Added TimeSkew to skew a time duration by a random flux

Signed-off-by: James Yonan <james@openvpn.net>

openvpn/time/skew.hpp

@@ -0,0 +1,53 @@
+//    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) 2012-2019 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 = 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;
+    }
+  };
+
+}