Midnight Traffic

The intercept

Western Front, mid-June 1940. You are the cipher clerk at an Army signals station. Your job is the half of the work that comes after the radio: a message arrives, and before anyone can read it you must find the right key sheet, set the machine, and take it apart.

The dispatcher hands you a slip and the station log entry for it.

Station log: received 18 June, 00:06.

Message

2358 = 1tle = 1tl = 085 = MRF KHD = RHDWY
MAKIP IEMNY EHXKB OODNQ NGURE GVTHQ REYZM CWFXZ
OQJBT LKTFK HVYLP LJUSE FCWYB BWCPR KVIUY FVSHI

The first line is the message header: the time the message was made ready, that it is the first part of one (1tle = 1tl), and its length in letters (085, counting the first group). Then comes the operator’s home position (Grundstellung) and his message key (Spruchschlüssel), enciphered. The last item, RHDWY, is the first group of the message (the Buchstabenkenngruppe), and it was not enciphered. The rest is the ciphertext.

The key sheets you hold for this month are in the Schlüsseltafeln panel on this page, one for the Army net and one for the Air Force net your station also works with. Pick the sheet and day that this message needs, or type a Kenngruppe into the Kenngruppe box to find its row. (Both sheets are invented for this site; they are modelled on the layout of real ones.)

The procedure

This is the procedure for Army and Air Force traffic from 1 May 1940. The Enigma touch manual (§2.3) describes it.

1. The first group, RHDWY, tells you which key was used. Its last three letters are one of the four Kenngruppen printed on a day’s row of a sheet. Find that row in the Schlüsseltafeln panel.
2. Set up the machine from that row with the setup modes: Modell, then Rotor for the reflector and rotor order, then Ring, then the plugboard.
3. Leave setup mode and turn the rotors to the home position from the header, MRF. Type the three letters KHD. This decrypts them into the operator’s message key.
4. Turn the rotors to the recovered message key and decrypt the ciphertext. Start after the first group: RHDWY is not part of the enciphered text.

Hint

Click for a hint

Two clocks are in play, and neither one chooses the key. The log gives the day the message arrived; the header gives the time it was made ready. The Kenngruppe names the row, whatever the date on the log.

Three rotors and ring settings written as numbers mean Modell I, whose rotor positions are numbered rather than lettered (A = 01 … Z = 26). The home position MRF is 13 18 06.

You can check the key you recover: a wrong sheet or a wrong day gives a message key that looks perfectly ordinary, and only the decrypted text tells you it was wrong.

Solution

Click to reveal the solution

Key used: Heer – Eisvogel, 17 June 1940. DWY is one of that day’s four Kenngruppen (IHQ LBI DWY IIA). The station log says 18 June, but the message was made ready at 23:58 and was keyed on the 17th; the 00:06 on the log is when it reached you.

Recovered message key: DZK (04 26 11 on the Enigma I). At MRF, the three letters KHD decode to DZK.

Plaintext: BRUCKEZWOKILOMETEROSTLIQDESORTESINSTANDGESETZTXNAQSQUBKOLONNENABVIERUHRFREIXXXXX. It reads “Brücke zwo Kilometer östlich des Ortes instandgesetzt. Nachschubkolonnen ab vier Uhr frei.” (“Bridge two kilometres east of the town repaired. Supply columns clear from four o’clock.”). Turn the rotors to DZK and decrypt the ciphertext, skipping RHDWY, to get it. The manual’s message-prep rules (§2.3) show up here:

• ü is written as u (Brücke → BRUCKE).
• The pair ch is written as q (östlich → OSTLIQ, Nachschub → NAQSQUB).
• The digit is spelled out: two is ZWO, four is VIER.
• The sentence ends with X, and four more X pad the text to a multiple of five.

What the wrong day gives

Trust the log’s date and use the 18 June row of Heer – Eisvogel (rotors II IV V). The header MRF KHD then decrypts to the message key BGL, which looks as good as any other. Set the rotors to BGL and the start of the ciphertext comes out as NLEVXRSFYWJJIFJ, which is not German.

Before May 1940 the doubled message key would have caught this: the two halves would not have matched. With the key enciphered only once, nothing on the page objects, and the only check left is whether the result is German.