CPE 645 / CS 685
Applied Cryptography
Calculators

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 remove the key and read the message anyway.

12 items

Run each classical cipher yourself. The next module shows how each one breaks.

Monoalphabeticdemo
Caesar Cipher

Shift every letter by the same number of places. The key is one number, so there are only 25 keys to try.

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Monoalphabeticdemo
General Monoalphabetic Substitution

Replace each letter with another letter, in any order. There are 26! keys, but letter counts still reveal the message.

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Digraphdemo
Playfair Cipher

Encrypt two letters at a time, using a 5x5 square of letters. Pairs hide the frequency of single letters.

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Polygraphicdemo
Hill Cipher

Multiply blocks of letters by a matrix, modulo 26. The cipher is linear, which later makes it easy to break.

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Polyalphabeticdemo
Vigenère Cipher

A keyword chooses a different Caesar shift at each position. The output looks flat, but the keyword repeats.

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Polyalphabeticdemo
Autokey Cipher

After the keyword, the message becomes its own key. The key never repeats, but it is ordinary English.

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Polyalphabeticdemo
Vernam Cipher and the One-Time Pad

Use a random key as long as the message. It cannot be broken if the key is random, secret, and used only once.

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Transpositiondemo
Rail Fence Cipher

Write the message in a zigzag over several rows, then read it row by row. The letters stay the same; only their positions change.

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Transpositiondemo
Columnar Transposition

Write the message into a grid. A keyword decides the order in which you read the columns.

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Compositiondemo
Product Ciphers

Substitute, then transpose, then repeat. This combines confusion and diffusion, the same way DES and AES do.

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Hidingdemo
Steganography

Hide that a message exists, instead of hiding what it says. There is no key, so this adds no secrecy by itself.

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Comparisondemo
Where Each One Breaks

One table for every cipher above: its type, its key, how many keys it has, and the attack that breaks it.

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15 items

Now the keys are gone. Every attack here uses structure that the cipher left in the text. None of them is brute force.

CryptanalysisFrequency analysisdemo
Letter Frequency Analysis

Paste any English text and count every letter. This pattern of counts is what a simple substitution cipher cannot hide.

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CryptanalysisChi-squaredemo
Breaking Caesar

Score all 26 shifts against English letter frequencies. A program picks the best shift, so nobody has to read 26 texts.

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CryptanalysisFrequency analysisdemo
Breaking Monoalphabetic Substitution

Compare the letter counts of the ciphertext with English. Then use pairs, triples, and guessed words to complete the key.

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CryptanalysisDigraph frequencydemo
Breaking Playfair

Single letters look flat here by design. So count letter pairs instead, because pairs are what this cipher encrypts.

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CryptanalysisKnown plaintextdemo
Breaking Hill

The cipher is linear. Two known plaintext blocks give two equations, and the key is C times the inverse of P, modulo 26.

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CryptanalysisPeriod findingdemo
Index of Coincidence

One number answers two questions: which kind of cipher produced this text, and how long its key is.

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CryptanalysisPeriod findingdemo
Kasiski Examination

Find repeated groups of letters, and measure the distances between them. Shared factors of those distances point to the key length.

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CryptanalysisCiphertext onlydemo
Breaking Vigenere

Split the text by the key length. Each part is a Caesar cipher again, so the first attack in this module finishes this one.

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CryptanalysisCrib draggingdemo
Breaking Autokey

There is no repeating key to find. But the key is English, so one correct guess at the beginning reveals the rest.

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CryptanalysisKey reusedemo
Two-Time Pad

XOR two messages that were encrypted with the same pad. The key cancels out, and both messages are still there.

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CryptanalysisTranspositiondemo
Breaking the Rail Fence

The letter statistics still look like English, because the letters did not change. That is how you recognise a transposition.

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CryptanalysisAnagrammingdemo
Breaking Columnar Transposition

Try each possible grid width. For every width, rank all column orders with a score based on letter pairs.

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CryptanalysisCompositiondemo
Breaking a Product Cipher

The letter statistics do not move, and the counts match one substitution used twice. Repeating a weak cipher does not make it strong.

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CryptanalysisDetectiondemo
Steganalysis

There is no key, so reading the message is not the attack. The attack is noticing characters that display as nothing.

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CryptanalysisComparisondemo
Score Card

One table for every cipher above: what it leaked, the attack that read it, and what the attacker needed.

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Randomness and Commitment

Where random numbers come from, and how to commit to a value before anyone knows what it decides.

Block Ciphers

Build a cipher that can be decrypted. Break it on purpose, and measure how far one changed bit travels.

The Data Encryption Standard

Take the real cipher apart: every permutation, every S-box, and the key schedule. Then see what an attacker can still use.

9 items

Run the real cipher one stage at a time. Then measure the three limits that ended it: key length, structure, and block size.

Block ciphersRound functiondemo
One Round of DES

Step through one DES round. Open the round function into its four stages, and change the subkey to see each stage move.

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Block ciphersKey scheduledemo
The DES Key Schedule

Type a key. Watch PC-1 remove the parity bits, the two halves rotate, and PC-2 select the subkey for each round.

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S-boxesDiffusiondemo
Inside a DES S-Box

Send six bits into any of the eight S-boxes. See which bits choose the row and the column, how many inputs share one output, and where those four bits go next.

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DecryptionRound tripdemo
Encrypting and Decrypting a Block

Encrypt the sample block through sixteen rounds, then decrypt it. Then turn off the subkey reversal and watch decryption fail.

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DiffusionAvalanchedemo
Avalanche in DES

Flip one bit of the plaintext, or one bit of the key. Count the differing bits after every round, and compare the two curves.

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Key propertiesWeak keysdemo
Weak Keys and the Complementation Property

Count how many different subkeys a key produces. Find the keys that produce only one, then test the complementation property yourself.

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Multiple encryptionKeying optionsdemo
Double DES and the Triple DES Keying Options

Build double DES and triple DES, stage by stage. Then find the key choices that reduce triple encryption back to single encryption.

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CryptanalysisMeet in the middledemo
Meeting in the Middle on Double and Triple DES

Attack double encryption from both ends. Two separate key searches replace one search over both keys. The page prices the attack in time, memory, and known pairs.

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CryptanalysisTime and data tradeoffdemo
Trading Known Plaintext for Time Against Two-Key TDEA

Guess the 64-bit value in the middle, then run the two key searches it opens. A slider shows how more known pairs reduce the work, and how that compares with the 80 bits NIST credits.

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Block Cipher Modes of Operation

One block is not a message. Encrypt the same picture five ways, and see which modes hide it.

1 item

Hash Functions

Take four hash functions apart, step by step. Then measure what it costs to find a digest.

Diffie–Hellman and Elliptic Curves

Agree on a key in public. Then do the same with points on a curve, and see why going backwards is hard.

The Quantum Threat

See what a qubit does, run Grover's search and the period finding inside Shor's algorithm, and decide which data is at risk now.

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