classical-cipher-analysis

1# SKILL: Classical Cipher Analysis — Expert Cryptanalysis Playbook
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3> **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.
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5## 0. RELATED ROUTING
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7- [symmetric-cipher-attacks](../symmetric-cipher-attacks/SKILL.md) when dealing with modern symmetric ciphers (AES/DES) rather than classical
8- [hash-attack-techniques](../hash-attack-techniques/SKILL.md) when the challenge involves hash-based constructions
9- [lattice-crypto-attacks](../lattice-crypto-attacks/SKILL.md) when knapsack-based ciphers are encountered
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11### Quick identification guide
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13| Observation | Likely Cipher | First Action |
14|---|---|---|
15| All uppercase letters, uneven frequency | Monoalphabetic substitution | Frequency analysis |
16| All uppercase, flat frequency distribution | Polyalphabetic (Vigenere) | IC + Kasiski |
17| Only A-Z shifted uniformly | Caesar/ROT | Brute force 25 shifts |
18| Base64 alphabet (A-Za-z0-9+/=) | Base64 encoded (decode first) | Base64 decode |
19| Hex string (0-9a-f) | Hex encoded (decode first) | Hex decode |
20| Binary (0s and 1s) | Binary encoded | Convert to ASCII |
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