CTF TOPIC · 6 FLAGS · 20 MIN

Introductory cryptography

ROT13, Caesar with an unknown shift, and the Atbash mirror cipher.

Classical ciphers move letters around by a rule. ROT13 shifts by 13 and is its own inverse. A Caesar cipher shifts by a fixed amount; with only 25 possible shifts you can try them all, and a known prefix like flag{ tells you the shift at once. Atbash mirrors the alphabet (A<->Z) and needs no key. Recognising flag{ in disguise (for example svnt{ is ROT13) is the core skill.

A first example

ROT13('flag') = synt ; Caesar('abc', 3) = def ; Atbash('a') = z.

First moves

1. Check whether braces and underscores survived (letter-only cipher)
2. Compare the first four letters with f-l-a-g to get the shift
3. Brute-force all 25 Caesar shifts when unsure
4. Try Atbash when no key is mentioned

The challenges

• Seven places away · Easy · 20 XP
• One byte of key · Medium · 30 XP
• A repeating key · Medium · 30 XP
• Rotate by thirteen · Easy · 20 XP
• Unknown shift · Medium · 30 XP
• Mirror alphabet · Easy · 20 XP