# Probabilistic polynomial time algorithms in MAC's

According to the Wiki article, a MAC is a triple of probabilistic polynomial time algorithms $(G, S, V)$ such that:

• G gives the key k on input 1^n, where n is the security parameter
• S outputs a tag on the key k and the input string x
• V outputs 'accepted' or 'rejected' on inputs: the key k, the string x and the tag t.

From my understanding, a probabilistic polynomial time algorithm is one that runs in polynomial time, and returns a probability.

I don't really see how the outputs of G, S and V count as 'probabilities'. What are they measuring the probability of?