Non–repudiable Authentication is a concept in which it can be verified that the sender and the receiver were, in fact, the two parties involved to send and receive the message respectively or we can say that Non repudiable authentication tries to prevent false denial of either of the two parties after the contract has been established [1]. Non Repudiation with proof of origin proves to the recipient that the claimed sender of the message is indeed the real sender. Non repudiation with proof of receipt proves to the sender that the message has been delivered to the intended recipient [2].
It seems non-repudiation is desirable but it’s not the case always. Sometimes people want plausible deniability. Consider an example where Alice is President and gives the orders to her underlings for some secret scheme (example: Assassinate Fidel Castro). The underlings must be absolutely certain that the orders came from the president, so it is necessary to have source authentication of the message, but Alice certainly wants plausible deniability so that if her underlings are caught or killed or the plan leaks out in public, president can disavow any knowledge of their actions [3].
Alice and Bob can choose to use short lived keys (Using timestamp) for non-repudiable authentication. Here when Alice sends a message to Bob, she must be able to prove not only that she was indeed the sender but also the time when the message was sent and once Bob knows all of the above, Alice can never deny having sent the message. Suppose Alice and Bob takes help of Tim the timestamper to thwart the villainous Eve’s attempt to interfere with their communication. Alice creates a message and digitally signs it using her private key and sends it to the Tim the timestamper. Tim just verifies that the Alice’s keys are still valid, and then adds a timestamp to the message, signs it and sends it back to Alice. Now Alice sends this letter to Bob. Now Bob uses Alice’s public key to check the signature on the message, to verify its integrity. Bob can also be sure that it was Alice who signed the message because the messaged is signed and dated by trustworthy Tim (Which means Alice’s keys were valid when Tim signed it). Since Bob knows that the Alice signed the message with her key and this key was neither expired nor revoked at the time she signed it, Alice can not deny having signed it [4].
References.
1. http://en.wikipedia.org/wiki/Non-repudiation
2. http://www.itsecurity.com/security.htm?s=538
3. Kaufman .C., Perlman . P., and Speciner. M., “ Network Security : Private
communication in a public world”
4. http://www.carillon.ca/library/nonrepudiation_1.2.pdf
