import hashlib


def _h_value(data):
    """H function of the DIGEST-MD5 algorithm (MD5 sum).

    :Parameters:
        - `data`: a byte string.
    :Types:
        - `data`: `bytes`

    :return: MD5 sum of the string.
    :returntype: `bytes`"""
    # pylint: disable-msg=E1101
    return hashlib.md5(data).digest()