Interpreter

Overview

This challenge exposes a small custom interpreter over the network (nc 20.198.224.34 8037). There is no source, so the approach is to experiment with the instruction set until the behaviour becomes clear. From poking at it, we worked out two commands:

• d — moves/steps backward.
• io — prints the text at the current position.

The idea is to walk the pointer backward and print characters until the flag is spelled out. By trial and error we tuned the exact number of d (backward) steps followed by repeated io (print) operations.

Payload

ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioioio

Sending this payload walks back the required distance and then prints out each character of the flag in sequence.