client-api: Add support for fallback keys

This implements support for MSC2732[1], fallback keys. Only support to
upload and get notifications about fallback keys via `/sync` is
implemented.

[1]: matrix-org/matrix-spec-proposals#2732

crates/ruma-client-api/src/r0/keys/upload_keys.rs

@@ -29,6 +29,11 @@ ruma_api! {
         /// One-time public keys for "pre-key" messages.
         #[serde(skip_serializing_if = "Option::is_none")]
         pub one_time_keys: Option<BTreeMap<Box<DeviceKeyId>, Raw<OneTimeKey>>>,
+
+        /// Fallback public keys for "pre-key" messages.
+        #[cfg(feature = "unstable-pre-spec")]
+        #[serde(skip_serializing_if = "Option::is_none", rename = "org.matrix.msc2732.fallback_keys")]
+        pub fallback_keys: Option<BTreeMap<Box<DeviceKeyId>, Raw<OneTimeKey>>>,
     }
 
     response: {

crates/ruma-client-api/src/r0/sync/sync_events.rs

@@ -93,6 +93,15 @@ ruma_api! {
         /// currently held on the server for a device.
         #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
         pub device_one_time_keys_count: BTreeMap<DeviceKeyAlgorithm, UInt>,
+
+        /// For each key algorithm, the number of unclaimed one-time keys
+        /// currently held on the server for a device.
+        ///
+        /// The presence of this field indicates that the server supports
+        /// fallback keys.
+        #[cfg(feature = "unstable-pre-spec")]
+        #[serde(rename = "org.matrix.msc2732.device_unused_fallback_key_types")]
+        pub device_unused_fallback_key_types: Option<Vec<DeviceKeyAlgorithm>>,
     }
 
     error: crate::Error
@@ -116,6 +125,8 @@ impl Response {
             to_device: Default::default(),
             device_lists: Default::default(),
             device_one_time_keys_count: BTreeMap::new(),
+            #[cfg(feature = "unstable-pre-spec")]
+            device_unused_fallback_key_types: None,
         }
     }
 }

crates/ruma-common/src/encryption.rs

@@ -83,12 +83,33 @@ pub struct SignedKey {
 
     /// Signatures for the key object.
     pub signatures: SignedKeySignatures,
+
+    /// Is this key considered to be a fallback key, defaults to false.
+    #[cfg(feature = "unstable-pre-spec")]
+    #[serde(default, skip_serializing_if = "is_false")]
+    pub fallback: bool,
+}
+
+#[cfg(feature = "unstable-pre-spec")]
+fn is_false(value: &bool) -> bool {
+    *value == false
 }
 
 impl SignedKey {
     /// Creates a new `SignedKey` with the given key and signatures.
     pub fn new(key: String, signatures: SignedKeySignatures) -> Self {
-        Self { key, signatures }
+        Self {
+            key,
+            signatures,
+            #[cfg(feature = "unstable-pre-spec")]
+            fallback: false,
+        }
+    }
+
+    /// Creates a new fallback `SignedKey` with the given key and signatures.
+    #[cfg(feature = "unstable-pre-spec")]
+    pub fn new_fallback(key: String, signatures: SignedKeySignatures) -> Self {
+        Self { key, signatures, fallback: true }
     }
 }