Disable Weak cipher check for DES KCV

[Jan Bilek]

Hi,

We have a problem here where I need to encrypt a block of data with zeros.

<>
  gcry_check_version (NULL);
  unsigned char key[] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
  unsigned char out[8];
  unsigned char data[8];
  gcry_error_t err = 0;
  gcry_cipher_hd_t hd = nullptr;
  err = gcry_cipher_open(&hd, GCRY_CIPHER_DES, GCRY_CIPHER_MODE_ECB, 0);
  //auto blklen = gcry_cipher_get_algo_blklen(GCRY_CIPHER_DES);
  //auto algolen = gcry_cipher_get_algo_keylen (GCRY_CIPHER_DES);
  err = gcry_cipher_setkey (hd, key, sizeof(key));
  std::cerr << "gpg_err_code: " << gpg_err_code(err) << std::endl;
  std::cerr << "gpg_strerror: " << gpg_strerror(err) << std::endl;
  gcry_cipher_encrypt(hd, out, sizeof(key), data, 8);
  if (err) {
    std::cerr << "Failed to perform cryptography" << std::endl;
    std::cerr << "  cipher:     " << static_cast<int>(GCRY_CIPHER_DES) << std::endl;
    std::cerr << "  mode:       " << static_cast<int>(GCRY_CIPHER_MODE_ECB) << std::endl;
    //std::cerr << "  keyBlock:   " << BinToHex<std::string>(key) << std::endl;
    //std::cerr << "  out:        " << BinToHex<std::string>(out) << std::endl;
    //std::cerr << "  data:       " << BinToHex<std::string>(encryptedData) << std::endl;
  }
</>

This blows on:

gpg_err_code: 43
gpg_strerror: Weak encryption key
cipher_encrypt: key not set

Tracked back t in a source to libcrypt / cipher / des.c

r. 1384 do_des_setkey
r. 1021 is_weak_key

  if (is_weak_key (key)) {
    _gcry_burn_stack (64);
    return GPG_ERR_WEAK_KEY;
  }

cipher.c
r.797

 rc = c->spec->setkey (&c->context.c, key, keylen, c);
  if (!rc) {

  } else
    c->marks.key = 0;

... then disallows weak key setting completely, resulting in a failure.

This has quite an impact on multiple (still) in-use KCV operations (e.g. KCV_METHOD_VISA) where key needs to be encrypted with a zero key to get its KCV.

May I propose a patch? (See in attachment).

Thanks & Cheers,
Jan
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