The user manual of the ALLU-encryption disc, version 1.0

1. General

The encryption disc ALLU,  which the radio operator of the long-range patrol used to quickly encrypt his message, is an application of the Albert-disc.
During the war the trainer of the long-range patrol, captain Matti Wihuri (OH2OH) and military expert Alvar Ahonen developed the encryption disc ALLU based on a mixed Tritheim table, the so called Mask box.

2. Construction

The ALLU disc is made up of two aluminium discs with the diameters 12 cm and 9 cm. The bigger, outer disc is divided into 26 sectors. On top of the aluminium discs two cardboard discs were placed. On top of these was put gutta-percha as protection against the weather.

The inner (smaller) cardboard disc has five concentric circles. The inner and outer cardboard discs can be locked to each other with a locking spring. The outer disc has two concentric circles.

The outermost circle had designations in plain text, for example, LeKo (airplane), H-auto (car), juna (train), silta (bridge) and numbers in the outermost circle. In the inner circle was the Finnish alphabet, 24 letters.

Moreover there were, on the outer disc, two sector marks, which were used to indicate switching to use either the inner or the outer circle. On the inner disc was the English alphabet mixed in five circles. These were drawn with the help of wooden balls, which were kept in a cloth bag.

Two sets of these cardboard discs were manufactured, one for the headquarters and one for the long-range patrol. Furthermore, yet another set of discs was manufacturd, in case the enemy should get hold of the encryption disc.

3. Encoding and decoding with the encryption disc ALLU.

This is how the the encryption disc ALLU was used:

The message in plain text could be, for example, the following:
SILTA TUHOTTU (BRIDGE DESTROYED)
SILTA is a plain text marking on the outermost circle of the outer disc in sector 24 (see attached picture).
TUHOTTU is a normal text, which should be encrypted.

As a starting point the agreed locking position between the discs is used (for example, the position on the attached picture)

The message should thus be SILTA TUHOTTU

To begin with, one reads the moving instruction on the first circle of the outer disc in the following way:
Go to sector 25 and read off the encrypted letter Q from the first circle of the inner disc as the first sign (which means that the next ”operation” should go the the outer circle of the outer disc).
The position SILTA is encrypted by passing on to sector 24 (where it says SILTA).

The rest of the message is read off from the second circle of the inner disc:

The encrypted message now reads QU

TUHOTTU is a normal text that should be encrypted, and therefore you must change to the inner circle of the outer disc.
This is done in the following way:

Go to sector 26 and read off the moving instruction from circle three of the inner disc, which is Y
The message now reads QUY

The plain text was TUHOTTU. You take the corresponding letter T from circle four of the inner disc, which gives Z. Now the encrypted message reads QUYZ

Then follows TUHOTTU, which is read off from circle five of the inner disc, which gives E

QUYZE

Thereafter the lock is realeased and the inner disc is moved one sector clockwise.

The encoding is continued in the same way.
In the corresponding way the decoding is done by reading off the letter Q from the first sector of the inner disc and looking for the equivalent on the outer disc etc.