Providing a secure and efficient communication system under network disruption without a trusted third party remains a challenging issue. To develop a secure and efficient system in such situations, we extend the conventional Off-The-Record (OTR) protocol into a new protocol named offline OTR (O 2 TR). O 2 TR provides end-to-end security between users without requiring the assumption that they are persistently connected to each other. To show the feasibility of the proposed protocol, we implemented a prototype to support O 2 TR based on the Gajim XMMP instant messaging platform. Our experiments showed that O 2 TR can
be used reliably even when the corresponding network party is temporarily broken down. Moreover, O 2 TR provides an efficient way to resume private sessions which is about 34% faster than the original OTR. We also proved the secrecy of O 2 TR using an automated verification tool called AVISPA.