func RandomString(length int) string {
	const characters = "abcdefghijklmnopqrstuvwxyzABCDEFGH" +
		"IJKLMNOPQRSTUVWXYZ0123456789-._~"
	b := make([]byte, length)
	for i := range b {
		b[i] = characters[rand.Intn(len(characters))]
	}
	return string(b)
}
RandomString generates a random string.
package pkce
import (
	"crypto/sha256"
	"encoding/base64"
	"math/rand"
	"time"
)
func init() {
	rand.Seed(time.Now().UnixNano())
}