syncthing/lib/rand/random.go
2019-02-02 12:16:27 +01:00

76 lines
2.3 KiB
Go

// Copyright (C) 2014 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.
// Package rand implements functions similar to math/rand in the standard
// library, but on top of a secure random number generator.
package rand
import (
"crypto/md5"
cryptoRand "crypto/rand"
"encoding/binary"
"io"
mathRand "math/rand"
)
// Reader is the standard crypto/rand.Reader, re-exported for convenience
var Reader = cryptoRand.Reader
// randomCharset contains the characters that can make up a randomString().
const randomCharset = "2345679abcdefghijkmnopqrstuvwxyzACDEFGHJKLMNPQRSTUVWXYZ"
var (
// defaultSecureSource is a concurrency safe math/rand.Source with a
// cryptographically sound base.
defaltSecureSource = newSecureSource()
// defaultSecureRand is a math/rand.Rand based on the secure source.
defaultSecureRand = mathRand.New(defaltSecureSource)
)
// String returns a strongly random string of characters (taken from
// randomCharset) of the specified length. The returned string contains ~5.8
// bits of entropy per character, due to the character set used.
func String(l int) string {
bs := make([]byte, l)
for i := range bs {
bs[i] = randomCharset[defaultSecureRand.Intn(len(randomCharset))]
}
return string(bs)
}
// Int63 returns a strongly random int63
func Int63() int64 {
return defaltSecureSource.Int63()
}
// Int64 returns a strongly random int64
func Int64() int64 {
var bs [8]byte
_, err := io.ReadFull(cryptoRand.Reader, bs[:])
if err != nil {
panic("randomness failure: " + err.Error())
}
return int64(binary.BigEndian.Uint64(bs[:]))
}
// Intn returns, as an int, a non-negative strongly random number in [0,n).
// It panics if n <= 0.
func Intn(n int) int {
return defaultSecureRand.Intn(n)
}
// SeedFromBytes calculates a weak 64 bit hash from the given byte slice,
// suitable for use a predictable random seed.
func SeedFromBytes(bs []byte) int64 {
h := md5.New()
h.Write(bs)
s := h.Sum(nil)
// The MD5 hash of the byte slice is 16 bytes long. We interpret it as two
// uint64s and XOR them together.
return int64(binary.BigEndian.Uint64(s[0:]) ^ binary.BigEndian.Uint64(s[8:]))
}