func NewLongTermIntegrity(username, realm, password string) MessageIntegrity {
	k := strings.Join([]string{username, realm, password}, credentialsSep)
	h := md5.New()
	fmt.Fprint(h, k)
	return MessageIntegrity(h.Sum(nil))
}
NewLongTermIntegrity returns new MessageIntegrity with key for long-term
credentials. Password, username, and realm must be SASL-prepared.
package stun
import (
	"crypto/md5"   
	"crypto/sha1"  
	"errors"
	"fmt"
	"strings"
	"github.com/pion/stun/internal/hmac"
)
const credentialsSep = ":"