import re


def validate_sha1_checksum(sha1_checksum):
    """
    Validates that string is truly a SHA1.
    :param sha1_checksum: str
    :return: dict with keys "success" (boolean) and "message" (str)
    """
    retVal = {'success': True, 'message': ''}

    if re.match("^[a-fA-F0-9]{40}$", sha1_checksum) == None:
        retVal['message'] += "The SHA1 digest needs to be 40 hex characters."
        retVal['success'] = False

    return retVal