Disable Weak cipher check for DES KCV

[Jussi Kivilinna]

Hello,

On 21.12.2019 3.40, 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.

I tried to find KCV specification where zero key is used to encrypt actual key as input block for KCV value, but all KCV algorithms I managed to find encrypt zero input block with the actual key as key. Can you check your documentation for KCV if zero key is really used and give pointer/link to that spec for us?

-Jussi

> 
> May I propose a patch? (See in attachment).
> 
> Thanks & Cheers,
> Jan
> 
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