Disable Weak cipher check for DES KCV
Jan Bilek
jan.bilek at eftlab.com.au
Sat Jan 4 11:33:36 CET 2020
Ping?
On 2019-12-21 11:40:06+10:00 Jan Bilek wrote:
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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