feat(crypto): Add support for the shared_history flag defined in MSC3061

This patch adds support for the `shared_history` flag from MSC3061 to
the `m.room_key` content, exported room keys, and backed-up room keys.

The flag is now persisted in our `InboundGroupSession`. Additionally,
when creating a new `InboundGroupSession`, we ensure the
`shared_history`  flag is set appropriately.

MSC3061: matrix-org/matrix-spec-proposals#3061

Author: poljar

Diff for: bindings/matrix-sdk-crypto-ffi/src/lib.rs

@@ -507,6 +507,7 @@ fn collect_sessions(
             imported: session.imported,
             backed_up: session.backed_up,
             history_visibility: None,
+            shared_history: false,
             algorithm: RustEventEncryptionAlgorithm::MegolmV1AesSha2,
         };
 

Diff for: crates/matrix-sdk-crypto/CHANGELOG.md

@@ -6,6 +6,17 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased] - ReleaseDate
 
+### Features
+
+- [**breaking**] Add support for the shared history flag defined in [MSC3061](https://github.com/matrix-org/matrix-spec-proposals/pull/3061).
+  The shared history flag is now respected when room keys are received as an
+  `m.room_key` event as well as when they are imported from a backup or a file
+  export. We also ensure to set the flag when we send out room keys. Due to
+  this, a new argument to the constructor for `room_key::MegolmV1AesSha2Content`
+  has been added and `PickledInboundGroupSession` has received a new
+  `shared_history` field that defaults to `false.`
+  ([#4700](https://github.com/matrix-org/matrix-rust-sdk/pull/4700))
+
 ## [0.10.0] - 2025-02-04
 
 ### Features

Diff for: crates/matrix-sdk-crypto/src/machine/tests/decryption_verification_state.rs

@@ -353,6 +353,7 @@ async fn test_verification_states_multiple_device() {
         SenderData::unknown(),
         EventEncryptionAlgorithm::MegolmV1AesSha2,
         None,
+        false,
     )
     .unwrap();
 
@@ -370,6 +371,7 @@ async fn test_verification_states_multiple_device() {
         SenderData::unknown(),
         EventEncryptionAlgorithm::MegolmV1AesSha2,
         None,
+        false,
     )
     .unwrap();
 

Diff for: crates/matrix-sdk-crypto/src/olm/account.rs

@@ -32,7 +32,7 @@ use ruma::{
             upload_signatures::v3::{Request as SignatureUploadRequest, SignedKeys},
         },
     },
-    events::AnyToDeviceEvent,
+    events::{room::history_visibility::HistoryVisibility, AnyToDeviceEvent},
     serde::Raw,
     DeviceId, DeviceKeyAlgorithm, DeviceKeyId, MilliSecondsSinceUnixEpoch, OneTimeKeyAlgorithm,
     OneTimeKeyId, OwnedDeviceId, OwnedDeviceKeyId, OwnedOneTimeKeyId, OwnedUserId, RoomId,
@@ -220,6 +220,7 @@ impl StaticAccountData {
 
         let sender_key = identity_keys.curve25519;
         let signing_key = identity_keys.ed25519;
+        let shared_history = shared_history_from_history_visibility(&visibility);
 
         let inbound = InboundGroupSession::new(
             sender_key,
@@ -229,6 +230,7 @@ impl StaticAccountData {
             own_sender_data,
             algorithm,
             Some(visibility),
+            shared_history,
         )?;
 
         Ok((outbound, inbound))
@@ -1511,6 +1513,34 @@ impl PartialEq for Account {
     }
 }
 
+/// Calculate the shared history flag from the history visibility as defined in
+/// [MSC3061]
+///
+/// The MSC defines that the shared history flag should be set to true when the
+/// history visibility setting is set to `shared` or `world_readable`:
+///
+/// > A room key is flagged as having been used for shared history when it was
+/// > used to encrypt a message while the room's history visibility setting
+/// > was set to world_readable or shared.
+///
+/// In all other cases, even if we encounter a custom history visibility, we
+/// should return false:
+///
+/// > If the client does not have an m.room.history_visibility state event for
+/// > the room, or its value is not understood, the client should treat it as if
+/// > its value is joined for the purposes of determining whether the key is
+/// > used for shared history.
+///
+/// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+pub(crate) fn shared_history_from_history_visibility(
+    history_visibility: &HistoryVisibility,
+) -> bool {
+    match history_visibility {
+        HistoryVisibility::Shared | HistoryVisibility::WorldReadable => true,
+        HistoryVisibility::Invited | HistoryVisibility::Joined | _ => false,
+    }
+}
+
 /// Expand the pickle key for an older version of dehydrated devices
 ///
 /// The `org.matrix.msc3814.v1.olm` variant of dehydrated devices used the

Diff for: crates/matrix-sdk-crypto/src/olm/group_sessions/inbound.rs

@@ -154,6 +154,12 @@ pub struct InboundGroupSession {
 
     /// Was this room key backed up to the server.
     backed_up: Arc<AtomicBool>,
+
+    /// Whether this [`InboundGroupSession`] can be shared with newly joined
+    /// users, allowing access to history, as defined in [MSC3061].
+    ///
+    /// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+    shared_history: bool,
 }
 
 impl InboundGroupSession {
@@ -176,6 +182,7 @@ impl InboundGroupSession {
     ///
     /// * `sender_data` - Information about the sender of the to-device message
     ///   that established this session.
+    #[allow(clippy::too_many_arguments)]
     pub fn new(
         sender_key: Curve25519PublicKey,
         signing_key: Ed25519PublicKey,
@@ -184,6 +191,7 @@ impl InboundGroupSession {
         sender_data: SenderData,
         encryption_algorithm: EventEncryptionAlgorithm,
         history_visibility: Option<HistoryVisibility>,
+        shared_history: bool,
     ) -> Result<Self, SessionCreationError> {
         let config = OutboundGroupSession::session_config(&encryption_algorithm)?;
 
@@ -208,6 +216,7 @@ impl InboundGroupSession {
             imported: false,
             algorithm: encryption_algorithm.into(),
             backed_up: AtomicBool::new(false).into(),
+            shared_history,
         })
     }
 
@@ -228,7 +237,13 @@ impl InboundGroupSession {
         signing_key: Ed25519PublicKey,
         content: &room_key::MegolmV1AesSha2Content,
     ) -> Result<Self, SessionCreationError> {
-        let room_key::MegolmV1AesSha2Content { room_id, session_id: _, session_key, .. } = content;
+        let room_key::MegolmV1AesSha2Content {
+            room_id,
+            session_id: _,
+            session_key,
+            shared_history,
+            ..
+        } = content;
 
         Self::new(
             sender_key,
@@ -238,6 +253,7 @@ impl InboundGroupSession {
             SenderData::unknown(),
             EventEncryptionAlgorithm::MegolmV1AesSha2,
             None,
+            *shared_history,
         )
     }
 
@@ -265,6 +281,7 @@ impl InboundGroupSession {
             backed_up: self.backed_up(),
             history_visibility: self.history_visibility.as_ref().clone(),
             algorithm: (*self.algorithm).to_owned(),
+            shared_history: self.shared_history,
         }
     }
 
@@ -317,6 +334,7 @@ impl InboundGroupSession {
             forwarding_curve25519_key_chain: vec![],
             sender_claimed_keys: (*self.creator_info.signing_keys).clone(),
             session_key,
+            shared_history: self.shared_history,
         }
     }
 
@@ -342,6 +360,7 @@ impl InboundGroupSession {
             backed_up,
             history_visibility,
             algorithm,
+            shared_history,
         } = pickle;
 
         let session: InnerSession = pickle.into();
@@ -362,6 +381,7 @@ impl InboundGroupSession {
             backed_up: AtomicBool::from(backed_up).into(),
             algorithm: algorithm.into(),
             imported,
+            shared_history,
         })
     }
 
@@ -511,6 +531,14 @@ impl InboundGroupSession {
     pub fn sender_data_type(&self) -> SenderDataType {
         self.sender_data.to_type()
     }
+
+    /// Whether this [`InboundGroupSession`] can be shared with newly joined
+    /// users, allowing access to history, as defined in [MSC3061].
+    ///
+    /// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+    pub fn shared_history(&self) -> bool {
+        self.shared_history
+    }
 }
 
 #[cfg(not(tarpaulin_include))]
@@ -556,6 +584,12 @@ pub struct PickledInboundGroupSession {
     /// The algorithm of this inbound group session.
     #[serde(default = "default_algorithm")]
     pub algorithm: EventEncryptionAlgorithm,
+    /// Whether this [`InboundGroupSession`] can be shared with newly joined
+    /// users, allowing access to history, as defined in [MSC3061].
+    ///
+    /// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+    #[serde(default)]
+    pub shared_history: bool,
 }
 
 fn default_algorithm() -> EventEncryptionAlgorithm {
@@ -574,6 +608,7 @@ impl TryFrom<&ExportedRoomKey> for InboundGroupSession {
             session_key,
             sender_claimed_keys,
             forwarding_curve25519_key_chain: _,
+            shared_history,
         } = key;
 
         let config = OutboundGroupSession::session_config(&key.algorithm)?;
@@ -596,6 +631,7 @@ impl TryFrom<&ExportedRoomKey> for InboundGroupSession {
             imported: true,
             algorithm: algorithm.to_owned().into(),
             backed_up: AtomicBool::from(false).into(),
+            shared_history: *shared_history,
         })
     }
 }
@@ -626,6 +662,7 @@ impl From<&ForwardedMegolmV1AesSha2Content> for InboundGroupSession {
             imported: true,
             algorithm: EventEncryptionAlgorithm::MegolmV1AesSha2.into(),
             backed_up: AtomicBool::from(false).into(),
+            shared_history: false,
         }
     }
 }
@@ -652,6 +689,7 @@ impl From<&ForwardedMegolmV2AesSha2Content> for InboundGroupSession {
             imported: true,
             algorithm: EventEncryptionAlgorithm::MegolmV1AesSha2.into(),
             backed_up: AtomicBool::from(false).into(),
+            shared_history: false,
         }
     }
 }
@@ -768,6 +806,7 @@ mod tests {
             SenderData::unknown(),
             EventEncryptionAlgorithm::MegolmV1AesSha2,
             Some(HistoryVisibility::Shared),
+            false,
         )
         .unwrap();
 
@@ -814,6 +853,7 @@ mod tests {
                 "room_id":"!test:localhost",
                 "imported":false,
                 "backed_up":false,
+                "shared_history":false,
                 "history_visibility":"shared",
                 "algorithm":"m.megolm.v1.aes-sha2"
             })

Diff for: crates/matrix-sdk-crypto/src/olm/group_sessions/mod.rs

@@ -98,6 +98,13 @@ pub struct ExportedRoomKey {
         serialize_with = "serialize_curve_key_vec"
     )]
     pub forwarding_curve25519_key_chain: Vec<Curve25519PublicKey>,
+
+    /// Whether this [`ExportedRoomKey`] can be shared with newly joined
+    /// users, allowing access to history, as defined in [MSC3061].
+    ///
+    /// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+    #[serde(default, rename = "org.matrix.msc3061.shared_history")]
+    pub shared_history: bool,
 }
 
 impl ExportedRoomKey {
@@ -115,6 +122,7 @@ impl ExportedRoomKey {
             session_key,
             sender_claimed_keys,
             forwarding_curve25519_key_chain,
+            shared_history,
         } = room_key;
 
         Self {
@@ -125,6 +133,7 @@ impl ExportedRoomKey {
             session_key,
             sender_claimed_keys,
             forwarding_curve25519_key_chain,
+            shared_history,
         }
     }
 }
@@ -152,13 +161,20 @@ pub struct BackedUpRoomKey {
     pub sender_claimed_keys: SigningKeys<DeviceKeyAlgorithm>,
 
     /// Chain of Curve25519 keys through which this session was forwarded, via
-    /// m.forwarded_room_key events.
+    /// `m.forwarded_room_key` events.
     #[serde(
         default,
         deserialize_with = "deserialize_curve_key_vec",
         serialize_with = "serialize_curve_key_vec"
     )]
     pub forwarding_curve25519_key_chain: Vec<Curve25519PublicKey>,
+
+    /// Whether this [`BackedUpRoomKey`] can be shared with newly joined
+    /// users, allowing access to history, as defined in [MSC3061].
+    ///
+    /// [MSC3061]: https://github.com/matrix-org/matrix-spec-proposals/pull/3061
+    #[serde(default, rename = "org.matrix.msc3061.shared_history")]
+    pub shared_history: bool,
 }
 
 impl TryFrom<ExportedRoomKey> for ForwardedRoomKeyContent {
@@ -228,6 +244,7 @@ impl From<ExportedRoomKey> for BackedUpRoomKey {
             session_key,
             sender_claimed_keys,
             forwarding_curve25519_key_chain,
+            shared_history,
         } = value;
 
         Self {
@@ -236,6 +253,7 @@ impl From<ExportedRoomKey> for BackedUpRoomKey {
             session_key,
             sender_claimed_keys,
             forwarding_curve25519_key_chain,
+            shared_history,
         }
     }
 }
@@ -261,6 +279,7 @@ impl TryFrom<ForwardedRoomKeyContent> for ExportedRoomKey {
                     sender_claimed_keys,
                     sender_key: content.claimed_sender_key,
                     session_key: content.session_key,
+                    shared_history: false,
                 })
             }
             #[cfg(feature = "experimental-algorithms")]
@@ -272,6 +291,7 @@ impl TryFrom<ForwardedRoomKeyContent> for ExportedRoomKey {
                 sender_claimed_keys: content.claimed_signing_keys,
                 sender_key: content.claimed_sender_key,
                 session_key: content.session_key,
+                shared_history: false,
             }),
             ForwardedRoomKeyContent::Unknown(c) => Err(SessionExportError::Algorithm(c.algorithm)),
         }

Diff for: crates/matrix-sdk-crypto/src/olm/group_sessions/outbound.rs

@@ -47,6 +47,7 @@ use super::SessionCreationError;
 #[cfg(feature = "experimental-algorithms")]
 use crate::types::events::room::encrypted::MegolmV2AesSha2Content;
 use crate::{
+    olm::account::shared_history_from_history_visibility,
     session_manager::CollectStrategy,
     store::caches::SequenceNumber,
     types::{
@@ -526,12 +527,15 @@ impl OutboundGroupSession {
 
     pub(crate) async fn as_content(&self) -> RoomKeyContent {
         let session_key = self.session_key().await;
+        let shared_history =
+            shared_history_from_history_visibility(&self.settings.history_visibility);
 
         RoomKeyContent::MegolmV1AesSha2(
             MegolmV1AesSha2RoomKeyContent::new(
                 self.room_id().to_owned(),
                 self.session_id().to_owned(),
                 session_key,
+                shared_history,
             )
             .into(),
         )

Diff for: crates/matrix-sdk-crypto/src/olm/group_sessions/sender_data_finder.rs

@@ -861,6 +861,7 @@ mod tests {
                 SenderData::unknown(),
                 EventEncryptionAlgorithm::MegolmV1AesSha2,
                 None,
+                false,
             )
             .unwrap();
             if options.session_is_imported {
@@ -1083,6 +1084,7 @@ mod tests {
                 room_id.to_owned(),
                 "mysession".to_owned(),
                 clone_session_key(session_key),
+                false,
             ))),
         )
     }