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: III – I – IV
Ringstellung: M N B
4. Walze (fest): Gamma Ring A Stand V
Umkehrwalze: B-thin
Steckerverbindungen: AD CN ET FL GI JV KZ PU QY WX
Anfangsstellung: Q R S

Ciphertext

ITADL XVZXT FLBFF NJYPZ LLPQV GMROZ LGCEK LSSUS IHHMA RB

Notes

Read every row of the sheet, including the reflector’s exact name, before you set up the machine. Everything you need to configure it correctly is right there.
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

Besides the three rotors under Walzenlage, the sheet has a separate “4. Walze (fest)” row, and the reflector is B-thin instead of plain B. Together those mean the Kriegsmarine’s four-rotor M4, not the three-rotor machine from Challenge 1. On the Enigma Touch, select M4 mode before dialing anything in; on a software simulator, pick the M4 machine type.

The fourth wheel sits between the leftmost rotor and the reflector. It adds its own fixed substitution but never steps. Only two such wheels existed, Beta and Gamma, and either one fits only alongside a thin reflector (B-thin or C-thin), never a full-width one.

If your simulator or Enigma Touch is decrypting to gibberish and you have selected M4 mode, the next most common mistake is entering the 4th wheel’s ring/position in the wrong field, or mixing it up with the third rotor’s settings.

Solution

Click to reveal the solution

Plaintext: GEGNERKONVOIGESICHTETQUADRATAJVIERSECHSKURSWEST — “Gegner Konvoi gesichtet, Quadrat AJ vier sechs, Kurs West” (“Enemy convoy sighted, square AJ46, course west”). Digits are spelled out as words (VIER = 4, SECHS = 6) rather than typed as numerals, since the machine’s keyboard only has letters.

Full trace

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

This board also 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 their original German labels. Walzenlage is the rotor order,Ringstellung the ring settings, Umkehrwalze the reflector, andSteckerverbindungen the plugboard pairs. Grundstellung orAnfangsstellung is the rotor starting position, and Netz/Datumare the radio net and date.

Rotors are listed left to right as installed in the machine, so the last one is the fast rotor nearest the keyboard. Ring settings and starting positions are given the same way, one letter per rotor. Plugboard pairs are unordered, so swapping the two ends of a cable changes nothing.

Enigma is reciprocal: the same settings that encrypted a message will decrypt it. Type the ciphertext in with the rotors dialed to the message's actual starting position and the plain text comes out the lampboard — there's no separate "decrypt mode" to find.

The Kriegsmarine's four-rotor M4 adds a thin wheel, Beta or Gamma, listed on a sheet as 4. Walze (fest). It sits between the leftmost rotor and a thin reflector (B-thin or C-thin). It never turns; it only adds one more fixed substitution to the signal path.

Spaces and punctuation aren't part of the alphabet the machine encrypts, so ciphertext and plaintext are both given as unbroken letter groups; add the spaces back in once decoded.