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+# MSC2732: Olm fallback keys
+
+Olm uses a set of one-time keys when initializing a session between two
+devices: Alice uploads one-time keys to her homeserver, and Bob claims one of
+them to perform a Diffie-Hellman to generate a shared key.  As implied by the
+name, a one-time key is only to be used once.  However, if all of Alice's
+one-time keys are claimed, Bob will not be able to create a session with Alice.
+
+This can be addressed by Alice uploading a fallback key that is used in place
+of a one-time key when no one-time keys are available.
+
+## Proposal
+
+A new request parameter, `fallback_keys`, is added to the body of the
+[`/keys/upload` client-server API](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-keys-upload), which is in the same format as the
+`one_time_keys` parameter with the exception that there must be at most one key
+per key algorithm.  If the user had previously uploaded a fallback key for a
+given algorithm, it is replaced -- the server will only keep one fallback key
+per algorithm for each user.
+
+When uploading fallback keys for algorithms whose key format is a signed JSON
+object, client should include a property named `fallback` with a value of
+`true`.
+
+Example:
+
+`POST /keys/upload`
+
+```json
+{
+  "fallback_keys": {
+    "signed_curve25519:AAAAAA": {
+      "key": "base64+public+key",
+      "fallback": true,
+      "signatures": {
+        "@alice:example.org": {
+          "ed25519:DEVICEID": "base64+signature"
+        }
+      }
+    }
+  }
+}
+```
+
+When Bob calls `/keys/claim` to claim one of Alice's one-time keys, but Alice
+has no one-time keys left, the homeserver will return the fallback key instead,
+if Alice had previously uploaded one.  Unlike with one-time keys, fallback keys
+are not deleted when they are returned by `/keys/claim`.  However, the server
+marks that they have been used.
+
+A new response parameter, `device_unused_fallback_key_types`, is added to
+`/sync`.  This is an array listing the key algorithms for which the server has
+an unused fallback key for the device.  If the client wants the server to have a
+fallback key for a given key algorithm, but that algorithm is not listed in
+`device_unused_fallback_key_types`, the client will upload a new key as above.
+
+The `device_unused_fallback_key_types` parameter must be present if the server
+supports fallback keys.  Clients can thus treat this field as an indication
+that the server supports fallback keys, and so only upload fallback keys to
+servers that support them.
+
+Example:
+
+`GET /sync`
+
+Response:
+
+```jsonc
+{
+  // other fields...
+  "device_unused_fallback_key_types": ["signed_curve25519"]
+}
+```
+
+## Security considerations
+
+Using a fallback key rather than a one-time key has security implications.  An
+attacker can replay a message that was originally sent with a fallback key, and
+the receiving client will accept it as a new message if the fallback key is
+still active.  Also, an attacker that compromises a client may be able to
+retrieve the private part of the fallback key to decrypt past messages if the
+client has still retained the private part of the fallback key.
+
+For this reason, clients should not store the private part of the fallback key
+indefinitely.  For example, client should only store at most two fallback keys:
+the current fallback key (that it has not yet received any messages for) and
+the previous fallback key, and should remove the previous fallback key once it
+is reasonably certain that it has received all the messages that use it (for
+example, one hour after receiving the first message that used it).
+
+For addressing replay attacks, clients can also keep track of inbound sessions
+to detect replays.
+
+## Unstable prefix
+
+The `fallback_keys` request parameter and the `device_unused_fallback_key_types`
+response parameter will be prefixed by `org.matrix.msc2732.`.