Some in-transit encrypted messaging suites, such as off-the-Record Messaging, offer deniable authentication which gives the participants plausible deniability of their conversations.
This is achieved by the fact that all information necessary to forge messages is appended to the encrypted messages - if an adversary is able to create digitally authentic messages in a conversation (see hash-based message authentication code (HMAC)), he is also able to forge messages in the conversation.