From 4118003622f1a7219602fc3717425039491ed0bc Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 17 Oct 2024 22:17:22 +0100
Subject: [PATCH] crypto: Add more UtdCauses

---
 crates/matrix-sdk-crypto/src/error.rs         |  2 +
 .../src/types/events/utd_cause.rs             | 68 +++++++++++++------
 2 files changed, 51 insertions(+), 19 deletions(-)

diff --git a/crates/matrix-sdk-crypto/src/error.rs b/crates/matrix-sdk-crypto/src/error.rs
index 89fe5c5edaf..4400da9d696 100644
--- a/crates/matrix-sdk-crypto/src/error.rs
+++ b/crates/matrix-sdk-crypto/src/error.rs
@@ -120,6 +120,8 @@ pub enum MegolmError {
     /// An encrypted message wasn't decrypted, because the sender's
     /// cross-signing identity did not satisfy the requested
     /// [`crate::TrustRequirement`].
+    ///
+    /// The nested value is the sender's current verification level.
     #[error("decryption failed because trust requirement not satisfied: {0}")]
     SenderIdentityNotTrusted(VerificationLevel),
 }
diff --git a/crates/matrix-sdk-crypto/src/types/events/utd_cause.rs b/crates/matrix-sdk-crypto/src/types/events/utd_cause.rs
index ffb6813cfde..eef81bfcea5 100644
--- a/crates/matrix-sdk-crypto/src/types/events/utd_cause.rs
+++ b/crates/matrix-sdk-crypto/src/types/events/utd_cause.rs
@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use matrix_sdk_common::deserialized_responses::{
+    DeviceLinkProblem, UnableToDecryptInfo, UnableToDecryptReason, VerificationLevel,
+};
 use ruma::{events::AnySyncTimelineEvent, serde::Raw};
 use serde::Deserialize;
 
@@ -27,13 +30,24 @@ pub enum UtdCause {
     /// We are missing the keys for this event, and the event was sent when we
     /// were not a member of the room (or invited).
     SentBeforeWeJoined = 1,
-    //
-    // TODO: Other causes for UTDs. For example, this message is device-historical, information
-    // extracted from the WithheldCode in the MissingRoomKey object, or various types of Olm
-    // session problems.
-    //
-    // Note: This needs to be a simple enum so we can export it via FFI, so if more information
-    // needs to be provided, it should be through a separate type.
+
+    /// The message was sent by a user identity we have not verified, but the
+    /// user was previously verified.
+    VerificationViolation = 2,
+
+    /// The [`crate::TrustRequirement`] requires that the sending device be
+    /// signed by its owner, and it was not.
+    UnsignedDevice = 3,
+
+    /// The [`crate::TrustRequirement`] requires that the sending device be
+    /// signed by its owner, and we were unable to securely  find the device.
+    ///
+    /// This could be because the device has since been deleted, because we
+    /// haven't yet downloaded it from the server, or because the session
+    /// data was obtained from an insecure source (imported from a file,
+    /// obtained from a legacy (asymmetric) backup, unsafe key forward, etc.
+    /// )
+    UnknownDevice = 4,
 }
 
 /// MSC4115 membership info in the unsigned area.
@@ -59,21 +73,37 @@ impl UtdCause {
         unable_to_decrypt_info: &UnableToDecryptInfo,
     ) -> Self {
         // TODO: in future, use more information to give a richer answer. E.g.
-        // is this event device-historical? Was the Olm communication disrupted?
-        // Did the sender refuse to send the key because we're not verified?
-
-        // Look in the unsigned area for a `membership` field.
-        if let Some(raw_event) = raw_event {
-            if let Ok(Some(unsigned)) = raw_event.get_field::<UnsignedWithMembership>("unsigned") {
-                if let Membership::Leave = unsigned.membership {
-                    // We were not a member - this is the cause of the UTD
-                    return UtdCause::SentBeforeWeJoined;
+        match unable_to_decrypt_info.reason {
+            UnableToDecryptReason::MissingMegolmSession
+            | UnableToDecryptReason::UnknownMegolmMessageIndex => {
+                // Look in the unsigned area for a `membership` field.
+                if let Some(raw_event) = raw_event {
+                    if let Ok(Some(unsigned)) =
+                        raw_event.get_field::<UnsignedWithMembership>("unsigned")
+                    {
+                        if let Membership::Leave = unsigned.membership {
+                            // We were not a member - this is the cause of the UTD
+                            return UtdCause::SentBeforeWeJoined;
+                        }
+                    }
                 }
+                UtdCause::Unknown
+            }
+
+            UnableToDecryptReason::SenderIdentityNotTrusted(
+                VerificationLevel::VerificationViolation,
+            ) => UtdCause::VerificationViolation,
+
+            UnableToDecryptReason::SenderIdentityNotTrusted(VerificationLevel::UnsignedDevice) => {
+                UtdCause::UnsignedDevice
+            }
+
+            UnableToDecryptReason::SenderIdentityNotTrusted(VerificationLevel::None(_)) => {
+                UtdCause::UnknownDevice
             }
-        }
 
-        // We can't find an explanation for this UTD
-        UtdCause::Unknown
+            _ => UtdCause::Unknown,
+        }
     }
 }