Add crypto_mod_exp() for GnuTLS (libgcrypt)

This allows WPS to be linked with GnuTLS.

src/crypto/crypto_gnutls.c

@@ -163,3 +163,38 @@ void aes_decrypt_deinit(void *ctx)
 	gcry_cipher_close(hd);
 }
 #endif /* EAP_TLS_FUNCS */
+
+
+int crypto_mod_exp(const u8 *base, size_t base_len,
+		   const u8 *power, size_t power_len,
+		   const u8 *modulus, size_t modulus_len,
+		   u8 *result, size_t *result_len)
+{
+	gcry_mpi_t bn_base = NULL, bn_exp = NULL, bn_modulus = NULL,
+		bn_result = NULL;
+	int ret = -1;
+
+	if (gcry_mpi_scan(&bn_base, GCRYMPI_FMT_USG, base, base_len, NULL) !=
+	    GPG_ERR_NO_ERROR ||
+	    gcry_mpi_scan(&bn_exp, GCRYMPI_FMT_USG, power, power_len, NULL) !=
+	    GPG_ERR_NO_ERROR ||
+	    gcry_mpi_scan(&bn_modulus, GCRYMPI_FMT_USG, modulus, modulus_len,
+			  NULL) != GPG_ERR_NO_ERROR)
+		goto error;
+	bn_result = gcry_mpi_new(modulus_len * 8);
+
+	gcry_mpi_powm(bn_result, bn_base, bn_exp, bn_modulus);
+
+	if (gcry_mpi_print(GCRYMPI_FMT_USG, result, *result_len, result_len,
+			   bn_result) != GPG_ERR_NO_ERROR)
+		goto error;
+
+	ret = 0;
+
+error:
+	gcry_mpi_release(bn_base);
+	gcry_mpi_release(bn_exp);
+	gcry_mpi_release(bn_modulus);
+	gcry_mpi_release(bn_result);
+	return ret;
+}