Source: rand_unix.go in package crypto/rand

+build aix darwin dragonfly freebsd linux netbsd openbsd plan9 solaris

Unix cryptographically secure pseudorandom number generator.


package rand

import (
	"bufio"
	"crypto/aes"
	"crypto/cipher"
	"encoding/binary"
	"io"
	"os"
	"runtime"
	"sync"
	"sync/atomic"
	"time"
)

const urandomDevice = "/dev/urandom"

Easy implementation: read from /dev/urandom. This is sufficient on Linux, OS X, and FreeBSD.


func init() {
	if runtime.GOOS == "plan9" {
		Reader = newReader(nil)
	} else {
		Reader = &devReader{name: urandomDevice}
	}
}

A devReader satisfies reads by reading the file named name.

type devReader struct {
	name string
	f    io.Reader
	mu   sync.Mutex
	used int32 // atomic; whether this devReader has been used
}

altGetRandom if non-nil specifies an OS-specific function to get urandom-style randomness.

var altGetRandom func([]byte) (ok bool)

func warnBlocked() {
	println("crypto/rand: blocked for 60 seconds waiting to read random data from the kernel")
}

func (r *devReader) Read(b []byte) (n int, err error) {

First use of randomness. Start timer to warn about being blocked on entropy not being available.

		t := time.AfterFunc(60*time.Second, warnBlocked)
		defer t.Stop()
	}
	if altGetRandom != nil && r.name == urandomDevice && altGetRandom(b) {
		return len(b), nil
	}
	r.mu.Lock()
	defer r.mu.Unlock()
	if r.f == nil {
		f, err := os.Open(r.name)
		if f == nil {
			return 0, err
		}
		if runtime.GOOS == "plan9" {
			r.f = f
		} else {
			r.f = bufio.NewReader(hideAgainReader{f})
		}
	}
	return r.f.Read(b)
}

var isEAGAIN func(error) bool // set by eagain.go on unix systems

hideAgainReader masks EAGAIN reads from /dev/urandom. See golang.org/issue/9205

type hideAgainReader struct {
	r io.Reader
}

func (hr hideAgainReader) Read(p []byte) (n int, err error) {
	n, err = hr.r.Read(p)
	if err != nil && isEAGAIN != nil && isEAGAIN(err) {
		err = nil
	}
	return
}

Alternate pseudo-random implementation for use on systems without a reliable /dev/urandom.

newReader returns a new pseudorandom generator that seeds itself by reading from entropy. If entropy == nil, the generator seeds itself by reading from the system's random number generator, typically /dev/random. The Read method on the returned reader always returns the full amount asked for, or else it returns an error. The generator uses the X9.31 algorithm with AES-128, reseeding after every 1 MB of generated data.

func newReader(entropy io.Reader) io.Reader {
	if entropy == nil {
		entropy = &devReader{name: "/dev/random"}
	}
	return &reader{entropy: entropy}
}

type reader struct {
	mu                   sync.Mutex
	budget               int // number of bytes that can be generated
	cipher               cipher.Block
	entropy              io.Reader
	time, seed, dst, key [aes.BlockSize]byte
}

func (r *reader) Read(b []byte) (n int, err error) {
	r.mu.Lock()
	defer r.mu.Unlock()
	n = len(b)

	for len(b) > 0 {
		if r.budget == 0 {
			_, err := io.ReadFull(r.entropy, r.seed[0:])
			if err != nil {
				return n - len(b), err
			}
			_, err = io.ReadFull(r.entropy, r.key[0:])
			if err != nil {
				return n - len(b), err
			}
			r.cipher, err = aes.NewCipher(r.key[0:])
			if err != nil {
				return n - len(b), err
			}
			r.budget = 1 << 20 // reseed after generating 1MB
		}
		r.budget -= aes.BlockSize

ANSI X9.31 (== X9.17) algorithm, but using AES in place of 3DES. single block: t = encrypt(time) dst = encrypt(t^seed) seed = encrypt(t^dst)

		ns := time.Now().UnixNano()
		binary.BigEndian.PutUint64(r.time[:], uint64(ns))
		r.cipher.Encrypt(r.time[0:], r.time[0:])
		for i := 0; i < aes.BlockSize; i++ {
			r.dst[i] = r.time[i] ^ r.seed[i]
		}
		r.cipher.Encrypt(r.dst[0:], r.dst[0:])
		for i := 0; i < aes.BlockSize; i++ {
			r.seed[i] = r.time[i] ^ r.dst[i]
		}
		r.cipher.Encrypt(r.seed[0:], r.seed[0:])

		m := copy(b, r.dst[0:])
		b = b[m:]
	}

	return n, nil