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feat(random): use crypto/rand for random string generator (#55)
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ichigozero authored Nov 9, 2023
1 parent d43909e commit 508eabf
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Showing 2 changed files with 98 additions and 7 deletions.
52 changes: 45 additions & 7 deletions random/random.go
Original file line number Diff line number Diff line change
@@ -1,13 +1,16 @@
package random

import (
"math/rand"
"bufio"
"crypto/rand"
"io"
"strings"
"time"
"sync"
)

type (
Random struct {
readerPool sync.Pool
}
)

Expand All @@ -27,20 +30,55 @@ var (
)

func New() *Random {
rand.Seed(time.Now().UnixNano())
return new(Random)
// https://tip.golang.org/doc/go1.19#:~:text=Read%20no%20longer%20buffers%20random%20data%20obtained%20from%20the%20operating%20system%20between%20calls
p := sync.Pool{New: func() interface{} {
return bufio.NewReader(rand.Reader)
}}
return &Random{readerPool: p}
}

func (r *Random) String(length uint8, charsets ...string) string {
charset := strings.Join(charsets, "")
if charset == "" {
charset = Alphanumeric
}

charsetLen := len(charset)
if charsetLen > 255 {
charsetLen = 255
}
maxByte := 255 - (256 % charsetLen)

reader := r.readerPool.Get().(*bufio.Reader)
defer r.readerPool.Put(reader)

b := make([]byte, length)
for i := range b {
b[i] = charset[rand.Int63()%int64(len(charset))]
rs := make([]byte, length+(length/4)) // perf: avoid read from rand.Reader many times
var i uint8 = 0

// security note:
// we can't just simply do b[i]=charset[rb%byte(charsetLen)],
// for example, when charsetLen is 52, and rb is [0, 255], 256 = 52 * 4 + 48.
// this will make the first 48 characters more possibly to be generated then others.
// so we have to skip bytes when rb > maxByte

for {
_, err := io.ReadFull(reader, rs)
if err != nil {
panic("unexpected error happened when reading from bufio.NewReader(crypto/rand.Reader)")
}
for _, rb := range rs {
if rb > byte(maxByte) {
// Skip this number to avoid bias.
continue
}
b[i] = charset[rb%byte(charsetLen)]
i++
if i == length {
return string(b)
}
}
}
return string(b)
}

func String(length uint8, charsets ...string) string {
Expand Down
53 changes: 53 additions & 0 deletions random/random_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -5,10 +5,63 @@ import (
"testing"

"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/require"
)

func Test(t *testing.T) {
assert.Len(t, String(32), 32)
r := New()
assert.Regexp(t, regexp.MustCompile("[0-9]+$"), r.String(8, Numeric))
}

func TestRandomString(t *testing.T) {
var testCases = []struct {
name string
whenLength uint8
expect string
}{
{
name: "ok, 16",
whenLength: 16,
},
{
name: "ok, 32",
whenLength: 32,
},
}

for _, tc := range testCases {
t.Run(tc.name, func(t *testing.T) {
uid := String(tc.whenLength, Alphabetic)
assert.Len(t, uid, int(tc.whenLength))
})
}
}

func TestRandomStringBias(t *testing.T) {
t.Parallel()
const slen = 33
const loop = 100000

counts := make(map[rune]int)
var count int64

for i := 0; i < loop; i++ {
s := String(slen, Alphabetic)
require.Equal(t, slen, len(s))
for _, b := range s {
counts[b]++
count++
}
}

require.Equal(t, len(Alphabetic), len(counts))

avg := float64(count) / float64(len(counts))
for k, n := range counts {
diff := float64(n) / avg
if diff < 0.95 || diff > 1.05 {
t.Errorf("Bias on '%c': expected average %f, got %d", k, avg, n)
}
}
}

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