Intercept #4

Armor Response

The intercept

Eastern Front, three days after the reconnaissance report that opened this campaign. The armor sighted east of Smolensk has been confirmed, and an Army station answers with orders of its own. Once again the operator has chosen his own starting position and sent it disguised at the head of the message — but this time there’s no second half to check it against.

The day’s key sheet (shared by every station on the net)

Transcription

Netz: Heer – Panzerabwehr Ost
Datum: 17. Juli 1943
Walzenlage: I – IV – II
Ringstellung: 05 17 15
Umkehrwalze: B
Steckerverbindungen: AF BQ CV DP EU GH IK LM NX TZ

Message header (sent ahead of the message): WFT KOO

Ciphertext

YTLES OSLPX VIKJH MNXSI WFHWY ZEDIA NNFPQ MAYQU BIVYW HLKXZ KUWSV

The procedure

This is the procedure the Enigma touch manual (§2.3) itself walks through — the version used from 1 May 1940 onward. Unlike an earlier procedure, the message key here was only ever encrypted once.

Set up the machine from the key sheet: Modell, then Rotor for the reflector and rotor order, then Ring, then the plugboard.
The first three letters of the header, WFT, were sent in the clear. They’re the operator’s own choice of home position (Grundstellung) for this message, not part of the day’s key. Leave setup mode and turn the rotors to WFT.
Type the next three letters, KOO. This decrypts once into the operator’s chosen message key — there’s no second copy to check it against, unlike the older doubled procedure.
Turn the rotors to the recovered 3-letter message key (not WFT) and decrypt the ciphertext.

Hint

Click for a hint

Three rotors and ring settings written as numbers mean Modell I. Convert the header’s home position, and the message key you recover from it, to the Enigma I’s numbered rotor positions (A = 01 … Z = 26, or read them off the plugboard sockets). WFT is 23 06 20.

There’s no matching second half to confirm the recovered key this time — if the rotors look wrong after step 4, the mistake is almost always in the key sheet setup, not the header.

Solution

Click to reveal the solution

Recovered message key: MDK (13 04 11 on the Enigma I). At WFT, the three letters KOO decode to MDK.

Plaintext: PANZERJAGERSOFORTNAQSMOLENSKVERLEGENXMUNITIONANFORDERNX. It reads “Panzerjäger sofort nach Smolensk verlegen. Munition anfordern.” (“Move anti-tank unit to Smolensk immediately. Request ammunition.”) Turn the rotors to MDK and decrypt the ciphertext to get it. Three of the manual’s message-prep rules (§2.3) show up here:

ä is written as a (Panzerjäger → PANZERJAGER).
The pair ch is written as q (nach → NAQ).
The two closing periods both become X, the second one also padding the message to a multiple of five.

Full trace

With the day’s key dialed in, the rotors at MDK, and the ciphertext typed in, the lamp field lights letter by letter:

Y->P   T->A   L->N   E->Z   S->E
O->R   S->J   L->A   P->G   X->E
V->R   I->S   K->O   J->F   H->O
M->R   N->T   X->N   S->A   I->Q
W->S   F->M   H->O   W->L   Y->E
Z->N   E->S   D->K   I->V   A->E
N->R   N->L   F->E   P->G   Q->E
M->N   A->X   Y->M   Q->U   U->N
B->I   I->T   V->I   Y->O   W->N
H->A   L->N   K->F   X->O   Z->R
K->D   U->E   W->R   S->N   V->X

Why no doubling this time: after 1 May 1940 the Wehrmacht dropped the doubled message key entirely. Encrypting the key twice had been meant to catch garbled transmissions — but it also gave every intercept a fixed relationship between the 1st and 4th letters of the indicator, which is exactly what let Rejewski’s team recover daily settings in the 1930s (see Challenge 3). The fix removed that structure by encrypting the key only once. The tradeoff was real: without a second copy to check against, a single mis-copied letter in the header now meant the whole message had to be sent again.

mediumindicator-proceduremessage-keyarmy-order

Solvable on: Enigma Touch, Physical Enigma replica, cryptii.com Universal Enigma — set the model to match the key sheet.

Reading the key sheets

The sheets use the German column names of real key sheets. Walzenlage is the rotor order, Ringstellung the ring settings, Umkehrwalze the reflector, andSteckerverbindungen the plugboard connections. Netz andDatum are the radio net and the date.

Rotors and ring settings are listed left to right, in the same order you enter them in the Enigma touch's Rotor and Ring setup modes. Plugboard pairs have no order: swapping the two ends of a cable changes nothing.

The Enigma I labels its ring settings and rotor positions with numbers (A = 01 … Z = 26). Message keys are still written as letters, so convert them before turning the rotors. The plugboard sockets are labelled with both.

The Kriegsmarine's four-rotor M4 has a fourth, thin "Greek rotor", β (Beta) or γ (Gamma). It is listed first under Walzenlage and has its own ring setting and rotor position. It never moves while you type. The reflector letter (B or C) is written the same way on every machine.

Enigma doesn't distinguish between encryption and decryption. Type the ciphertext in with the rotors at the message key and the plaintext lights up in the lamp field. Spaces aren't part of the Enigma's alphabet, so add them back once the message is decoded.