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// OpenVPN -- An application to securely tunnel IP networks
// over a single port, with support for SSL/TLS-based
// session authentication and key exchange,
// packet encryption, packet authentication, and
// packet compression.
//
// Copyright (C) 2012-2022 OpenVPN Inc.
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License Version 3
// as published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program in the COPYING file.
// If not, see <http://www.gnu.org/licenses/>.
#pragma once
#include <string>
#include <atomic>
namespace openvpn {
namespace crypto {
// Compare strings in a way that is more resistant to timing attacks.
// s1 should be the string provided by the user, while s2 is the
// "secret" string that we are comparing s1 against.
// Our goal is to prevent timing data from leaking info about the
// length or content of s2.
// https://nachtimwald.com/2017/04/02/constant-time-string-comparison-in-c/
inline bool str_neq(const char *s1, const char *s2)
{
unsigned int neq = 0;
size_t i = 0;
size_t j = 0;
while (true)
{
neq |= s1[i] ^ s2[j];
if (s1[i] == '\0')
break;
i++;
atomic_thread_fence(std::memory_order_acq_rel);
if (s2[j] != '\0')
j++;
atomic_thread_fence(std::memory_order_acq_rel);
}
atomic_thread_fence(std::memory_order_acq_rel);
return bool(neq);
}
inline bool str_neq(const std::string &s1, const std::string &s2)
{
return str_neq(s1.c_str(), s2.c_str());
}
} // namespace crypto
} // namespace openvpn
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