Create SECRET_CHAT.md

SECRET_CHAT.md

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+# Secret Chat
+
+E2E is opt-in only. Most people probably don’t use Secret Chat since `In a secret chatrooom, features including free calling, polls, events and chatroom album are currently not available`.
+
+Main implementation in package `com.kakao.talk.secret` and the `LocoCipherHelper ` class.
+
+MITM PoC:
+
+- Sender's RSA public key pair in `TalkKeyStore.preferences.xml`
+- Receiver's public keys in `KakaoTalk.db`
+- PoC how-to:
+  - Delete all public keys from `KakaoTalk.db` database
+  - Start mitmproxy and Frida script
+  - Create new Secret Chat room
+	  - `GETLPK` packet gets intercepted -> Maybe we don't need that?
+	  - `SCREATE` packet gets intercepted (shouldn't include a shared secret, otherwise we remove it)
+	  - Bad signature check of MITM public key doesn't seem to have any implications
+    - Sender sends a `SETSK` packet (mitmproxy grabs shared secret)
+    - Dump `SWRITE` packets
+
+Questions:
+- How to attack an already existing E2E chat room?
+- How to fix maldformed `SCREATE` packets?
+- Check public key fingerprints if they have changed
+
+- Test CFB bit flipping
+- Test Secret Chat interception with `mitmproxy` script
+  * Use value from `pt` field to compute the nonce
+  * Does a warning pop up?
+  * What about the master secret?