# Zero Knowledge Proofs in BPP

I understand that there are non-interactive (static) ZK proofs for every language in BPP, but I am not sure I fully understand this.

Can someone give an example of a Zero Knowledge Proof in BPP?

A zero-knowledge proof for a language in BPP is trivial: a statement in BPP can be verified in probabilistic polynomial time (by definition). So, for any language $L$ in BPP, and any $x \in L$, here is a trivial proof that $x\in L$: the prover sends nothing, and the verifier checks whether $x\in L$ (this takes polynomial time) and output yes iff this holds. Correctness is trivial, soundness too (the prover cannot cheat as he does not send anything at all), and zero-knowledge as well (it requires simulating an empty transcript...).
Here is an example: consider the language of prime numbers. Given a number $n$, the prover wants to convince the verifier that $n$ (which they both know) is prime. To do so, he comfortably stays at home and watches a movie, while the verifier checks by herself that $n$ is prime, using any polytime primality test of her choice. When she's done, she can just tell the prover "ok, you convinced me".