openssl Keys Generate RSA Key

Example

In order to generate an RSA key, an EVP_PKEY must first be allocated with EVP_PKEY_new:

EVP_PKEY *pkey;
pkey = EVP_PKEY_new();

An exponent for the key is also needed, which will require allocating a BIGNUM with BN_new and then assigning with BN_set_word:

BIGNUM *bn;
bn = BN_new();
BN_set_word(bn, RSA_F4);

To generate the key, create a new RSA with RSA_new and call RSA_generate_key_ex:

RSA *rsa;
rsa = RSA_new();
RSA_generate_key_ex(
    rsa,  /* pointer to the RSA structure */
    2048, /* number of bits for the key - 2048 is a good value */
    bn,   /* exponent allocated earlier */
    NULL, /* callback - can be NULL if progress isn't needed */
);

To assign the newly generated key to the EVP_PKEY structure, call EVP_PKEY_assign_RSA:

EVP_PKEY_assign_RSA(pkey, rsa);

The RSA structure will be automatically freed when the EVP_PKEY structure is freed. This is done with EVP_PKEY_free:

EVP_PKEY_free(pkey);