Tech pages/OMEMO

XEP-0384: OMEMO Encryption is an end-to-end encryption mechanism for use with XMPP. It is based around the Double Ratchet Algorithm as known from the messaging application Signal.

More background information can be found on Wikipedia.

Mind the Namespace

There are different, incompatible versions of OMEMO existing in practice, which can be seen in the XEP's revision history.

"siacs" OMEMO

Protocol specification versions 0.2.0 to 0.3.0 are identified by the namespace of eu.siacs.conversations.axolotl.

Those are the versions with (currently) the greatest adoption in the ecosystem. If you need to have an implementation with compatibility to the status quo, you should implement this version.

Note however, that you might end up needing to rework your implementation if the ecosystem moves to the next iteration.

"OMEMO:1"

The specification versions 0.4 up to the 0.7.0 are identified by the namespace of urn:xmpp:omemo:1 and describe an improved, yet sparsely adopted version of the protocol. If you intend to implement a future proof encryption protocol, you should go with "OMEMO:1".

"OMEMO:2"

The specification versions 0.8 up to the 0.8.3 are identified by the namespace of urn:xmpp:omemo:2 Informally it's also called Twomemo or OMEMO 2.

"OMEMO:0"

Identified by the namespace urn:xmpp:omemo:0, specification versions 0.0.2 and 0.1.0 describe a protocol which has no known implementations. You should not implement this version.

Criticism

• The use of the term "Identity Key" is unfortunate. Better suited would be "device key", as it does not make sense that a user has multiple identities.

Further resources

• Siacs OMEMO Quickstart
• Using PEP with a different access model
• Usage in MUCs
• Here a third party audit report on Omemo v. 0.3.0