TEC‑01 the enigma challengeenigma touch first, every simulator welcome
Intercept #6

Rail Movement

The intercept

Eastern Front, Saturday 24 July 1943, a week after the anti-tank unit was ordered to Smolensk and asked for ammunition. The request has been passed back along the supply chain, and an Army supply net answers it. You are the cipher clerk at the receiving station, which works two Army nets, and a message has just come in.

Station log: received 24 July, 09:21.

Message

0915 = 1tle = 1tl = 085 = RZV GUJ = HLPTN
VWXVQ EQSUQ CRBQY SVERS OOFLV YAVTD CREWE PVQCX
EYTGV VOCUS PRJFN MPSGJ QSHDB DFMSO FOQUQ OWRGI

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, HLPTN, 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, one for each of your two nets, are in the Schlüsseltafeln panel on this page. Find the sheet and day 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, HLPTN, 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, RZV. Type the three letters GUJ. 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: HLPTN is not part of the enciphered text.

Hint

Click for a hint

The Kenngruppe names the row, so it also tells you which of your two nets sent the message. You don’t need to guess the net or trust the date on the log: look the group up.

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 RZV is 18 26 22.

Ten plugboard pairs is a lot of cables. If the recovered message key looks like nonsense, or the decrypted text does, check the plugboard first. One wrong pair garbles everything.

Solution

Click to reveal the solution

Key used: Heer – Kormoran, 24 July 1943. PTN is one of that day’s four Kenngruppen (FBV PTN GZU SRB). The log date and the Kenngruppe agree this time, and the group shows the message came in on the Kormoran net, not Rebhuhn.

Recovered message key: HBT (08 02 20 on the Enigma I). At RZV, the three letters GUJ decode to HBT.

Plaintext: MUNITIONSZUGTRIFFTSONNTAGFRUHSMOLENSKEINXENTLADUNGSOFORTXPANZERJAGEREINWEISENXXX. It reads “Munitionszug trifft Sonntag früh Smolensk ein. Entladung sofort. Panzerjäger einweisen.” (“Ammunition train arrives at Smolensk Sunday morning. Unload at once. Brief the anti-tank unit.”) Turn the rotors to HBT and decrypt the ciphertext, skipping HLPTN, to get it. Three of the manual’s message-prep rules (§2.3) show up here:

  • ü and ä are written as plain u and a (früh → FRUH, Panzerjäger → PANZERJAGER).
  • Each sentence ends in X for its period.
  • The last XX is padding to a multiple of five letters.
Full trace

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

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

Where it leads: the train is due on Sunday 25 July, and the anti-tank unit now has a place to meet it. The Soviet offensive against the Smolensk sector opened on 7 August, two weeks later.

mediumkey-sheet-lookupkenngruppeindicator-proceduresupply

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