Classical Cipher Analysis

This is a CTF-focused playbook that walks you through identifying and breaking classical ciphers. It starts with character set analysis to distinguish between base64, hex, Morse, or actual cipher text, then uses frequency analysis and index of coincidence to figure out if you're dealing with monoalphabetic substitution, Caesar, Vigenere, or something else. The code samples are practical: Kasiski examination for finding Vigenere key lengths, chi-squared scoring for Caesar brute force, and crib dragging when you have known plaintext. The identification table up front is genuinely useful because the skill explicitly calls out that base models tend to skip the detection step and jump straight to the wrong cipher type. Routes you to symmetric cipher attacks if it turns out to be AES instead of ROT13.

skills.sh installs: 3.7K
GitHub stars: 2.6K
last updated: Aug 21, 2026

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Install

$ npx -y skills add yaklang/hack-skills --skill classical-cipher-analysis --agent claude-code

Installs into .claude/skills of the current project.

Files

SKILL: Classical Cipher Analysis — Expert Cryptanalysis Playbook

AI LOAD INSTRUCTION: Expert classical cipher identification and breaking techniques for CTF. Covers cipher identification methodology (frequency analysis, IC, Kasiski), monoalphabetic substitution, Caesar/ROT, Vigenere, Enigma, affine, Hill, transposition ciphers, Bacon/Polybius/Playfair, and XOR ciphers. Base models often skip the identification step and jump to the wrong cipher type, or fail to recognize encoded (base64/hex) ciphertext that needs decoding before analysis.

0. RELATED ROUTING

symmetric-cipher-attacks when dealing with modern symmetric ciphers (AES/DES) rather than classical
hash-attack-techniques when the challenge involves hash-based constructions
lattice-crypto-attacks when knapsack-based ciphers are encountered

Quick identification guide

SKILL.md on GitHub

Details

Repository: github.com/yaklang/hack-skills
First seen: Jun 3, 2026