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Intercept #7

Before the Barrage

The intercept

Eastern Front, the night of Saturday 31 July 1943. The ammunition train has been and gone, and the anti-tank unit is dug in east of Smolensk. A week from now the Soviet offensive will open. Tonight a forward observation post reports on what it can hear and see across the lines, and the report is passed back on the net you serve: the same Army net that carried the supply message. You are the cipher clerk at the receiving station, and a message has just come in.

Station log: received 1 August, 00:04.

Message

2352 = 1tle = 1tl = 135 = LUC EFE = KWTGA
ZYBCE UEGSS KXSEO NXJEK AATKV VLMUZ KJVBJ UUIWO
KMTTO OIGFV GRJPN XYZGT IBCMI MIAOU PGKEV BMEFH
PYVPD DTBBK QIVUC ZALQL QNVVJ PSEJP DKUNN TFTHM
EZVZD OOZOH

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 (135, counting the first group). Then comes the operator’s home position (Grundstellung) and his message key (Spruchschlüssel), enciphered. The last item, KWTGA, is the first group of the message (the Buchstabenkenngruppe), and it was not enciphered. The rest is the ciphertext.

The key sheets you hold are in the Schlüsseltafeln panel on this page: this net’s sheets for July and for August. 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, KWTGA, 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, LUC. Type the three letters EFE. 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: KWTGA is not part of the enciphered text.

Hint

Click for a hint

The message was made ready eight minutes before midnight and reached you four minutes after it. Between those two moments the month changed, and the Kenngruppe names the row whatever the date on the log says.

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 LUC is 12 21 03.

A wrong row still gives a message key that looks perfectly ordinary. Only the decrypted text tells you it was wrong.

Solution

Click to reveal the solution

Key used: Heer – Kormoran, 31 July 1943. TGA is one of that day’s four Kenngruppen (ZYX TGA GHE BDD), on the July sheet. The station log says 1 August, but the message was made ready at 23:52 on the 31st and was keyed that day; the 00:04 on the log is when it reached you.

Recovered message key: ZEP (26 05 16 on the Enigma I). At LUC, the three letters EFE decode to ZEP.

Plaintext: RUSSISQEARTILLERIEFEUERTEINZELSQUSSEAUFUNSEREGRABENXSTARKETRUPPENBEWEGUNGBEINAQTAUFDERROLLBAHNXANGRIFFINWENIGENTAGENZUERWARTENXXXX. It reads “Russische Artillerie feuert Einzelschüsse auf unsere Gräben. Starke Truppenbewegung bei Nacht auf der Rollbahn. Angriff in wenigen Tagen zu erwarten.” (“Russian artillery is firing single rounds at our trenches. Heavy troop movement at night on the main road. Attack expected in a few days.”) Turn the rotors to ZEP and decrypt the ciphertext, skipping KWTGA, to get it. The manual’s message-prep rules (§2.3) show up here:

  • ü and ä are written as plain u and a (Einzelschüsse → EINZELSQUSSE, Gräben → GRABEN).
  • The pair ch is written as q, including inside sch (Russische → RUSSISQE, Einzelschüsse → EINZELSQUSSE, Nacht → NAQT).
  • Each sentence ends in X for its period.
  • The last four X after the final period are padding to a multiple of five letters.
What the wrong row gives

Trust the log’s date and use the 1 August row of Heer – Kormoran (rotors I IV II). The header LUC EFE then decrypts to the message key DIU, which looks as good as any other. Set the rotors to DIU and the start of the ciphertext comes out as MKPMVVZRWDARHNQVESKTLGVA, which is not German.

With the key enciphered only once, nothing on the page objects to the wrong row. The only check left is whether the result is German.

Full trace

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

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

Where it leads: the observation post has heard what it expected to hear. Seven days later, on 7 August, the Soviet offensive against the Smolensk sector opened.

mediumkey-sheet-lookupkenngruppeindicator-procedurefront-line-report

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