bitmessage.network.Broadcast.GetPayload C# (CSharp) Method

        private Payload GetPayload()
        {
            lock(_lock4GetPayload)
            if (_payload == null)
            {
                if (_version == 2)
                {
                    PrivateKey privkey = PrivateKey.GetPrivateKey(_bm.DB, Key);
                    if (privkey == null) throw new Exception("PrivateKey not found");

                    MemoryStream payload = new MemoryStream(1000 + Subject.Length + Body.Length); // TODO realy 1000?
                    Random rnd = new Random();
                    ulong dt = DateTime.UtcNow.ToUnix() + (ulong)rnd.Next(600) - 300;

                    payload.Write(dt);
                    payload.WriteVarInt(Version);
                    payload.WriteVarInt(Stream);

                    MemoryStream dataToEncrypt = new MemoryStream(1000 + Subject.Length + Body.Length); // TODO realy 1000?
                    dataToEncrypt.WriteVarInt(Version);

                    byte[] publicAddress = privkey.GetPayload4Broadcast();
                    dataToEncrypt.Write(publicAddress, 0, publicAddress.Length);

                    Byte encodingType = (byte)EncodingType;
                    dataToEncrypt.Write(encodingType);
                    dataToEncrypt.WriteVarStr("Subject:" + Subject + "\nBody:" + Body);

                    byte[] signature = privkey.Sign(dataToEncrypt.ToArray());

                    dataToEncrypt.WriteVarInt((UInt64)signature.Length);
                    dataToEncrypt.Write(signature, 0, signature.Length);

                    var privEncryptionKey = privkey.Sha512VersionStreamHashFirst32();
                    var pubEncryptionKey = ECDSA.PointMult(privEncryptionKey);

                    byte[] bytesToEncrypt = dataToEncrypt.ToArray();
                    byte[] encrypt = ECDSA.Encrypt(bytesToEncrypt, pubEncryptionKey);

                    payload.Write(encrypt, 0, encrypt.Length);

                    _payload = new Payload("broadcast", ProofOfWork.AddPow(payload.ToArray()));
                }
            }
            return _payload;
        }