Course Modules

Pick a module, then the demo you want. Everything runs in the browser.

Classical Ciphers

13 items

Drive each classical cipher yourself, then see what breaks it.

Monoalphabeticdemo

Caesar Cipher

Shift every letter by a fixed amount. One number is the whole key, and the whole keyspace fits on one screen.

Open

Monoalphabeticdemo

General Monoalphabetic Substitution

An arbitrary permutation of the alphabet: 26! keys, and letter frequency still hands it over.

Open

Digraphdemo

Playfair Cipher

Encrypt letter pairs through a 5×5 key square, the first cipher here that flattens single-letter frequency.

Open

Polygraphicdemo

Hill Cipher

A linear cipher that multiplies blocks of letters by a matrix mod 26.

Open

Polyalphabeticdemo

Vigenère Cipher

A repeating keyword picks a different Caesar shift at every position. Flat-looking output, and a period hiding underneath it.

Open

Polyalphabeticdemo

Autokey Cipher

The plaintext extends its own key, so the key stream never repeats. It is also written in English.

Open

Polyalphabeticdemo

Vernam Cipher and the One-Time Pad

A random key as long as the message. Unconditionally secure under stated conditions, and worthless the moment a pad is reused.

Open

Transpositiondemo

Rail Fence Cipher

Write the message in a zigzag across rails and read it off row by row. Same letters, new positions.

Open

Transpositiondemo

Columnar Transposition

Write the message into a grid and read the columns out in the order a keyword dictates.

Open

Compositiondemo

Product Ciphers

Substitute, then transpose, then repeat. Confusion and diffusion together: the shape DES and AES inherit.

Open

Hidingdemo

Steganography

Hide the existence of a message rather than its content. No key, so no security service, but it denies the adversary traffic knowledge.

Open

Comparisondemo

Where Each One Breaks

Type, key, keyspace, and the attack that ends it, side by side for every cipher above.

Open

Cryptanalysisdemoplanned

Breaking Classical Ciphers

Chi-squared, index of coincidence, Kasiski, crib dragging, and pad reuse: the same ciphers with the keys hidden.