Today I moved my web application to .net 4.0 and Forms Auth just stopped working. After several hours of digging into my SqlMembershipProvider (simplified version of built-in SqlMembershipProvider), I found that HMACSHA256 hash is not consistent. This is the encryption method:

internal string EncodePassword(string pass, int passwordFormat, string salt)
{
    if (passwordFormat == 0) // MembershipPasswordFormat.Clear
        return pass;

    byte[] bIn = Encoding.Unicode.GetBytes(pass);
    byte[] bSalt = Convert.FromBase64String(salt);
    byte[] bAll = new byte[bSalt.Length + bIn.Length];
    byte[] bRet = null;

    Buffer.BlockCopy(bSalt, 0, bAll, 0, bSalt.Length);
    Buffer.BlockCopy(bIn, 0, bAll, bSalt.Length, bIn.Length);
    if (passwordFormat == 1)
    { // MembershipPasswordFormat.Hashed
        HashAlgorithm s = HashAlgorithm.Create( Membership.HashAlgorithmType );
        bRet = s.ComputeHash(bAll);
    } else
    {
        bRet = EncryptPassword( bAll );
    }

    return Convert.ToBase64String(bRet);
}

Passing the same password and salt twice returns different results!!! It was working perfectly in .NET 3.5

Anyone aware of any breaking changes, or is it a known bug?

UPDATE: When I specify SHA512 as hashing algorithm, everything works fine, so I do believe it's a bug in .NET 4.0 crypto

Thanks! Andrey