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At the 2004 Workshop on Privacy in the Electronic Society (WPES), Borisov, Goldberg and Brewer, presented "Off the Record Messaging" (OTR), a protocol designed to add end-to-end security and privacy to Instant Messaging protocols. An open-source implementation of OTR is available and has achieved considerable success.In this paper we present a security analysis of OTR showing that, while the overall concept of the system is valid and attractive, the protocol suffers from security shortcomings due to the use of an insecure key-exchange protocol and other problematic design choices.On the basis of these findings, we propose alternative designs and improvements that strengthen the security of the system and provide the originally intended features of the protocol, including deniability, in a sound and well-defined sense.