Enigma orrery · dboudreau.dev

Stage I · one rotor

A wheel of wires

A rotor is a disc with 26 contacts on each face. Inside, 26 wires join every contact on one face to a contact on the other in a fixed, scrambled order. On its own that's a plain substitution: hold the rotor still and the same key always lights the same lamp.

But it never holds still. Every key press turns the rotor one place first and only then sends the current through, so each letter meets the wiring at a new angle. Press the same key twice and you get two different lamps.

This stage uses a one-rotor teaching rig. In the drawing, the keys are the outer ring, the rotor is the wide ring with the wires, and the lamps are the inner ring. The small triangle on the rotor is its notch. In the full machine, when the notch letter shows in the window, the next key press also steps the rotor to its left by one place.

Your task, step by step

Press Set up the rig, just under these steps. The drawing (to the right of this cardbelow this card) then shows rotor I with A in its window, at the top.
Type ROTOR on the keyboard further down the page, or on your own keyboard. Or press Type ROTOR for me.
Watch the strip under these steps fill in: each key you type, and the lamp it lit.
Copy the lit letters, in order, into the answer box below and press Check.

Since the page opened: nothing typed yet

Typed

Lit

Nothing typed yet.

Watch for itROTOR has two R's and two O's. In the strip, each repeated letter lights a different lamp every time, because the rotor turned in between.

Hints

DefenceA lone rotor repeats every 26 letters, so letters 26 apart share one substitution and enough ciphertext falls to plain frequency counting. Enigma's three rotors stretch that to 16,900 letters. A modern stream cipher like ChaCha20 builds its keystream from a 256-bit key and a fresh nonce, and that keystream doesn't repeat within a message.

Stage II · the key sheet

Read the intercept

The full machine adds three things. A plugboard swaps pairs of letters on the way in and again on the way out (wartime keys used ten pairs). Three rotors, picked from five, turn almost like the wheels of a mileage counter. The middle rotor double-steps, so the cycle is 26 × 25 × 26 = 16,900 letters instead of 17,576. And a reflector at the far end sends the current back through the rotors by a different route.

Sender and receiver had to set their machines identically, so every net shared a printed key sheet with a line for each day. This one follows the layout used until September 1938, when the day's start position was part of the key (it borrows the two extra rotors issued that December). The columns:

Walzenlage: which rotors, in which order, left to right
Ringstellung: where each alphabet ring sits on its wiring, 01 to 26 (01 is A)
Steckerverbindungen: the plugboard pairs
Grundstellung: the start position: the letters to turn into the windows

The reflector makes the machine its own inverse: set it the same way, type the ciphertext, and the plaintext lights up.

Key sheetSynthetic · made for this page

Tag	Walzenlage	Ringstellung	Steckerverbindungen	Grundstellung

Your task, step by step

The intercept's header says it was sent on day 23. Find that day's line on the key sheet above.
Open the Machine key panel under the drawing (to the right of this cardbelow this card, and tap Machine key if it's folded). Set Walzenlage (left, middle, right), Ringstellung and Steckerverbindungen to that line. Leave the reflector on B.
In the same panel, turn the three Window letters to that line's Grundstellung.
Press Type the intercept in, under the intercept above. The page presses every key for you.
Read the output in the machine check below. X stands for a space between words, and the key phrase comes after KEY and PHRASE. Enter it in the box below and press Check.

Machine checkupdates as you work

Output

A simplificationHere the whole message starts from the day's Grundstellung, so every message sent that day would share one start. Stage III adds the step the real procedure used to avoid that.

Hints

DefenceThe whole net rested on paper, and one captured sheet opened a month of traffic. TLS 1.3 agrees a fresh key for every session using ephemeral Diffie-Hellman, so there's no sheet to steal and a stolen long-term key can't read past sessions.

Stage III · the message key

A fresh start for every message

If every message of the day started from the Grundstellung, they'd all run through the same rotor positions, letter for letter. Stack them up and every column is enciphered the same way. Codebreakers call that depth, and it's exactly what they want.

So each operator picked a new start for every message, the message key, and sent it hidden in the indicator at the head of the message. Until September 1938 the Army and Air Force procedure went like this:

Set the day's key, with the Grundstellung in the windows.
Type your chosen message key twice, for example RTZRTZ. The six letters that light are the indicator.
Turn the windows to the message key itself and encipher the text.

The receiver works it backwards: Grundstellung in the windows, type the indicator, read the message key twice, turn the windows to it, then type the text.

The procedure changed over the years. From September 1938 operators picked the start for the indicator too and sent it in the clear, and from May 1940 the message key was typed only once.

Indicator

Your task, step by step

This intercept is from day 23 too. Keep that day's Walzenlage, Ringstellung and Steckerverbindungen in the Machine key panel under the drawing (to the right of this cardbelow this card). The machine check below ticks each one.
In the same panel, turn the three Window letters to the day's Grundstellung.
Press Type the indicator in, under the intercept above. Six lamps light: one group of three letters, twice. That group is the message key.
Turn the three Window letters to the message key.
Press Type the text in. Read the output in the machine check below: the key phrase comes after KEY and PHRASE. Enter it in the box below and press Check.

Machine checkupdates as you work

Output of the text

Hints

DefenceThe message key is the ancestor of the nonce: a fresh value for every message, so no two share a keystream (people still reuse nonces anyway lol). Modern protocols send the nonce in the clear (AES-GCM's 96 bits must never repeat under one key) and catch tampering with an authentication tag instead of repeating anything. The doubled message key was the opening Marian Rejewski used to break the machine.

Stage IV · the reflector

A letter never enciphers to itself

The reflector is what makes Enigma its own inverse, and it has a cost. The current has to come back by a different wire than it went in, so a letter can never encipher to itself. At any position the whole machine is just 13 swapped pairs. Turn on Show all 13 pairs under the drawing to see them.

Codebreakers turned that into a sieve. Take a crib (a word you expect in the message) and slide it along the ciphertext. Any position where a crib letter lands on the same ciphertext letter is a crash, and a crash rules that position out.

Your task, step by step

This weather report came from another net, under a key you don't have. Weather messages kept to a fixed pattern, so the analyst expects WETTERVORHERSAGE (weather forecast) to start at one of the first six letters.

Look at the strip just below this card. The top row is the start of the intercept, the bottom row is the crib, and the small numbers mark the six possible starts.
Slide the crib one letter at a time with the ◀ and ▶ buttons under the strip, or drag the slider between them.
At each position, look for a crash: a crib letter directly under the same ciphertext letter, marked in red. The line under the slider names it.
Enter the only position with no crash in the Position box below and press Check.

Example: at position 1 the crib's fourth letter, T, sits under a T. No Enigma setting turns T into T, so the crib can't start there.

Ciphertext, with the start positionsCrib below

Hints

Why it mattered

The Polish Cipher Bureau broke Enigma first. At the end of 1932 Marian Rejewski worked out the rotor wiring by mathematics, using the doubled message keys and German documents obtained by French intelligence. In July 1939 the Poles shared their methods with Britain and France.

At Bletchley Park, Alan Turing designed the bombe, an electromechanical machine that searched for the rotor settings consistent with a crib. Gordon Welchman improved it. Stock phrases in weather reports were favourite cribs, and the rule you just used placed them before a bombe ran.

The keys were the real secret. Key sheets were printed a month at a time, with the days running from the end of the month down. Each used line could then be cut off and destroyed. Every day brought a new Walzenlage, Ringstellung and plugboard, and ten plugboard cables alone allow 150,738,274,937,250 pairings. Captured sheets and codebooks let Bletchley read traffic straight away, which is why they were guarded so closely.

DefenceModern ciphers don't have a weak spot like that. AES has no rule against a block encrypting to itself, and knowing some plaintext doesn't help an attacker find the key. Keys aren't printed a month ahead: they're generated fresh, agreed over the network or kept in hardware, and changed without anyone carrying paper.

A wheel of wires

Read the intercept

A fresh start for every message

A letter never enciphers to itself

Why it mattered

The machine

Lamps, keys and the pad