MAGIC KEYS

There is a service listening on target host that can decrypt your messages. You can connect with socat or similar tool to it and use the text-based menu for operating it. The service is a custom piece of code, written in Python. Unfortunately implementing your own cryptography often ends up with buggy code that is easy to exploit, and result is complete loss of secrecy.

Steal the keys! Source code of the service is available for your inspection over HTTP on the target machine.