def scrypt(password, salt, N=SCRYPT_N, r=SCRYPT_r, p=SCRYPT_p, olen=64):
    """Returns a key derived using the scrypt key-derivarion function

    N must be a power of two larger than 1 but no larger than 2 ** 63 (insane)
    r and p must be positive numbers such that r * p < 2 ** 30

    The default values are:
    N -- 2**14 (~16k)
    r -- 8
    p -- 1

    Memory usage is proportional to N*r. Defaults require about 16 MiB.
    Time taken is proportional to N*p. Defaults take <100ms of a recent x86.

    The last one differs from libscrypt defaults, but matches the 'interactive'
    work factor from the original paper. For long term storage where runtime of
    key derivation is not a problem, you could use 16 as in libscrypt or better
    yet increase N if memory is plentiful.
    """

    check_args(password, salt, N, r, p, olen)

    # Everything is lists of 32-bit uints for all but pbkdf2
    try:
        B  = _pbkdf2('sha256', password, salt, 1, p * 128 * r)
        B  = list(struct.unpack('<%dI' % (len(B) // 4), B))
        XY = [0] * (64 * r)
        V  = [0] * (32 * r * N)
    except (MemoryError, OverflowError):
        raise ValueError("scrypt parameters don't fit in memory")

    for i in xrange(p):
        smix(B, i * 32 * r, r, N, V, XY)

    B = struct.pack('<%dI' % len(B), *B)
    return _pbkdf2('sha256', password, B, 1, olen)