Re: EVP and Elliptic curve

[Matt Caswell]

> On Thu, Mar 14, 2013, Leon Brits wrote:
>
> > Hi List,
> >
> > I just want to verify: Elliptic curve functions are not encapsulated by
> the
> > EVP functions - correct?  If so, what is the
> > EVP_PKEY_CTX_set_ec_paramgen_curve_nid function then used for?  If NOT
> so,
> > then please help with an example since I could only find the normal
> > EC_{KEY,GROUP}* type of example code?
> >
> > I am required to perform ECDSA and was hoping I could use EVP which is
> now
> > working for DSA and RSA (sans the padding problem).
> >
>

As Steve has said, yes you can use ECDSA using EVP. The
EVP_PKEY_CTX_set_ec_paramgen_curve_nid is used during parameter generation.
Here is some sample code that uses it. Once you have an EVP_PKEY set up
with a type of EVP_PKEY_EC, then the code to do signatures should be pretty
much identical for DSA and ECDSA. This code generates a new key but you can
equally set it directly using EVP_PKEY_set1_EC_KEY. See the man page for
further details:

http://www.openssl.org/docs/crypto/EVP_PKEY_set1_RSA.html



EVP_PKEY *generate_key(int type)
{
    EVP_PKEY_CTX *pctx = NULL, *kctx = NULL;
    EVP_PKEY *params = NULL, *key = NULL;

    /* Check whether we need to generate parameters first */
    if(type == EVP_PKEY_EC || type == EVP_PKEY_DSA || type == EVP_PKEY_DH)
    {
        /* 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:
            if(!EVP_PKEY_CTX_set_ec_paramgen_curve_nid(pctx,
NID_X9_62_prime256v1)) goto err;
            break;
        case EVP_PKEY_DSA:
            if(!EVP_PKEY_CTX_set_dsa_paramgen_bits(pctx, 2048)) goto
err;
            break;
        case EVP_PKEY_DH:
            if(!EVP_PKEY_CTX_set_dh_paramgen_prime_len(pctx, 2048)) goto
err;
        }


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

        /* Create context for the key generation */
        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;
    }

    /* Set the keygen parameters according to the type */
    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(type == EVP_PKEY_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;

        if (EVP_PKEY_CTX_ctrl(kctx, -1, EVP_PKEY_OP_KEYGEN,
            EVP_PKEY_CTRL_SET_MAC_KEY,
            /*key length*/32, "01234567890123456789012345678901") <= 0)
            goto err;
    }

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

   goto end;
err:

  ERR_print_errors_fp(stderr);

end:

    if(pctx) EVP_PKEY_CTX_free(pctx);
    if(params) EVP_PKEY_free(params);
    if(kctx) EVP_PKEY_CTX_free(kctx);

    return key;
}


Regards

Matt