Chris Spackman's NeoCities Page

Cryptanalysis: Breaking Ciphers

The traditional cryptography pages show how to use ciphers — how to encrypt and decrypt messages. These pages show the other side: how to break them. This is cryptanalysis, and it is at least as old as the ciphers themselves.

The techniques here come from a long tradition of practical codebreaking. Parker Hitt's 1916 Manual for the Solution of Military Ciphers — the first American military cryptanalysis textbook — laid out many of these methods in clear, practical terms. William Friedman later formalized them mathematically. But the core idea is simple: every cipher leaks information, and a careful analyst can exploit those leaks.

The Attacker's Toolkit

Every attack starts with a question: what kind of cipher am I looking at? The answer determines which tools to use.

• Frequency analysis — count how often each letter appears. If the distribution looks like English (one letter around 13%, others clustered in predictable ways), you are looking at a monoalphabetic substitution or a transposition cipher. If the distribution is flat, it is polyalphabetic.
• Index of Coincidence — a single number that measures the "roughness" of the frequency distribution. English text: ~0.067. Random text: ~0.038. This number tells you immediately whether you are dealing with substitution (IC near English) or polyalphabetic (IC near random).
• Digraph and trigraph analysis — common letter pairs (TH, HE, IN) and triples (THE, AND, ING) give much stronger clues than single letters. If the text has English-like digraph frequencies but is unreadable, it is probably a transposition cipher.
• Kasiski examination — for polyalphabetic ciphers, repeated sequences in the ciphertext reveal the key length. Once you know the key length, each column becomes a simple Caesar cipher.
• Chi-squared testing — a statistical test that measures how well a frequency distribution matches English. Try all 26 shifts; the one with the lowest chi-squared score is probably correct.

The Pages

1. Breaking Substitution Ciphers — frequency analysis, pattern recognition, and an interactive solver where you build a letter mapping to decode the message one letter at a time.
2. Breaking the Vigenère Cipher — a guided 5-step attack: detect the cipher type with the IC, find the key length with Kasiski, split into columns, break each column, and recover the plaintext.
3. Breaking Transposition Ciphers — brute-force rail fence breaking and columnar transposition column-reordering using digraph frequency scoring.

Section Glossary — definitions of terms used on these pages.

How to Use These Pages

The best way to use these pages is alongside the cipher pages. Go to a cipher page, encrypt a message, then come back here and try to break it. When you see how the attack works against a cipher you understand, the techniques make much more sense.

Each page also has pre-loaded challenge texts if you just want to practice breaking things.