My question is related to:
Question: When we use a pseudorandom function to pad a message do we need to worry about the pseudorandom function's output range and the probability of collision?
Assume we blind (read: “add padding to”) a message $m$ as $r_1=m\cdot PRF(k,1)$, and next time we blind it as $r_2=m\cdot PRF(k',1)$, where $k,k'$ are two independent random keys. We give $r_1,r_2$ to an adversary.
If our PRF is not collision resistant then when $PRF(k,1)=PRF(k',1)$ then the adversary can learn the message $m$.