def deserialize_tag(stream, header, verifier=None):
    """Deserialize the Tag value from a non-framed stream.

    :param stream: Source data stream
    :type stream: io.BytesIO
    :param header: Deserialized header
    :type header: aws_encryption_sdk.structures.MessageHeader
    :param verifier: Signature verifier object (optional)
    :type verifier: aws_encryption_sdk.internal.crypto.Verifier
    :returns: Tag value for body
    :rtype: bytes
    """
    (data_tag,) = unpack_values(
        format_string=">{auth_len}s".format(auth_len=header.algorithm.auth_len), stream=stream, verifier=verifier
    )
    return data_tag