CTF TOPIC · 8 FLAGS · 22 MIN

Reversing fundamentals

Reading a check, strings built at runtime, and undoing a single-byte XOR.

Reverse engineering means working out what a program does, often without its source. Start by reading any logic you can see: client-side password checks reveal their own answer. Watch for flags assembled from parts at runtime to defeat a simple search. And learn XOR: data XOR-ed with a key is recovered by XOR-ing with the same key again. These ideas scale up to real binaries later.

A first example

if (input === expected) ; a + b + c ; byte XOR 0x42 twice = byte.

First moves

1. Run strings before anything else
2. Find the comparison: what is input checked against?
3. Trace values built at runtime
4. Invert transforms in reverse order
5. XOR again with the same key to undo XOR

The challenges

• Read the check backwards · Easy · 20 XP
• Trace every byte · Medium · 30 XP
• Follow the accumulator · Medium · 30 XP
• Read the check · Easy · 20 XP
• Rebuild the string · Easy · 20 XP
• Undo the XOR · Hard · 40 XP
• String.fromCharCode · Medium · 30 XP
• Undo the transforms · Hard · 40 XP