/// <summary>
        /// Signieren des äußeren Umschlages
        /// </summary>
        /// <param name="outer_envelope">äußer Umschlag</param>
        /// <param name="privateKey">Privater Schlüssel</param>
        public static string createSignOuterEnvelope(Models.Message envelope, string timestamp, string receiver, string privateKey)
        {
            string content = envelope.sender + envelope.cipher + envelope.iv + envelope.key_recipient_enc + envelope.sig_recipient + timestamp + receiver;

            return(createSign(content, privateKey));
        }
        /// <summary>
        /// Signieren des inneren Umschlages
        /// </summary>
        /// <param name="inner_envelope">Innere Umschlag</param>
        /// <param name="privateKey">Privater Schlüssel</param>
        public static string createSignInnerEnvelope(Models.Message envelope, string privateKey)
        {
            string content = envelope.sender + envelope.cipher + envelope.iv + envelope.key_recipient_enc;

            return(createSign(content, privateKey));
        }