A system and method is disclosed that provides lightweight non-repudiability for networked computer systems. Each party to a two-party communication maintains hashes on its incoming and outgoing messages. At its discretion, either party can request that the other party commit to the conversation. The second party (if it agrees) then sends signed hashes that third parties can use to verify the content of the conversation. The party requesting the commitment stores its corresponding hashes when it sends the request. If the hashes from both parties are the same for the same positions in their conversation, the two parties can verify that their conversation is error-free. If the sending party also maintains logs of both sides (incoming and outgoing) of the conversation and stores hashes corresponding to the beginning of the logs, the sending party is also able to verify to a third party that the logged portion of the conversation was between the first party and the second party. Non-repudiability for entire conversations consisting of millions of messages can therefore be provided using a single pair of commit message and commitment/signature messages.