strneq.hpp

//    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) 2012-2020 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;
      size_t k = 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);
	  if (s2[j] == '\0')
	    k++;
	}
      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());
    }
  }
}