This was a question on an exam: Two persons are using a one way communication ch

Question

This was a question on an exam:

Two persons are using a one way communication channel and the HMAC functionality (concretely HMAC-SHA1). Choose the correct statement below:

1) We can prove who the sender is, because HMAC-SHA1 uses a shared secret, which is known to both of them. Even more, the communication channel only works in one way.

2) We can prove who the sender is, because HMAC-SHA1 uses a shared secret, which is known to both of them.

3) The sender can deny that he sent the messagge, because confidentiality wasn't provided.

4) Nothing of the above.

The questions on this exam are known to be tricky. I'm thinking answer 3 or 4 is correct ( you need a digital signature to provide non-repudiation and confidentiality has nothing to do with non-repudiation), but am leaning towards the fourth answer.

Which one would you choose?

Explanation / Answer

I would say 2.

The reason: Theres 2 types of non-repudiation: One that you can prove to others, and one that you cannot.

There is schemes out there that are specifically made to prevent the possibility to prove to someone else that X sent it. Its called non-transferable PGP signatures, and goes on this way:

You hash the message with a MAC, using a random k. Then you sign the random k with your private key, and the encrypt it with the recipients public key.

This means the receiver are able to prove (to himself) that the sender sent the message since the receiver know he didnt write the message itself, preventing man-in-the-middle forgery. But since also the receiver knows the MAC key, and he must also reveal the MAC key to the verifier, he cannot prove to others what the sender did send, because he could aswell create that message itself.

In this case, both parties already have a shared key, so a key Exchange using a signed, encrypted random k is not neccessary. You could aswell run with HMAC directly on the shared key. But the same principle applies, that the receiver can prove to himself that sender sent the message, gaining non-repudiation this way, but he cannot prove to others that he sent the message.

But the mentioned MAC scheme does not gain non-repudiation to others, you cannot prove to other people that somone sent something.

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