Using Poly1305 MAC key multiple times I know that for Poly1305-ChaCha20 reusing the key does reveal information about the plaintext. How it is for the Poly1305 MAC algorithm? Revealing information about the plaintext is not a problem at all, as it is available in plaintext anyways.

Question: Does a reuse of the Mac key reveal information about the key itself ?

Source: https://tools.ietf.org/html/rfc8103
Does a reuse of the Mac key reveal information about the key itself ?


One would certainly hope not - Message Authenticate Codes are supposed to be able to be safely used with the same key multiple times.

There is one caveat - the Poly1305 algorithm does need a nonce, which must be different whenever you MAC a message (specifically, you cannot reuse the same (key, nonce) pair to MAC two different messages) - if you do, you do leak enough information for someone to generate a forgery.