copy from docker

generateID.go

package main

import (
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"github.com/docker/docker/pkg/random"
	"io"
	"strconv"
	"strings"
)

const shortLen = 12

func main() {
	for i := 0; i < 10; i++ {
		fmt.Println(generateID(true))
	}
}

func generateID(crypto bool) string {
	b := make([]byte, 32)
	r := random.Reader
	if crypto {
		r = rand.Reader
	}
	for {
		if _, err := io.ReadFull(r, b); err != nil {
			panic(err) // This shouldn't happen
		}
		id := hex.EncodeToString(b)
		// if we try to parse the truncated for as an int and we don't have
		// an error then the value is all numeric and causes issues when
		// used as a hostname. ref #3869
		if _, err := strconv.ParseInt(TruncateID(id), 10, 64); err == nil {
			continue
		}
		return id
	}
}

func TruncateID(id string) string {
	if i := strings.IndexRune(id, ':'); i >= 0 {
		id = id[i+1:]
	}
	trimTo := shortLen
	if len(id) < shortLen {
		trimTo = len(id)
	}
	return id[:trimTo]
}