def _raw_bsdi_crypt(secret, rounds, salt):
    """pure-python backend for bsdi_crypt"""

    # decode salt
    salt_value = h64.decode_int24(salt)

    # gotta do something - no official policy since this predates unicode
    if isinstance(secret, unicode):
        secret = secret.encode("utf-8")
    assert isinstance(secret, bytes)

    # forbidding NULL char because underlying crypt() rejects them too.
    if _BNULL in secret:
        raise uh.exc.NullPasswordError(bsdi_crypt)

    # convert secret string into an integer
    key_value = _bsdi_secret_to_key(secret)

    # run data through des using input of 0
    result = des_encrypt_int_block(key_value, 0, salt_value, rounds)

    # run h64 encode on result
    return h64big.encode_int64(result)