Classical Ciphers

Why classics appear in CTFs

Organisers use classical ciphers as accessible entry points: the math is pre-computer, the ciphertext is printable text, and the “aha” moment is satisfying. Recognising the cipher family is half the work.

Caesar / ROT13

The Caesar cipher shifts every letter by a fixed amount. ROT13 is Caesar with a shift of 13 — applying it twice gives back the original.

KHOOR ZRUOG  →  shift −3  →  HELLO WORLD

If the flag looks like naq gur synt vf..., it’s ROT13 — paste it in CyberChef’s ROT13 operation and you’re done.

Brute-forcing: only 25 shifts to try. CyberChef’s ROT13 Brute Force or a simple Python loop:

ct = "KHOOR ZRUOG"
for shift in range(26):
    print(shift, ''.join(chr((ord(c) - 65 - shift) % 26 + 65) if c.isupper() else c for c in ct))

Vigenère

A polyalphabetic substitution: a keyword repeats over the plaintext and each letter gets a different Caesar shift. The key signature is repeating patterns separated by the key length.

Attack — Kasiski / Index of Coincidence:

1. Find repeated sequences in the ciphertext; their distance is a multiple of the key length.
2. Try key lengths 2–20; pick the one whose IC is closest to English (~0.065).
3. For each key position, collect every n-th character and frequency-analyse.

CyberChef’s Vigenère Decode accepts the key once you’ve recovered it. dcode.fr has an auto-solve.

Monoalphabetic substitution

Every letter maps to a fixed different letter. No key — just 26 unknowns. The attack is frequency analysis:

• English frequency order: E T A O I N S H R D L C U M W F G Y P B V K J X Q Z
• Map the most-frequent ciphertext letter to E, next to T, etc., then tweak by hand using bigrams (TH, HE, IN).

quipqiup.com automates this with near-perfect accuracy.

Recognising the cipher family

Clue	Likely cipher
Only letters, uniform distribution	Substitution or Caesar
Two-character alphabet (. -)	Morse
Base64-looking but offset	Vigenère or XOR
Repeating short word boundaries	Vigenère
Flag wrapped as ROT13(...)	Apply ROT13 once

CyberChef workflow

Paste ciphertext → Magic operation → enable “Intensive Mode” → CyberChef will try dozens of operations and flag likely decryptions automatically.