classical-cipher-analysis

A classical cipher analysis playbook aimed at CTF challenges, emphasizing that the first job is correct identification before attempting to break anything. It provides a quick-identification guide that maps observable traits (all-uppercase with uneven frequency, flat frequency distribution, uniform letter shifts, Base64/hex/binary alphabets, Morse dots and dashes, digit pairs, scrambled letters, or non-printable XOR-like bytes) to a likely cipher and a first action, and warns that encoded ciphertext such as Base64 or hex must be decoded before cryptanalysis.

The identification methodology is spelled out in code: character-set analysis to detect encodings, letter-frequency analysis against English norms, the Index of Coincidence to distinguish monoalphabetic (IC around 0.065) from polyalphabetic (IC around 0.038) ciphers, and Kasiski examination to recover a Vigenere key length from repeated-sequence distances. From there it gives breaking techniques for the major classical families: monoalphabetic substitution via frequency mapping and known-plaintext crib dragging (for example a known flag prefix), and Caesar/ROT via 25-shift brute force scored with chi-squared against English frequencies.

Use it when a challenge presents substitution, Vigenere, transposition, XOR, or otherwise encoded text and you need a systematic path from raw ciphertext to plaintext. Per its load instruction, the skill also covers Vigenere, Enigma, affine, Hill, transposition, and Bacon/Polybius/Playfair ciphers alongside XOR (single and repeating key). It notes automated aids such as quipqiup.com and dcode.fr for substitution solving, and routes to related skills for modern symmetric ciphers (AES/DES), hash-based constructions, and lattice or knapsack attacks when the challenge falls outside the classical domain.