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classical-cipher-analysis

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Classical cipher analysis playbook. Use when encountering substitution ciphers, Vigenere, transposition, XOR, or encoded text in CTF challenges that requires frequency analysis, Kasiski examination, or known-plaintext cryptanalysis.

skills/classical-cipher-analysis/SKILL.md

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

### Quick identification guide

| Observation | Likely Cipher | First Action |
|---|---|---|