Ox: Change rpad from random length to length dependent on message length

The length of message + the length of the rpad will always be a
total length of a multiple of 100.

helpers.go

@@ -28,11 +28,11 @@ func readFile(path string) (*bytes.Buffer, error) {
 	return buffer, nil
 }
 
-func getRpad() string {
+func getRpad(messageLength int) string {
 	var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.,;?!+-_§$%&/()=")
 	lettersLength := int64(len(letters))
-	length, _ := rand.Int(rand.Reader, big.NewInt(30))
-	s := make([]rune, length.Int64())
+	length := 100 - messageLength%100
+	s := make([]rune, length)
 	for i := range s {
 		pos, _ := rand.Int(rand.Reader, big.NewInt(lettersLength))
 		s[i] = letters[pos.Int64()]

ox.go

@@ -533,7 +533,7 @@ func oxEncrypt(client *xmpp.Client, oxPrivKey *crypto.Key, recipient string,
 	oxCryptMessageScTime := oxCryptMessageSc.CreateElement("time")
 	oxCryptMessageScTime.CreateAttr("stamp", time.Now().UTC().Format("2006-01-02T15:04:05Z"))
 	oxCryptMessageScRpad := oxCryptMessageSc.CreateElement("rpad")
-	oxCryptMessageScRpad.CreateText(getRpad())
+	oxCryptMessageScRpad.CreateText(getRpad(len(message)))
 	oxCryptMessageScPayload := oxCryptMessageSc.CreateElement("payload")
 	oxCryptMessageScPayloadBody := oxCryptMessageScPayload.CreateElement("body")
 	oxCryptMessageScPayloadBody.CreateAttr("xmlns", nsJabberClient)