mirror of
https://gitlab.com/yawning/obfs4.git
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113 lines
3.1 KiB
Go
113 lines
3.1 KiB
Go
/*
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* Copyright (c) 2014, Yawning Angel <yawning at torproject dot org>
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are met:
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*
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* * Redistributions of source code must retain the above copyright notice,
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* this list of conditions and the following disclaimer.
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*
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* * Redistributions in binary form must reproduce the above copyright notice,
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* this list of conditions and the following disclaimer in the documentation
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* and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
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* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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* POSSIBILITY OF SUCH DAMAGE.
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*/
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package obfs4
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import (
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"fmt"
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"math/rand"
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"github.com/yawning/obfs4/csrand"
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"github.com/yawning/obfs4/drbg"
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)
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const (
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minBuckets = 1
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maxBuckets = 100
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)
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// wDist is a weighted distribution.
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type wDist struct {
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minValue int
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maxValue int
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values []int
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buckets []int64
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totalWeight int64
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rng *rand.Rand
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}
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// newWDist creates a weighted distribution of values ranging from min to max
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// based on a HashDrbg initialized with seed.
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func newWDist(seed *drbg.Seed, min, max int) (w *wDist) {
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w = new(wDist)
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w.minValue = min
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w.maxValue = max
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if max <= min {
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panic(fmt.Sprintf("wDist.Reset(): min >= max (%d, %d)", min, max))
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}
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w.reset(seed)
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return
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}
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// sample generates a random value according to the distribution.
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func (w *wDist) sample() int {
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retIdx := 0
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var totalWeight int64
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weight := csrand.Int63n(w.totalWeight)
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for i, bucketWeight := range w.buckets {
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totalWeight += bucketWeight
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if weight <= totalWeight {
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retIdx = i
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break
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}
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}
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return w.minValue + w.values[retIdx]
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}
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// reset generates a new distribution with the same min/max based on a new seed.
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func (w *wDist) reset(seed *drbg.Seed) {
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// Initialize the deterministic random number generator.
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drbg := drbg.NewHashDrbg(seed)
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w.rng = rand.New(drbg)
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nBuckets := (w.maxValue + 1) - w.minValue
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w.values = w.rng.Perm(nBuckets)
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if nBuckets < minBuckets {
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nBuckets = minBuckets
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}
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if nBuckets > maxBuckets {
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nBuckets = maxBuckets
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}
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nBuckets = w.rng.Intn(nBuckets) + 1
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w.totalWeight = 0
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w.buckets = make([]int64, nBuckets)
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for i, _ := range w.buckets {
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prob := w.rng.Int63n(1000)
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w.buckets[i] = prob
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w.totalWeight += prob
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}
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w.buckets[len(w.buckets)-1] = w.totalWeight
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}
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/* vim :set ts=4 sw=4 sts=4 noet : */
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