Generate secure passwords in Go

passgen.go

package main

import (
	"crypto/rand"
	"flag"
	"fmt"
	"math/big"
)

func GenerateSecurePassword(n int, alphabet string) (string, error) {
	alphabetLength := big.NewInt(int64(len(alphabet)))
	ret := make([]byte, n)
	for i := 0; i < n; i++ {
		charIndex, err := rand.Int(rand.Reader, alphabetLength)
		if err != nil {
			return "", err
		}
		ret[i] = alphabet[charIndex.Int64()]
	}
	return string(ret), nil
}

func GenerateAlphaNumericPassword(n int) (string, error) {
	const alphaNumericAlphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	return GenerateSecurePassword(n, alphaNumericAlphabet)
}

func main() {
	n := flag.Int("n", 12, "password length")
	flag.Parse()

	password, err := GenerateAlphaNumericPassword(*n)
	if err != nil {
		panic(err)
	}
	fmt.Println("Password:", password)
}