HMAC (SHA-256) implementation

hmac_sha256.py

# HMAC with SHA-256

def sha256(message):
    # SHA-256 implementation
    import hashlib
    return hashlib.sha256(message).digest()

def xor(a, b):
    # XOR two byte strings
    return bytes([x ^ y for x, y in zip(a, b)])

def hmac_sha256(key, message):
    """
    HMAC-SHA256 implementation

    >>> text = b"Hi There"
    >>> key = b'\x0b' * 20
    >>> a = hmac_sha256(key, text).hex()
    >>> import hmac
    >>> b = hmac.new(key, text.encode('ascii'), hashlib.sha256).digest().hex()
    >>> a == b
    True

    >>> text = b"Hello" * 100
    >>> key = b'1234567890' * 100
    >>> a = hmac_sha256(key, text).hex()
    >>> import hmac
    >>> b = hmac.new(key, text.encode('ascii'), hashlib.sha256).digest().hex()
    >>> a == b
    True

    :param key:
    :param message:
    :return:
    """

    if len(key) > 64:
        key = sha256(key)

    if len(key) < 64:
        key = key + b'\x00' * (64 - len(key))

    o_key_pad = xor(b'\x5c' * 64, key)
    i_key_pad = xor(b'\x36' * 64, key)

    return sha256(o_key_pad + sha256(i_key_pad + message))