crypto: Add OlmMachine::device_creation_time

Turns out this is useful for element-hq/element-meta#2313.

crates/matrix-sdk-crypto/CHANGELOG.md

@@ -19,6 +19,9 @@ Breaking changes:
 
 Additions:
 
+- Expose new method `OlmMachine::device_creation_time`.
+  ([#3275](https://github.com/matrix-org/matrix-rust-sdk/pull/3275))
+
 - Log more details about the Olm session after encryption and decryption.
   ([#3242](https://github.com/matrix-org/matrix-rust-sdk/pull/3242))
 

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

@@ -40,8 +40,8 @@ use ruma::{
         AnyToDeviceEvent, MessageLikeEventContent,
     },
     serde::Raw,
-    DeviceId, DeviceKeyAlgorithm, OwnedDeviceId, OwnedDeviceKeyId, OwnedTransactionId, OwnedUserId,
-    RoomId, TransactionId, UInt, UserId,
+    DeviceId, DeviceKeyAlgorithm, MilliSecondsSinceUnixEpoch, OwnedDeviceId, OwnedDeviceKeyId,
+    OwnedTransactionId, OwnedUserId, RoomId, TransactionId, UInt, UserId,
 };
 use serde_json::value::to_raw_value;
 use tokio::sync::Mutex;
@@ -346,6 +346,16 @@ impl OlmMachine {
         &self.inner.device_id
     }
 
+    /// The time at which the `Account` backing this `OlmMachine` was created.
+    ///
+    /// An [`Account`] is created when an `OlmMachine` is first instantiated
+    /// against a given [`Store`], at which point it creates identity keys etc.
+    /// This method returns the timestamp, according to the local clock, at
+    /// which that happened.
+    pub fn device_creation_time(&self) -> MilliSecondsSinceUnixEpoch {
+        self.inner.store.static_account().creation_local_time()
+    }
+
     /// Get the public parts of our Olm identity keys.
     pub fn identity_keys(&self) -> IdentityKeys {
         let account = self.inner.store.static_account();
@@ -2410,8 +2420,13 @@ pub(crate) mod tests {
 
     #[async_test]
     async fn test_create_olm_machine() {
+        let test_start_ts = MilliSecondsSinceUnixEpoch::now();
         let machine = OlmMachine::new(user_id(), alice_device_id()).await;
 
+        let device_creation_time = machine.device_creation_time();
+        assert!(device_creation_time <= MilliSecondsSinceUnixEpoch::now());
+        assert!(device_creation_time >= test_start_ts);
+
         let cache = machine.store().cache().await.unwrap();
         let account = cache.account().await.unwrap();
         assert!(!account.shared());