Agent skill · research science · yaklang
classical-cipher-analysis
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.
Why this skill is useful
Provides specific techniques and scripts for identifying and breaking classical ciphers that the AI wouldn't reliably generate on its own.
What it needs
About 11k tokens when loaded. Last updated 2026-06-16. 1,563 stars on the source repository.
What this skill does
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 Observation Likely Cipher First Action --- --- --- All uppercase letters, uneven frequency Monoalphabetic substitution Frequency analysis All uppercase, flat frequency distribution Polyalphabetic (Vigenere) IC + Kasiski Only A-Z shifted uniformly Caesar/ROT Brute force 25 shifts Base64 alphabet (A-Za-z0-9+/=) Base64 encoded (decode first) Base64 decode Hex string (0-9a-f) Hex encoded (decode first) Hex decode Binary (0s and 1s) Binary encoded Convert to ASCII Dots and dashes Morse code Morse decode Raised/normal text pattern Bacon cipher Map to A/B, decode 2-digit number pairs (11-55) Polybius square Grid lookup Text appears scrambled (right letters, wrong order) Transposition Anagram analysis Non-printable bytes XOR-like XOR cipher Single/repeating key XOR analysis --- 1. CIPHER IDENTIFICATION METHODOLOGY 1.1 Step 1: Character Set Analysis 1.2 Step 2: Frequency Analysis 1.3 Step 3: Index of Coincidence (IC) 1.4 Step 4: Kasiski Examination (for Polyalphabetic) --- 2. MONOALPHABETIC SUBSTITUTION 2.1 Frequency Analysis Attack 2.2 Known Plaintext (Crib Dragging) If part of the plaintext is known (e.g., "flag{" prefix): --- 3. …
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills yaklang/classical-cipher-analysis