CPE 645 / CS 685
Applied Cryptography

Course Modules

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

Classical Ciphers

Build each cipher by hand, then take the keys away and read the message anyway.

12 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

15 items

Now take the keys away. Every attack here reads structure the cipher left behind, and not one of them is brute force.

CryptanalysisFrequency analysisdemo
Letter Frequency Analysis

Paste any English paragraph and count every letter. The shape you get is the shape a substitution cipher cannot hide.

Open
CryptanalysisChi-squaredemo
Breaking Caesar

All 26 shifts scored against English letter frequency, so a loop picks the winner instead of a person.

Open
CryptanalysisFrequency analysisdemo
Breaking Monoalphabetic Substitution

The English histogram beside the ciphertext histogram, then singles, digraphs, trigraphs, and cribs.

Open
CryptanalysisDigraph frequencydemo
Breaking Playfair

Single letters are flat by design, so count the pairs the cipher actually operates on.

Open
CryptanalysisKnown plaintextdemo
Breaking Hill

The cipher is linear, so two known blocks are two equations: K = C·P inverse, mod 26.

Open
CryptanalysisPeriod findingdemo
Index of Coincidence

One statistic doing two jobs: which family of cipher produced a text, and then how long its key is.

Open
CryptanalysisPeriod findingdemo
Kasiski Examination

Repeated n-grams, the distances between them, and a vote on their factors scored as excess over chance.

Open
CryptanalysisCiphertext onlydemo
Breaking Vigenere

Split by the period and each stream is a Caesar cipher again, so the first attack of the module finishes this one.

Open
CryptanalysisCrib draggingdemo
Breaking Autokey

No period to count, but the key stream is English: one correct guess at the opening unzips the rest.

Open
CryptanalysisKey reusedemo
Two-Time Pad

XOR two ciphertexts sent under one pad and the key cancels, leaving both plaintexts still in there.

Open
CryptanalysisTranspositiondemo
Breaking the Rail Fence

Index of coincidence and chi-square both come back saying English. That is the detection, not a failure.

Open
CryptanalysisAnagrammingdemo
Breaking Columnar Transposition

Candidate widths from the divisors, then every one of the m! orderings ranked by a bigram score.

Open
CryptanalysisCompositiondemo
Breaking a Product Cipher

Index of coincidence and chi-square at every stage, unmoved, and letter counts identical to one substitution applied twice.

Open
CryptanalysisDetectiondemo
Steganalysis

No key, so recovering the message is not the attack. Counting the code points that render as nothing is.

Open
CryptanalysisComparisondemo
Score Card

What leaked, the attack that read it, and what the attacker had to hold, side by side for every cipher above.

Open
Static site. Every demo computes in your browser, and nothing you type is sent anywhere.