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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 <algorithm>
#include <openvpn/time/time.hpp>
#include <openvpn/random/randapi.hpp>
namespace openvpn {
struct TimeSkew
{
// Skew factors (+/- %).
// Pass these to skew() via factor parameter.
enum
{
PCT_50 = 0,
PCT_25 = 1,
PCT_12_5 = 2,
PCT_6_25 = 3,
PCT_3_125 = 4,
PCT_1_5625 = 5,
};
// Skew a duration by some random flux.
static Time::Duration skew(const Time::Duration &dur, const unsigned int factor, RandomAPI &prng)
{
const std::uint32_t bms = static_cast<uint32_t>(std::min(dur.to_binary_ms() >> factor, oulong(0x40000000))); // avoid 32-bit overflow in next step
const int flux = int(prng.randrange32(bms)) - int(bms / 2);
return dur + flux;
}
};
} // namespace openvpn
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