# 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 ?

• @poncho Poly1305 takes a 256-bit key consisting of two 128-bit partitions (r,s). The public nonce n is not part of the core Poly1305 algorithm. However, Poly1305 has strict requirements for s: The pair (r,s) should be unique, and MUST be unpredictable for each invocation (that is why it was originally obtained by encrypting a nonce), while "r" MAY be constant. In Poly1305-AES, s is derived from from a public nonce n and a 128-bit secret key k as s = AES(k,n). This was simply chosen as a practical way to exchange a suitable (secret, unique and unpredictable) s. – Lukas Feb 27 '20 at 23:49