How to decrypt secret chat messages?

[dustessavdh]

Hi everyone,

I have a question on how to work with secret chats. I'm currently struggling to get the decryption to work.
When you receive an UpdateNewEncryptedMessage it should contain the data somewhere. The data I'm looking for is probably inside the message key. For now I'm only interested in the message with type EncryptedMessage since I think these contain actual message data like when someone sends you a message.
For context the object looks as follows:

{
  CONSTRUCTOR_ID: 314359194,
  SUBCLASS_OF_ID: 2676568142,
  className: 'UpdateNewEncryptedMessage',
  classType: 'constructor',
  message: {
    CONSTRUCTOR_ID: 3977822488,
    SUBCLASS_OF_ID: 597634641,
    className: 'EncryptedMessage',
    classType: 'constructor',
    randomId: Integer { value: 7062284521859799285n },
    chatId: 60847,
    date: 1678568005,
    bytes: <Buffer 1c 6c f6 87 0b 82 f3 de 64 5b 79 d3 b9 81 77 52 2b 00 03 a5 62 dc 56 ba 13 00 26 40 c5 5b 85 29 70 74 0c 0a 0a 8e 0d 81 85 8f 19 c1 a4 85 4b 46 74 f1 ... 86 more bytes>,
    file: {
      CONSTRUCTOR_ID: 3256830334,
      SUBCLASS_OF_ID: 2217371584,
      className: 'EncryptedFileEmpty',
      classType: 'constructor'
    }
  },
  qts: 443369380
}

So I'm trying to decrypt the buffer, but I'm getting nowhere. I tried following this library from the same create as gramjs, but written in python.
This is what I ended up with, which is not the best translation of the python code from this function:

private async decryptMtproto2(
  _chatId: number,
  encryptedMessage: Buffer,
): Promise<unknown> {
  const mTProtoState = new MTProtoState();
  const messageKey = encryptedMessage.subarray(8, 24);
  const encryptedData = encryptedMessage.subarray(24);
  const authKey = this.telegramClientService.client.session
    .getAuthKey()
    ?.getKey();
  if (authKey !== undefined) {
    const { key, iv } = await mTProtoState._calcKey(
      authKey,
      messageKey,
      true,
    );

    const iGE = new IGE(key, iv);
    const decryptedData = iGE.decryptIge(encryptedData);
    // const messageDataLength = decryptedData.subarray(0, 4).readUint16BE();

    const messageData = decryptedData.subarray(4);

    const test = new BinaryReader(messageData);

    console.log(test.tgReadObject());
  }
  return 'Trying to get this to work';
}

This gives me the following error:

TypeNotFoundError: Could not find a matching Constructor ID for the TLObject that was supposed to be
        read with ID 3915666473. Most likely, a TLObject was trying to be read when
         it should not be read. Remaining bytes: 108

The things I couldn't get to work are:

• The peer painor gets in the python code
• The correct message data length

How would I implement this in TypeScript or am I doing it completly wrong?