Classical ciphers — overview

Classical ciphers — overview

TL;DR: Pre-computer ciphers split into substitution (Caesar, Vigenere, Playfair, mono-alphabetic) and transposition (rail fence, columnar) families; recognise the family from ciphertext shape, then apply the right break.

What it is

Classical ciphers act on letters, not bits. Substitution ciphers map symbols to other symbols (mono- or poly-alphabetic). Transposition ciphers permute symbol order without changing the alphabet. Both leave statistical fingerprints — letter frequencies, index of coincidence, n-gram distributions — that modern analysis tears apart in milliseconds. They appear in CTFs as warm-ups, in puzzle hunts, and occasionally hidden inside binaries or steganography layers.

Preconditions / where it applies

  • Ciphertext is ASCII letters or digits, length usually 30+ characters (less makes statistics noisy).
  • No headers/magic bytes — first run encoding-base-family decoders to peel encoding layers.
  • Optional crib: a known word (“FLAG”, “CTF”, language hint).

Technique

Triage by quick statistics, then branch:

  1. Index of coincidence (IC): English ~0.067, random ~0.038. IC near 0.067 → mono-alphabetic substitution or transposition. IC near 0.04 → poly-alphabetic (vigenere-cipher, Beaufort).
  2. Letter histogram: matches English-shifted → shift-ciphers (try all 25 Caesar offsets). Matches English but reordered → fixed-substitution-ciphers (use quipqiup or hill-climbing solver). Flat → poly-alphabetic or transposition.
  3. Transposition check: histogram is exactly English but plaintext is unreadable → try rail-fence (2..10 rails), columnar transposition (try key lengths 2..12 sorted by anagram score).
  4. Bigram-aware: digrams like TH, HE, IN map to a single Playfair pair → spot 5x5 grid hints (no J, pairs of letters).
  5. Linear/affine variants: see linear-mapping-ciphers for y = a·x + b mod 26.
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from collections import Counter
def ic(s):
    s = [c for c in s.upper() if c.isalpha()]
    N = len(s); f = Counter(s)
    return sum(n*(n-1) for n in f.values()) / (N*(N-1))

Universal toolkits: CyberChef, dCode, cipher-identifier libraries.

Detection and defence

  • Classical ciphers offer no real confidentiality — never use them for anything but puzzles and CTF flavour.
  • For obfuscation use case (e.g. cookies, config), classical ciphers are trivially reversed offline; even simple frequency analysis on captured tokens breaks them.

References