EVP Key and Parameter Generation

The EVP functions support the ability to generate parameters and keys if required for EVP_PKEY objects. Since these functions use random numbers you should ensure that the random number generator is appropriately seeded as discussed here.

Parameter Generation

Parameter generation is supported for the following EVP_PKEY types only:

• EVP_PKEY_EC (for ECDSA and ECDH keys)
• EVP_PKEY_DSA
• EVP_PKEY_DH

The following sample code shows an example of how to generate parameters for each of these key types:

/* Create the context for generating the parameters */
if(!(pctx = EVP_PKEY_CTX_new_id(type, NULL))) goto err;
if(!EVP_PKEY_paramgen_init(pctx)) goto err;

/* Set the paramgen parameters according to the type */
switch(type)
{
case EVP_PKEY_EC:
  /* Use the NID_X9_62_prime256v1 named curve - defined in obj_mac.h */
  if(!EVP_PKEY_CTX_set_ec_paramgen_curve_nid(pctx, NID_X9_62_prime256v1)) goto err;		
  break;

case EVP_PKEY_DSA:
  /* Set a bit length of 2048 */
  if(!EVP_PKEY_CTX_set_dsa_paramgen_bits(pctx, 2048)) goto err;		
  break;

case EVP_PKEY_DH:
  /* Set a prime length of 2048 */
  if(!EVP_PKEY_CTX_set_dh_paramgen_prime_len(pctx, 2048)) goto err;
}

/* Generate parameters */
if (!EVP_PKEY_paramgen(pctx, &params)) goto err;

Key Generation

The following sample code shows an example of how to generate keys with the exception of EVP_PKEY_HMAC and EVP_PKEY_CMAC keys:

if(*params != NULL)
{
  if(!(kctx = EVP_PKEY_CTX_new(params, NULL))) goto err; 
}
else
{
  /* Create context for the key generation */
  if(!(kctx = EVP_PKEY_CTX_new_id(type, NULL))) goto err;
}

/* RSA keys set the key length during key generation rather than parameter generation! */
if(type == EVP_PKEY_RSA)
{
  if(!EVP_PKEY_CTX_set_rsa_keygen_bits(kctx, 2048)) goto err;
}

/* Generate the key */

if(!EVP_PKEY_keygen_init(kctx)) goto err; 
if (!EVP_PKEY_keygen(kctx, &key)) goto err;

CMAC keys are generated in a simlar fashion:

if(!(kctx = EVP_PKEY_CTX_new_id(type, NULL))) goto err;


if(!EVP_PKEY_keygen_init(kctx)) goto err;

/* Set the cipher to be used for the CMAC */
if (EVP_PKEY_CTX_ctrl(kctx, -1, EVP_PKEY_OP_KEYGEN,
  EVP_PKEY_CTRL_CIPHER,
  0, (void *)EVP_aes_256_ecb()) <= 0)
  goto err;

/* Set the key data to be used for the CMAC */
if (EVP_PKEY_CTX_ctrl(kctx, -1, EVP_PKEY_OP_KEYGEN,
  EVP_PKEY_CTRL_SET_MAC_KEY,
  /*key length*/32, "01234567890123456789012345678901") <= 0)
  goto err;

/* Generate the key */ 
if (!EVP_PKEY_keygen(kctx, &key)) goto err;

HMAC keys can be generated in the same way as for CMAC keys but do not take a cipher. A convenience function which wraps this process exists to simplify HMAC key generation:

key = EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, "password", strlen("password"));

See also

• EVP
• Libcrypto API