static int ubsec_rsa_mod_exp(BIGNUM *r0, const BIGNUM *I, RSA *rsa)
	{
	BN_CTX *ctx;
	int to_return = 0;

	if((ctx = BN_CTX_new()) == NULL)
		goto err;

	if(!rsa->p || !rsa->q || !rsa->dmp1 || !rsa->dmq1 || !rsa->iqmp)
		{
		UBSECerr(UBSEC_F_UBSEC_RSA_MOD_EXP, UBSEC_R_MISSING_KEY_COMPONENTS);
		goto err;
		}

	/*
	 * Do in software if argument is too large for hardware.
	 */
	if ((BN_num_bits(rsa->p)+BN_num_bits(rsa->q)) > 1024) {
		const RSA_METHOD *meth = RSA_PKCS1_SSLeay();
		to_return = (*meth->rsa_mod_exp)(r0, I, rsa);
	} else {
		to_return = ubsec_mod_exp_crt(r0, I, rsa->p, rsa->q, rsa->dmp1,
			rsa->dmq1, rsa->iqmp, ctx);
	}
err:
	if(ctx)
		BN_CTX_free(ctx);
	return to_return;
	}