kakaotalk_analysis/doc/SECRET_CHAT.md

# Secret Chat

*Secret Chat* is KakaoTalk's E2E encryption feature. It was added on top of the existing [LOCO protocol](https://kth.diva-portal.org/smash/get/diva2:1046438/FULLTEXT01.pdf#page=77) which has a couple of [flaws](https://kth.diva-portal.org/smash/get/diva2:1046438/FULLTEXT01.pdf#page=100) including missing integrity protection of the ciphertext.

*Secret Chat* is opt-in only and not enabled by default in the KakaoTalk mobile app. Most users might not use *Secret Chat* as it doesn't have the same feature set as regular non-E2E encrypted chat rooms.

We've created a simple script to man-in-the-middle *Secret Chat* communications with `Frida` and `mitmproxy`. It demonstrates a well-known server-side attack in which the operator (i.e. KakaoTalk) can spoof a client's public key to intercept and read E2E encrypted chat messages. 

After the MITM attack there's no immediate warning message that is prompted to the user. Only if both parties go to `Chatroom Settings` -> `Public Key` and compare their public key fingerprints, the attack can be detected.

This is how one can run the PoC:

- Assumption: You've already set up your test environment (see setup description [here](./SETUP.md))
- Wipe all entries in the `public_key_info` and `secret_key_info` tables from the `KakaoTalk.db` database
- Start `mitmproxy`: `$ mitmdump -m wireguard -s mitm_secret_chat.py`
- Start `Frida`: `$ frida -U -l loco-tracer.js -f com.kakao.talk`
- Create a new *Secret Chat* room in the KakaoTalk app and send a message
- View message in `mitmproxy` terminal window

How it works:

- Server-side `GETLPK` packet gets intercepted -> Inject MITM public key
- Server-side `SCREATE` packet gets intercepted -> Remove an already existing shared secret (if any)
- Sender sends a `SETSK` packet -> `mitmproxy` script grabs shared secret and re-encrypts it with the recipient's original public key
- Using the shared secret, the script computes the E2E encryption key
- `MSG` and `SWRITE` packets are decrypted and dumped in the `mitmproxy` terminal

Known issues / TODOs:

- Mitmproxy splits large `SCREATE` packets into multiple messages. There's no built-in packet reassembly. As such, fragmented `SCREATE` packets lead to parsing errors -> work-around:
  - Delete the Secret Chat chatroom and delete all rows from the `public_key_info` and `secret_key_info` tables in the `KakaoTalk.db` database
  - Restart the mitmproxy and Frida scripts
  - Create a new chatroom. Also try to create a new chatroom with a different friend.
- If the MITM public key can't be injected, the shared secret can't be decrypted and the script fails with a `ValueError` exception (`Encryption/decryption failed`) -> just try again :wink:
- The removal of server-side stored shared secrets doesn't seem to work in some cases (flaky)

Android implementation specifics:

- Main *Secret Chat* implementation in package `com.kakao.talk.secret` and in the `LocoCipherHelper ` class
- Sender's RSA public key pair in `TalkKeyStore.preferences.xml`
- Receiver's public keys in table `public_key_info` of `KakaoTalk.db` database
- Shared secret stored in table `secret_key_info` of `KakaoTalk.db` database

TODOS:

- How are the msgId and chatId generated? -> nonce for CTR mode!

Demo:

![MITM](https://github.com/stulle123/kakaotalk_analysis/tree/main/doc/secret_chat_demo.gif?raw=true)