Enigma-uhr

ADDRESS: Gneisenaustr. 6, 38102 Braunschweig GERMANY.

ABSTRACT: The construction and the function of the Enigma-Uhr are demonstrated. An example is given.

KEYWORDS: Cryptology, German keying-system, Enigma.

During the late phase of WW II, the German cipher-machine Enigma was refined in several cipher nets by an accessory, the Enigma-Uhr, designed to change the function of the plugboard (Figure 1). The plugboard caused a reciprocal super-encipherment, whereas the uhr got rid of this reciprocity of the plugboard. The reciprocity of the encipherment by the Enigma as a whole was not violated. The reciprocity of the plugboard had been the basis of the technique of Welchman's diagonal board, which, incorporated in the British cryptological bomb, had helped the Allied cryptological services to break the Enigma.

In 1993 through the kindness of Professor Ulvensoen, the Forsvarsmuseet in Oslo NORWAY, gave the author the opportunity to examine the uhr.

The uhr is housed in a wooden box of about 15cm x 18cm x 8cm (Figure 2). It bears the label

Enigma-Uhr

Ln. H. R. B. (GL)1/III

GeratE. U.

WerkNr. 1171

Another uhr in the museum of the Marineschule in Murwik GERMANY, bears the WerkNr. 1193. The label leads to the assumption that these uhren had been used by units of the Luftwaffe signal corps. With high probability the abbreviation H. R. B. stands for Heimsoeth & Rinke, Berlin-Wilmersdorf (manufacturer of the Enigma), GL for DC (as the letter W is also found, which points to AC).[1]

The main part of the uhr is a circular scrambler-disk of non-conducting material. It is shielded by a covering plate with a square window and is on an axle, which bears a wooden knob with an arrow on it to show its position (Figures 3,4). The possible positions (00 to 39) are printed around the circumference of a circular plate, screwed to the disk, so that one at a time can be seen through the window. The numbering runs anti-clockwise, so that in turning the knob clockwise the numbers 00 to 39 appear in ascending order.

The disk has 40 teeth at its rim to define 40 positions. The disk contains two concentric circles of 40 contact-areas each on the lower side. On the upper side, they are connected by wires in pairs, each contact of the outer circle with one of the inner circle in a pseudo-random manner, thus forming a permutation. Figures 5 and 6 show the disk from above and from underneath. Table 1 describes the wiring of the scrambler disk.

Figure 7 gives a sketch of the wiring of the disk as seen from above. The contact-areas can be sensed by contact-pins, arranged in two concentric circles of 20 pins each at the bottom of the box (Figures 8 and 9).

The uhr's input and output are managed by 20 cables. Both ends of each cable emerge from a compartment in the box. On one end, each cable leads to two of the above-mentioned contact-pins, either neighbored on the outer circle or on the inner circle. On the other end of each cable there is a plug with two pins of different diameters (4 mm and 3 mm ), thus fitting into the females of different diameters of the plugboard of the Enigma without the possibility of putting them in the wrong way. The plugs are labelled a1 (red), b1 (white); a2 (red), b2 (white), ..., a10 (red), b10 (white). Tables 2 and 3 give the connections of the plugs to the contact pins of Figure 9. The wiring is shown for positions 00 and 03 in Figure 11. (For the sake of clarity, the circular construction has been flattened to a linear one.) The thick pins of the plugs always lead (through the plugboard) to the keyboard or to the lamp field; the thin ones to the scrambler unit of the Enigma. Thus, the uhr's permutation function takes place between the keyboard and the scrambler and then again between the scrambler and the lamp field, but - contrary to the normal plugs - not in a reciprocal way.

On the inside of the lid of the box, a plaque (Figure 10), tells the user:

1. Clamp the uhr to the right side of the Enigma.

2. Install the plug-connections, lOa and lOb being one connection.

3. Keep to the order la-lb, 2a-2b, ..., lOa-lOb, installing the connections corresponding to the daily key from left to right.

The plaque furthermore shows two alphabets, which translate the plug-pairs of the daily key into uhr-positions. (E. g.: A/I leads to 03, P/S leads to 27.) It must be noted that uhrposition 00 is the neutral one, not changing the plugpairing and remaining reciprocal. (Table 4). Every position p with p = 0 mod 4 has the property of preserving reciprocity, but with different letter-pairs. Table 5 gives the result of the uhr's permutation for position 03.

No written instructions for using the uhr have been found by the author so far. Therefore only guesswork is possible for the length of the periods, a certain position of the uhr was kept constant. Perhaps the uhr was turned to the next allowed position after a certain number of messages, perhaps after a certain time. In any case, the loss of reciprocity in the plugs in most messages (pnot divisible by 4), the frequent change of the uhr-induced superencipherment, must have caused headaches in the teams in Great Britain and in the USA, though Hinsley plays down this effect [2].