CometProxy

A MITM proxy for Better Than Adventure (BTA!) servers, written in Kotlin. It sits between a BTA client and a real server, decodes every packet that passes through, and lets you inspect or modify the connection.

How

CometProxy poses as a BTA! server, accepts a client connection, opens its own connection to the target server, and relays packets in both directions. Packets are fully parsed and inspected through a handler pipeline before being resent.

CometProxy is also able to read otherwise encrypted chat packets:

• On PacketLogin, the proxy swaps the client's real RSA public key for its own before forwarding to the server. It also keeps the client's real key.
• When the server responds with the AES session key, the proxy decrypts it with its own private key, then re-encrypts it with the client's real public key before forwarding.
• The proxy now holds the shared AES key for that session, so it can decrypt chat messages passing through in either direction.

Architecture

• network/proxy/Proxy.kt: accepts incoming TCP connections and spawns a Bridge per session.
• network/proxy/Bridge.kt: runs two coroutines per connection (client->server, server->client), each reading packets and pushing them through the pipeline.
• network/pipeline/: a packet processing pipeline (middleware), which allows the proxy to insepct/modify packets.
• network/pipeline/handlers/: the built-in handlers: HandlerLogin, HandlerAESSendKey, HandlerMessage, HandlerPingHandshake.
• network/packet/: packet protocol. Each packet implements its serialization strategy, as BTA! does not have packet length headers, most packets must be read field by field from the stream, however a known/fixed size optimization is implemented for reading certain packets of known size.
• nbt/: a standalone NBT reader/writer (Minecraft/BTA! binary data format).
• network/warmup/JvmWarmup.kt: runs dummy serialization/crypto/socket work at startup to warm up the JIT.
• ui/: a desktop GUI (Compose Multiplatform + Jewel) showing active connections (with player heads), a log panel, and the ability to send arbitrary messages to client/server.

Building and running

Requires JDK 25.

./gradlew packageUberJarForCurrentOS

Run with the GUI:

./gradlew run

Or headless, with CLI args (-h to see all args):

./gradlew run --args="--headless --target-server example.com --target-port 25565 --host-port 25566"

In headless mode, typing stop shuts the proxy down.

Testing

Unit tests for packet serialization, NBT encoding, and JVM warmup routine. Run with:

./gradlew test

License

MIT. See LICENSE.