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Ciphers

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Nine ways to hide a message, each one animated a step at a time. Type your own message and press run it. Print the sheets to work on paper.

Rail Fence

Message

Direction

Press Run it to start the machine.

0 / 0 Step time

Result —

How it works

The story

Crack these

Take one apart

Twenty-six of them, each built a step at a time.

Zig-zag the letters down the rails, then read each rail on its own. Nothing is swapped for anything else.

Write the message into a grid, then collect the letters along a secret path: a plow path, a spiral, or straight up the columns.

Key Word Columns

A key word numbers the columns, and the columns come out in that order. Change the key word and the whole cipher changes.

Slide the whole alphabet along by a secret number. Julius Caesar used this one, and it takes a code-breaker about a minute.

The shift changes with every letter, so the same letter rarely looks the same twice. Much harder to break than a plain shift.

Key Word Alphabet

Build a scrambled alphabet out of a key word. Nothing to carry, nothing to lose, and you can rebuild it from memory.

Every letter becomes the shape of the pen it lives in. The page stops looking like writing at all.

Polybius Squares

Turn every letter into two numbers: which row, then which column. A message made of nothing but digits.

Lewis Carroll's Table

A square of 26 alphabets. The key word picks a different one for every letter, which is why it held for centuries.

Letters travel in pairs, so counting single letters tells a code-breaker almost nothing. It lasted into two world wars.

The alphabet backwards, so A becomes Z. No key at all, and scribes were using it thousands of years ago.

Every letter becomes five units of two kinds, which can then be hidden inside an ordinary sentence.

Route with Nulls

A grid where some letters are nonsense on purpose, so even the right path gives a code-breaker noise.

Numbers pointing at words in a book you both own. Nothing to break, because the key is on your shelf.

A fresh random shift for every letter, used once and destroyed. Unbreakable, and almost unusable.

Simple Substitution

Any letter for any letter, with no pattern at all. The strongest one-letter cipher there is, and the hardest to remember.

Shift by one more letter every time. No key word, and no letter ever looks the same twice.

On or off, eight times over, and that is a letter. Everything you have ever typed is this underneath.

The Polybius square sent by knocking on a wall. Nothing to write, and it goes straight through.

A square and a transposition stacked together, with five letters chosen because they sound nothing alike in morse.

Four grids at once, and pairs of letters read across them. Playfair without the awkward X.

Cut every letter into two numbers, mix the numbers with the neighbours, then put them back together.

Homophonic Substitution

Common letters get several symbols each, so counting them tells a code-breaker nothing.

One agreed word standing for a whole sentence. Nothing is scrambled, and there is nothing to break.

Half cipher, half codebook. Important words vanish entirely, and the rest is enciphered letter by letter.

Every cipher, sorted by how it works

This is our list. For a longer one, with an article on each, Wikipedia keeps a category of every classical cipher, and Practical Cryptography explains each with worked examples.

The full category Worked examples and code

Show all 46

Substitution

CaesarSlide the whole alphabet along by a secret number AtbashThe alphabet reversed, so A becomes Z. No key at all Simple substitutionAny letter for any letter, and nothing to remember it by Keyword cipherA key word builds a whole scrambled alphabet PigpenLetters become the walls of the pen they live in Homophonic substitutionCommon letters get several symbols each, to fool the counters

Transposition

ScytaleA rod and a strip: the oldest cipher machine there is Rail fenceZig-zag the letters down the rails, then read each rail Route cipherA grid, and a secret path through it Route with nullsSome cells are nonsense on purpose Columnar transpositionA key word decides the order the columns come out

Coordinate

Polybius squareEvery letter becomes two numbers, a row and a column Tap codeThe same square, sent by tapping on a wall ADFGXA Polybius square with a transposition on top, used in 1918

Polyalphabetic

Alberti diskTurn the wheel mid-message and the cipher changes with it TrithemiusShift by one more letter every time, no key word needed VigenereA key word picks a different alphabet for every letter Date shiftThe date supplies the shifts, and they change every letter

Digraph and fractionation

PlayfairLetters travel in pairs, which kills the counting trick Four-squareFour grids at once, and pairs read across them BifidBreak each letter into coordinates, shuffle them, put them back

Binary and hiding

Bacon's cipherFive units of two kinds, hidden in two typefaces BinaryOn and off, which is the same idea with different names

Codebook systems

Book cipherNumbers pointing at words in a book you both own Code wordsOne agreed word standing for a whole sentence NomenclatorsHalf cipher, half codebook, and they ran Europe for centuries

Mechanical

Cipher disksTwo rings of letters and a pin through the middle

Rotor machinesWheels that step after every letter, so nothing ever repeats

Modern cryptography

One-time padRandom shifts, used once, and provably unbreakable The DIANA tableThe one-time pad as US Special Forces actually used it

XORThe same operation hides a message and reveals it

AESWhat actually protects your messages today

RSATwo keys: one you hand out, one you never show anyone

Diffie-HellmanTwo people agree a secret while everyone is listening

HashingA fingerprint for a message, easy forwards, impossible back

Digital signaturesProof of who sent it, which no cipher gives you

When there is no code

Codes that were never thereShakespeare, the Bible code and pi: patterns nobody put there PiThe digits run forever and say nothing at all

Machines and code-breaking

Can a computer break it?What a laptop ends in a second, and the two that hold Artificial intelligenceVery good at the ciphers here, no use at all against AES

Cryptanalysis

Frequency analysisCount the letters: E, then T, then A, then O, then N Pattern analysisA four-letter word shaped X Y Z X is usually THAT Brute forceTry every key, when there are few enough keys to try Known plaintextGuess a word that must be in there, and pull the thread Cipher identificationNumbers, groups of four, or shapes: what am I even looking at? KeyspaceHow many keys exist, and why that number decides everything

Hidden in plain sight

Some messages are hidden by scrambling them. Others are hidden by looking completely ordinary.

Read more elsewhere

Longer, harder and written for grown-ups. Worth a look when this page runs out.

Every classical cipher Worked examples and code