Daniel Boudreau · Enigma orrery

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 is a plain substitution: hold the rotor still and the same key always lights the same lamp.

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

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 carries the rotor to its left on 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 itself every 26 letters: letters 26 apart share one substitution, so enough ciphertext falls to plain frequency counting. Enigma's three rotors stretch that cycle to 16,900 letters. Modern stream ciphers such as ChaCha20 build their keystream from a 256-bit key and a fresh nonce, and it never repeats within a message.

Stage II · the key sheet

Read the intercept

The full machine adds three things. A plugboard swaps pairs of letters (wartime keys used ten pairs), on the way in and again on the way out. Three rotors, chosen 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, not 17,576). And a reflector at the far end sends the current back through the rotors by another 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):

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

Because of the reflector the machine is 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; tap Machine key if it is 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 against exactly that.

Hints

DefenceThe whole net rested on paper that could be lost, copied or captured, and one captured sheet could open the rest of that month's traffic. Modern protocols agree a fresh key for every session over the network itself (TLS 1.3 uses an ephemeral Diffie-Hellman exchange), so there is no sheet to steal, and a stolen long-term key does not unlock past sessions.

Stage III · the message key

A fresh start for every message

If every message of the day started from the Grundstellung, all of them would run through the same rotor positions, letter for letter. Stack them up and each column is enciphered the same way. Codebreakers call that depth, and it is a gift.

So each operator chose 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 was:

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 chose 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. Modern protocols send the nonce in the clear, because it need not be secret (AES-GCM takes a 96-bit nonce that must never repeat under one key), and they catch errors and 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

No letter is ever itself

The reflector is what makes Enigma its own inverse, and it comes at a price. 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. Wherever a crib letter sits on the same ciphertext letter, a crash, that position is impossible.

Your task, step by step

This weather report came from another net, under a key you do not have. Weather messages kept to a fixed pattern, and 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 cannot 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, improved by Gordon Welchman: an electromechanical machine that searched for the rotor settings consistent with a crib. 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 and destroyed after use; the days ran from the end of the month down, so each used line could be cut off. 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 at once, which is why they were guarded so closely.

DefenceModern ciphers leave no such tell. In AES nothing forbids a block from encrypting to itself, and knowing some plaintext does not help an attacker find the key. Keys are not printed for a month: they are 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

No letter is ever itself

Why it mattered

The machine

Lamps, keys and the pad