Intercept #2

Convoy in Sight

The intercept

North Atlantic, 1942. A lookout has a convoy in sight, and the radio room sends a contact report back to headquarters. Read the sheet carefully before you touch any dials. This one isn’t set up quite the same way as the last transmission.

Transcription

Netz: Kriegsmarine – U-Boot-Funknetz Atlantik
Datum: 9. Oktober 1942
Walzenlage: Beta – III – I – IV
Ringstellung: A M N B
Umkehrwalze: B
Steckerverbindungen: AD CN ET FL GI JV KZ PU QY WX

Message key: VQRS

Ciphertext

TAWKP GTBVK XTJLT KPWPJ LGXZQ OCJCE ZFFZM TBGTN HDXNJ

Notes

Read every entry under Walzenlage and Ringstellung before you set up the machine. Everything you need to configure it is on the sheet.
The grid square in the plaintext (AJ) refers to the real Kriegsmarine Quadratkarte naval grid system, which divided the ocean into lettered squares for exactly this kind of contact report. The specific square and course here are invented for this exercise, not lifted from a real sighting.

Hint

Click for a hint

Walzenlage lists four rotors, starting with the Greek rotor Beta, and there are four ring settings. That means the Kriegsmarine’s four-rotor M4, not the machine from Challenge 1.

On the Enigma touch:

Modell: M4.
Rotor: reflector B and Greek rotor β in the leftmost window, then III I IV.
Ring: A M N B. The leftmost window is the Greek rotor’s ring. The M4 uses letters, not the Enigma I’s numbers.
Leave setup mode and turn the rotors to the message key V Q R S with the sprockets. The Greek rotor is the leftmost one. It never moves while you type, but its position still counts.

On a software simulator, pick the M4 machine type and the same settings.

Solution

Click to reveal the solution

Plaintext: GEGNERKONVOIGESIQTETQUADRATAJVIERSEQSKURSWEST. It reads “Gegner Konvoi gesichtet, Quadrat AJ vier sechs, Kurs West” (“Enemy convoy sighted, square AJ46, course west”). Two of the manual’s message-prep rules (§2.3) show up here. The letter pair ch is written as q (gesichtet → GESIQTET, sechs → SEQS), and digits are spelled out as words (VIER = 4, SEQS = 6). At 45 letters it’s already a multiple of five, so no padding is needed.

Full trace

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

This board uses 10 plugboard pairs, which was typical Kriegsmarine practice. 20 of the 26 letters are swapped; B, H, M, O, R, and S pass through unplugged.

mediumconvoy-reportatlantic

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.