# Tag Info

It looks fine; whether you use the secret $S_0, S_1$ as the HMAC key, or whether you use the random value $r$ as the HMAC key; if $t' = t$, it implies that either $S_0 = S_1$, or we found a collision in the underlying hash function. I would personally suggest you use $S_0, S_1$ as the key. With HMAC, it doesn't really matter; however if we extend this to ...