Exclude insecure devices on Olm encryption

bindings/matrix-sdk-crypto-ffi/src/machine.rs

@@ -18,7 +18,8 @@ use matrix_sdk_crypto::{
     olm::ExportedRoomKey,
     store::types::{BackupDecryptionKey, Changes},
     types::requests::ToDeviceRequest,
-    DecryptionSettings, LocalTrust, OlmMachine as InnerMachine, UserIdentity as SdkUserIdentity,
+    CollectStrategy, DecryptionSettings, LocalTrust, OlmMachine as InnerMachine,
+    UserIdentity as SdkUserIdentity,
 };
 use ruma::{
     api::{
@@ -832,6 +833,7 @@ impl OlmMachine {
         device_id: String,
         event_type: String,
         content: String,
+        share_strategy: CollectStrategy,
     ) -> Result<Option<Request>, CryptoStoreError> {
         let user_id = parse_user_id(&user_id)?;
         let device_id = device_id.as_str().into();
@@ -840,8 +842,11 @@ impl OlmMachine {
         let device = self.runtime.block_on(self.inner.get_device(&user_id, device_id, None))?;
 
         if let Some(device) = device {
-            let encrypted_content =
-                self.runtime.block_on(device.encrypt_event_raw(&event_type, &content))?;
+            let encrypted_content = self.runtime.block_on(device.encrypt_event_raw(
+                &event_type,
+                &content,
+                share_strategy,
+            ))?;
 
             let request = ToDeviceRequest::new(
                 user_id.as_ref(),

crates/matrix-sdk-crypto/CHANGELOG.md

@@ -6,13 +6,8 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased] - ReleaseDate
 
-## [0.13.0] - 2025-07-10
-
 ### Features
 
-- [**breaking**] Add a new `VerificationLevel::MismatchedSender` to indicate that the sender of an event appears to have been tampered with.
-  ([#5219](https://github.com/matrix-org/matrix-rust-sdk/pull/5219))
-
 - [**breaking**]: When in "exclude insecure devices" mode, refuse to decrypt
   incoming to-device messages from unverified devices, except for some
   exceptions for certain event types. To support this, a new variant has been
@@ -25,6 +20,17 @@ All notable changes to this project will be documented in this file.
   Affected methods are `OlmMachine::receive_sync_changes`,
   `RehydratedDevice::receive_events`, and several internal methods.
   ([#5319](https://github.com/matrix-org/matrix-rust-sdk/pull/5319)
+- [**breaking**] The `Device::encrypt_event_raw` and (experimental)
+  `OlmMachine::encrypt_content_for_devices` have new `share_strategy` parameters
+  to ensure that the recipients are sufficiently trusted.
+  ([#5457](https://github.com/matrix-org/matrix-rust-sdk/pull/5457/))
+
+## [0.13.0] - 2025-07-10
+
+### Features
+
+- [**breaking**] Add a new `VerificationLevel::MismatchedSender` to indicate that the sender of an event appears to have been tampered with.
+  ([#5219](https://github.com/matrix-org/matrix-rust-sdk/pull/5219))
 
 ### Refactor
 

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

@@ -77,6 +77,10 @@ pub enum OlmError {
     #[error("encryption failed due to an error collecting the recipient devices: {0}")]
     SessionRecipientCollectionError(SessionRecipientCollectionError),
 
+    /// Encrypted content is withheld from this device
+    #[error("encryption content is withheld from this: {0}")]
+    Withheld(WithheldCode),
+
     /// Refused to decrypt because the sender was not verified or did not meet
     /// the required VerificationLevel.
     #[error(

crates/matrix-sdk-crypto/src/identities/device.rs

@@ -43,6 +43,7 @@ use crate::{
     olm::{
         InboundGroupSession, OutboundGroupSession, Session, ShareInfo, SignedJsonObject, VerifyJson,
     },
+    session_manager::{withheld_code_for_device_for_share_strategy, CollectStrategy},
     store::{
         caches::SequenceNumber,
         types::{Changes, DeviceChanges},
@@ -462,6 +463,8 @@ impl Device {
     /// * `event_type` - The type of the event to be sent.
     /// * `content` - The content of the event to be sent. This should be a type
     ///   that implements the `Serialize` trait.
+    /// * `share_strategy` - The share strategy to use to determine whether we
+    ///   should encrypt to the device.
     ///
     /// # Returns
     ///
@@ -472,7 +475,19 @@ impl Device {
         &self,
         event_type: &str,
         content: &Value,
+        share_strategy: CollectStrategy,
     ) -> OlmResult<Raw<ToDeviceEncryptedEventContent>> {
+        if let Some(withheld_code) = withheld_code_for_device_for_share_strategy(
+            &self.inner,
+            share_strategy,
+            &self.own_identity,
+            &self.device_owner_identity,
+        )
+        .await?
+        {
+            return Err(OlmError::Withheld(withheld_code));
+        }
+
         let (used_session, raw_encrypted) = self.encrypt(event_type, content).await?;
 
         // Persist the used session

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

@@ -63,6 +63,8 @@ use vodozemac::{
     Curve25519PublicKey, Ed25519Signature,
 };
 
+#[cfg(feature = "experimental-send-custom-to-device")]
+use crate::session_manager::split_devices_for_share_strategy;
 use crate::{
     backups::{BackupMachine, MegolmV1BackupKey},
     dehydrated_devices::{DehydratedDevices, DehydrationError},
@@ -1159,15 +1161,17 @@ impl OlmMachine {
         devices: Vec<DeviceData>,
         event_type: &str,
         content: &Value,
+        share_strategy: CollectStrategy,
     ) -> OlmResult<(Vec<ToDeviceRequest>, Vec<(DeviceData, WithheldCode)>)> {
-        // TODO: Use a `CollectStrategy` arguments to filter our devices depending on
-        // safety settings (like not sending to insecure devices).
         let mut changes = Changes::default();
 
+        let (allowed_devices, mut blocked_devices) =
+            split_devices_for_share_strategy(&self.inner.store, devices, share_strategy).await?;
+
         let result = self
             .inner
             .group_session_manager
-            .encrypt_content_for_devices(devices, event_type, content.clone(), &mut changes)
+            .encrypt_content_for_devices(allowed_devices, event_type, content.clone(), &mut changes)
             .await;
 
         // Persist any changes we might have collected.
@@ -1182,7 +1186,10 @@ impl OlmMachine {
             );
         }
 
-        result
+        result.map(|(to_device_requests, mut withheld)| {
+            withheld.append(&mut blocked_devices);
+            (to_device_requests, withheld)
+        })
     }
     /// Collect the devices belonging to the given user, and send the details of
     /// a room key bundle to those devices.

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

@@ -39,6 +39,7 @@ use tokio::sync::Mutex;
 use crate::{
     machine::tests,
     olm::PrivateCrossSigningIdentity,
+    session_manager::CollectStrategy,
     store::{types::Changes, CryptoStoreWrapper, MemoryStore},
     types::{
         events::ToDeviceEvent,
@@ -194,7 +195,7 @@ pub async fn send_and_receive_encrypted_to_device_test_helper(
         sender.get_device(recipient.user_id(), recipient.device_id(), None).await.unwrap().unwrap();
 
     let raw_encrypted = device
-        .encrypt_event_raw(event_type, content)
+        .encrypt_event_raw(event_type, content, CollectStrategy::AllDevices)
         .await
         .expect("Should have encrypted the content");
 

crates/matrix-sdk-crypto/src/machine/tests/send_encrypted_to_device.rs

@@ -34,6 +34,7 @@ use crate::{
         tests::{self, decryption_verification_state::mark_alice_identity_as_verified_test_helper},
     },
     olm::SenderData,
+    session_manager::CollectStrategy,
     types::{
         events::{
             room::encrypted::ToDeviceEncryptedEventContent, EventType as _, ToDeviceCustomEvent,
@@ -443,7 +444,7 @@ async fn test_processed_to_device_variants() {
 
     let device = alice.get_device(bob.user_id(), bob.device_id(), None).await.unwrap().unwrap();
     let raw_encrypted = device
-        .encrypt_event_raw(custom_event_type, &custom_content)
+        .encrypt_event_raw(custom_event_type, &custom_content, CollectStrategy::AllDevices)
         .await
         .expect("Should have encryted the content");
 
@@ -600,7 +601,7 @@ async fn test_send_encrypted_to_device_no_session() {
         .await
         .unwrap()
         .unwrap()
-        .encrypt_event_raw(custom_event_type, &custom_content)
+        .encrypt_event_raw(custom_event_type, &custom_content, CollectStrategy::AllDevices)
         .await;
 
     assert_matches!(encryption_result, Err(OlmError::MissingSession));
@@ -700,3 +701,50 @@ async fn make_alice_unverified(alice: &OlmMachine, bob: &OlmMachine) {
     alice.receive_keys_query_response(&TransactionId::new(), &kq_response).await.unwrap();
     bob.receive_keys_query_response(&TransactionId::new(), &kq_response).await.unwrap();
 }
+
+#[async_test]
+/// Test that when we get an error when we try to encrypt to a device that
+/// doesn't satisfy the share strategy.
+async fn test_share_strategy_prevents_encryption() {
+    use matrix_sdk_common::deserialized_responses::WithheldCode;
+    use matrix_sdk_test::test_json::keys_query_sets::KeyDistributionTestData as DataSet;
+    use ruma::TransactionId;
+
+    use crate::CrossSigningKeyExport;
+
+    // Create the local user (`@me`), and import the public identity keys
+    let machine = OlmMachine::new(DataSet::me_id(), DataSet::me_device_id()).await;
+    let keys_query = DataSet::me_keys_query_response();
+    machine.mark_request_as_sent(&TransactionId::new(), &keys_query).await.unwrap();
+
+    // Also import the private cross signing keys
+    machine
+        .import_cross_signing_keys(CrossSigningKeyExport {
+            master_key: DataSet::MASTER_KEY_PRIVATE_EXPORT.to_owned().into(),
+            self_signing_key: DataSet::SELF_SIGNING_KEY_PRIVATE_EXPORT.to_owned().into(),
+            user_signing_key: DataSet::USER_SIGNING_KEY_PRIVATE_EXPORT.to_owned().into(),
+        })
+        .await
+        .unwrap();
+
+    let keys_query = DataSet::dan_keys_query_response();
+    let txn_id = TransactionId::new();
+    machine.mark_request_as_sent(&txn_id, &keys_query).await.unwrap();
+
+    let custom_event_type = "m.new_device";
+
+    let custom_content = json!({
+            "device_id": "XYZABCDE",
+            "rooms": ["!726s6s6q:example.com"]
+    });
+
+    let encryption_result = machine
+        .get_device(DataSet::dan_id(), DataSet::dan_unsigned_device_id(), None)
+        .await
+        .unwrap()
+        .unwrap()
+        .encrypt_event_raw(custom_event_type, &custom_content, CollectStrategy::OnlyTrustedDevices)
+        .await;
+
+    assert_matches!(encryption_result, Err(OlmError::Withheld(WithheldCode::Unverified)));
+}

crates/matrix-sdk-crypto/src/session_manager/group_sessions/mod.rs

@@ -35,8 +35,12 @@ use ruma::{
     UserId,
 };
 use serde::Serialize;
-pub(crate) use share_strategy::CollectRecipientsResult;
+#[cfg(feature = "experimental-send-custom-to-device")]
+pub(crate) use share_strategy::split_devices_for_share_strategy;
 pub use share_strategy::CollectStrategy;
+pub(crate) use share_strategy::{
+    withheld_code_for_device_for_share_strategy, CollectRecipientsResult,
+};
 use tracing::{debug, error, info, instrument, trace, warn, Instrument};
 
 use crate::{