60 lines
1.8 KiB
Go
60 lines
1.8 KiB
Go
// Copyright (C) 2014 The Syncthing Authors.
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//
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// This Source Code Form is subject to the terms of the Mozilla Public
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// License, v. 2.0. If a copy of the MPL was not distributed with this file,
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// You can obtain one at http://mozilla.org/MPL/2.0/.
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package main
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import (
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"crypto/md5"
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cryptoRand "crypto/rand"
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"encoding/binary"
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"io"
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mathRand "math/rand"
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)
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// randomCharset contains the characters that can make up a randomString().
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const randomCharset = "01234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-"
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// predictableRandom is an RNG that will always have the same sequence. It
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// will be seeded with the device ID during startup, so that the sequence is
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// predictable but varies between instances.
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var predictableRandom = mathRand.New(mathRand.NewSource(42))
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func init() {
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// The default RNG should be seeded with something good.
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mathRand.Seed(randomInt64())
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}
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// randomString returns a string of random characters (taken from
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// randomCharset) of the specified length.
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func randomString(l int) string {
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bs := make([]byte, l)
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for i := range bs {
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bs[i] = randomCharset[mathRand.Intn(len(randomCharset))]
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}
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return string(bs)
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}
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// randomInt64 returns a strongly random int64, slowly
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func randomInt64() int64 {
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var bs [8]byte
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_, err := io.ReadFull(cryptoRand.Reader, bs[:])
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if err != nil {
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panic("randomness failure: " + err.Error())
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}
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return seedFromBytes(bs[:])
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}
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// seedFromBytes calculates a weak 64 bit hash from the given byte slice,
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// suitable for use a predictable random seed.
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func seedFromBytes(bs []byte) int64 {
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h := md5.New()
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h.Write(bs)
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s := h.Sum(nil)
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// The MD5 hash of the byte slice is 16 bytes long. We interpret it as two
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// uint64s and XOR them together.
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return int64(binary.BigEndian.Uint64(s[0:]) ^ binary.BigEndian.Uint64(s[8:]))
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}
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