From 8641ef299e74f7535fe31add91efe7bb56c21f2b Mon Sep 17 00:00:00 2001 From: Mark Haines Date: Fri, 21 Oct 2016 15:25:33 +0100 Subject: [PATCH] Document the requirement that clients track the message_index --- supporting-docs/guides/2016-10-18-e2e_implementation.rst | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/supporting-docs/guides/2016-10-18-e2e_implementation.rst b/supporting-docs/guides/2016-10-18-e2e_implementation.rst index 30876df9..164888ab 100644 --- a/supporting-docs/guides/2016-10-18-e2e_implementation.rst +++ b/supporting-docs/guides/2016-10-18-e2e_implementation.rst @@ -326,6 +326,11 @@ Encrypted events using this algorithm should have ``sender_key``, Megolm session (see `below`__), the ciphertext can be decrypted by passing the ciphertext into ``olm_group_decrypt``. +In order to avoid replay attacks a client should remember the megolm +``message_index`` of each event they decrypt for each session. If the client +decrypts an event with the same ``message_index`` as one that it has already +decrypted using that session then it should fail decryption. + __ `m.room_key`_ The client should check that the sender's fingerprint key matches the