Generating an unpredictable random value in Go

There are a lot of examples to use math/rand. However, should use crypto/rand if you want to generate an unpredictable random value. That's because crypto/rand uses getrandom(2) if available, /dev/urandom otherwise on Linux.

As a real world example, UUID v4 uses crypto/rand internally. UUID v4) is randomly generated. An implementation of UUID v4 in Go uses crypto/rand internally.

• https://github.com/google/uuid/blob/master/version4.go#L29-L38
• https://github.com/google/uuid/blob/master/uuid.go#L36

Here is an example to generate random values with crypto/rand.

Generate random integer

func generateInt(n int64) (int64, error) {
    r, err := rand.Int(rand.Reader, big.NewInt(n))
    if err != nil {
        return 0, err
    }
    return r.Int64(), nil
}

Generate random string

func generateString(length int) (string, error) {
    b := make([]byte, length)
    _, err := rand.Read(b)
    if err != nil {
        return "", err
    }
    return base64.URLEncoding.EncodeToString(b), nil
}

Whole source code

package main

import (
    "crypto/rand"
    "encoding/base64"
    "fmt"
    "log"
    "math/big"
)

func main() {
    v, err := generateInt(1000)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("random int = %v\n", v)

    s, err := generateString(10)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("random string = %v\n", s)
}

func generateInt(n int64) (int64, error) {
    r, err := rand.Int(rand.Reader, big.NewInt(n))
    if err != nil {
        return 0, err
    }
    return r.Int64(), nil
}

func generateString(length int) (string, error) {
    b := make([]byte, length)
    _, err := rand.Read(b)
    if err != nil {
        return "", err
    }
    return base64.URLEncoding.EncodeToString(b), nil
}