SWEDISH SA TELEPRINTER CIPHER SYSTEM

Cryptologia, Jan 2004 by Zetterstrom, Urban

ABSTRACT: Discussion of an on-/off-line teleprinter cipher system used by the Swedish defense for securing all levels of teleprinter communications. The system was in use from 1959-1988.

KEYWORDS: Pinwheels, Electro-mechanical, Teleprinter, Irregular stepping, System variation box, Sweden.

INTRODUCTION

In the period following World War II Swedish defense branches were in need of updating their cipher equipment. The achievements made by the FRA (National Defense Radio Institute) during the war, including the breaking of German Siemens und Halske T52(1) and Lorenz SZ40(2) teleprinter ciphersfl], clearly demonstrated that new ideas where needed. Cipher machines in use at the time[2] were all lug and pin machines(3) originating from Boris Hagelin's company AB Crypto Teknik4. The equipment could no longer be considered safe from a cryptological viewpoint and furthermore it was not particularly suited to handle novel (at the time) communication modes, e. g. teleprinter.

Defense units addressed the problems with various administrative measures such as limiting the number of characters encrypted with the same external key[4]; needless to say this was not an adequate solution. Being a country with a policy of neutrality, Sweden could not seek assistance from foreign nations; up until the late 1970's expertise in the cryptographic area could almost exclusively be found in the nations SIGINT organizations.

In 1948 the inventors P-E Ahlman,A Lindegren, and V Lindstein filed a patent application for a teleprinter cipher machine which was known by the abbreviation SA5. Boris Hagelin and his company AB Crypto Teknik, on which Sweden had relied on as a supplier, were in the process of relocating[3] to Switzerland, consequently, before long, Sweden would be without a domestic cipher company. In a joint effort to find a solution, Swedish defense staff represented by lieutenant Colonel Harald Werneman and FRA's director general Rear Admiral Torgil Thoren drafted a position in which they inter alia encouraged the formation of a private cipher company.

On 9 April 1951 P-E Ahlman, B Florin, W Jonsson, and V Lundstein founded Transvertex. The company was run by the four founders as an independent company between 1951 to 1969. The two top priorities for Transvertex were to build a successor to the Hagelin "C" type of machines and to develop a Teleprinter ciphering machine.6

The SA system has been succinctly described in various articles on the Internet7 so the main principles of this particular machine should perhaps not be entirely unfamiliar to the reader. The SA machines where used by the Swedish defense between 1957 and 1988.

In the Swedish defense work the machine was designated as Kryptering s apparat 1018(Kryapp 101), Transvertex own designation was SA-48 M later SA-48 Mb.

The machine could be used for on-line as well as off-line en-/de-ciphering. Although of electro-mechanical design, some components of the machine are electronic; the logic circuitry built with solid state electronics rather than mechanical relays.

CIPHERING SYSTEM

The SA ciphering system is perhaps most easy to understand if it is broken down into the five individual parts that make up the system:

1. Pinwheels with reading devices

2. Key stream generating mechanism

3. Inverter

4. Stepping mechanism

5. System variation box

Pinwheels and reading devices

The machine has a total of ten pinwheels divided into two sets of five pinwheels each. We will refer to these two sets as [alpha] and [beta] and to the individual wheels as w^sub 1^-w^sub 0^. Each set of wheels has a period of 19, 21, 23, 25 and 26 teeth. It is important to differentiate between the [alpha] and [beta] sets, since the stepping control mechanism operates differently on each set of wheels. For the time being let us assume that the [alpha]-wheels are w^sub 2^, w^sub 4^, w^sub 5^, w8 and w^sub 0^ and [beta]-wheels are w^sub 1^, w^sub 3^, w^sub 6^, w^sub 7^ and w^sub 9^.

Each wheel has a set of pins that can be pushed to the left or active position, a pin remaining in the right (inactive) position will be regarded as a zero (0). The setting of the pins make up the machine's internal key.

Around the perimeter of each wheel are letters inscribed a-s for wheels w^sub 1^ and w^sub 6^, a-u for wheels w^sub 2^ and w^sub 7^ and so forth. The operator of the machine sets its external or message key by aligning the desired letter(s) on each wheel with an indicator on the pinwheel cover.

In order to read the active or passive pins each wheel is equipped with two binary switches i^sub s^ with i^sub s^=0 or i^sub s^=l and s=1...20, which reads two different positions on each wheel. The first ten switches [i^sub 1^-i^sub 10^] read active or passive pins at the indicator position, while the second set of switches [i^sub 11^-i^sub 20^] read two positions back from the indicator position. If the wheels are set a-a-a-a-a-a-a-a-a-a; switches i^sub 1^-i^sub 10^ will read the pins at the indicator position and switches i^sub 11^-i^sub 20^ will read the pins at the r-t-v-y-y-r-t-v-y-y position.