[go-nuts] [ANN] An implementation of SRP

Hi,

srp.go:322+

func (ss *ServerSession) VerifyClientAuthenticator(cauth []byte) bool {
  M := computeClientAutneticator(ss.SRP.HashFunc(), ss.SRP.Group, ss.username, ss.salt, ss._A.Bytes(), ss._B.Bytes(), ss.key)
  return subtle.ConstantTimeCompare(M, cauth) == 0
}

If the intent is to return true if M equals cauth, the above code is
wrong. I guess you want to use '== 1' (see
http://golang.org/pkg/crypto/subtle/#ConstantTimeCompare) here, but then
all the tests fail.

Same comment applies to ClientSession.VerifyServerAuthenticator.

Thanks for pointing that out. For some reason I thought ConstantTimeCompare
returned -1, 0, or 1. And now I can't figure out why I thought that.

I've fixed the errors in the implementation and made an important
modification to the API.
It is not possible to specify a separate key derivation function. This was
needed in order to be able to use key derivation functions like pbkdf2 or
scrypt.
I've added srp/pbkdf2 anbd srp/scrypt which each contain a convenience
function that returns a key derivation function in the form expected by the
srp package.