  This paper proves that several interactive proof systems are zero-knowledge
against quantum attacks. This includes a few well-known classical
zero-knowledge proof systems as well as quantum interactive proof systems for
the complexity class HVQSZK, which comprises all problems having "honest
verifier" quantum statistical zero-knowledge proofs. It is also proved that
zero-knowledge proofs for every language in NP exist that are secure against
quantum attacks, assuming the existence of quantum computationally concealing
commitment schemes. Previously no non-trivial proof systems were known to be
zero-knowledge against quantum attacks, except in restricted settings such as
the honest-verifier and common reference string models. This paper therefore
establishes for the first time that true zero-knowledge is indeed possible in
the presence of quantum information and computation.
