(":").ok_or(MegolmError::StateKeyVerificationFailed)?;
+
+        // Deserialize the decrypted event.
+        let AnyTimelineEvent::State(inner) =
+            decrypted.deserialize().map_err(MegolmError::JsonError)?
+        else {
+            return Err(MegolmError::StateKeyVerificationFailed);
+        };
+
+        // Check event types match, discard if not.
+        let inner_event_type = inner.event_type().to_string();
+        if outer_event_type != inner_event_type {
+            return Err(MegolmError::StateKeyVerificationFailed);
+        }
+
+        // Check state keys match, discard if not.
+        if outer_state_key != inner.state_key() {
+            return Err(MegolmError::StateKeyVerificationFailed);
+        }
+        Ok(())
     }
 
     /// Try to decrypt the events bundled in the `unsigned` object of the given
@@ -3032,6 +3085,8 @@ fn megolm_error_to_utd_info(
         JsonError(_) => UnableToDecryptReason::PayloadDeserializationFailure,
         MismatchedIdentityKeys(_) => UnableToDecryptReason::MismatchedIdentityKeys,
         SenderIdentityNotTrusted(level) => UnableToDecryptReason::SenderIdentityNotTrusted(level),
+        #[cfg(feature = "experimental-encrypted-state-events")]
+        StateKeyVerificationFailed => UnableToDecryptReason::StateKeyVerificationFailed,
 
         // Pass through crypto store errors, which indicate a problem with our
         // application, rather than a UTD.

crates/matrix-sdk-crypto/src/types/events/room/mod.rs

@@ -36,6 +36,9 @@ where
     /// The globally unique identifier for this event.
     pub event_id: OwnedEventId,
 
+    /// Present if and only if this event is a state event.
+    pub state_key: Option<String>,
+
     /// The body of this event, as created by the client which sent it.
     pub content: C,