OpenSSL::PKey::PKey#derive

static VALUE
ossl_pkey_derive(int argc, VALUE *argv, VALUE self)
{
    EVP_PKEY *pkey, *peer_pkey;
    EVP_PKEY_CTX *ctx;
    VALUE peer_pkey_obj, str;
    size_t keylen;
    int state;

    GetPKey(self, pkey);
    rb_scan_args(argc, argv, "1", &peer_pkey_obj);
    GetPKey(peer_pkey_obj, peer_pkey);

    ctx = EVP_PKEY_CTX_new(pkey, /* engine */NULL);
    if (!ctx)
        ossl_raise(ePKeyError, "EVP_PKEY_CTX_new");
    if (EVP_PKEY_derive_init(ctx) <= 0) {
        EVP_PKEY_CTX_free(ctx);
        ossl_raise(ePKeyError, "EVP_PKEY_derive_init");
    }
    if (EVP_PKEY_derive_set_peer(ctx, peer_pkey) <= 0) {
        EVP_PKEY_CTX_free(ctx);
        ossl_raise(ePKeyError, "EVP_PKEY_derive_set_peer");
    }
    if (EVP_PKEY_derive(ctx, NULL, &keylen) <= 0) {
        EVP_PKEY_CTX_free(ctx);
        ossl_raise(ePKeyError, "EVP_PKEY_derive");
    }
    if (keylen > RB_LEN_MAX) {
        EVP_PKEY_CTX_free(ctx);
        rb_raise(ePKeyError, "derived key would be too large");
    }
    str = ossl_str_new(NULL, (rb_len_t)keylen, &state);
    if (state) {
        EVP_PKEY_CTX_free(ctx);
        rb_jump_tag(state);
    }
    if (EVP_PKEY_derive(ctx, (unsigned char *)RSTRING_PTR(str), &keylen) <= 0) {
        EVP_PKEY_CTX_free(ctx);
        ossl_raise(ePKeyError, "EVP_PKEY_derive");
    }
    EVP_PKEY_CTX_free(ctx);
    rb_str_set_len(str, keylen);
    return str;
}