CTF TOPIC · 6 FLAGS · 25 MIN

Scripting for CTFs

Script repetitive decoding: shifts, brute-forcing a Caesar, repeating-key XOR.

A little Python turns slow hand-decoding into instant answers. Read bytes or numbers from a file, transform them in a loop, and print the result. Three patterns cover most beginner challenges: apply a per-character shift, brute-force a small keyspace (like all 26 Caesar shifts) and keep the readable candidate, and XOR bytes against a repeating key. Write them on your own machine; the in-browser tools cover the simple cases.

A first example

''.join(chr((n-5)%256) for n in nums) ; for k in range(26): ... ; b ^ key[i%len(key)]

First moves

1. Read the input from a file
2. Write one transformation as a function
3. Loop over keys or layers
4. Stop when the output starts with flag{
5. Print candidates so you can see near misses

The challenges

• Automate the shift · Medium · 30 XP
• Brute every shift · Medium · 30 XP
• Repeating-key XOR · Hard · 40 XP
• Twelve layers · Medium · 30 XP
• Brute a one-byte key · Hard · 40 XP
• Most frequent token · Medium · 30 XP