Computer Security and Cryptography

6.11 THE LORENZ SCHLÜSSELZUSATZ

The Lorenz Schlüsselzusatz (Fig. 6.16) is an additive encipherment cryptosystem; a key stream determined by more than 501 key bits is XORed to 5-bit Baudot-coded plaintext. The SZ40 was used to encipher German High Command communications. SZ40 ciphertext traffic was referred to as fish [Tutte, 1998]. Both the cryptographic device and the special processor used to carry out the cryptanalysis of the SZ40 were referred to as tunny; this first generation processor was designed by the British General Communications Headquarters (GCHQ) located in Bletchley Park outside London, where the SZ40 cryp-tanalysis activities took place. The SZ40 saga is described in the book by Hinsley and Stripp [2001].

The SZ40 and a succeeding model (SZ42) were manufactured by Lorenz; they were generalizations of the Vernam–Vigenère stream cipher system. The SZ40 encipherment equation is

Figure 6.16 The Lorenz Schlüsselzusatz (Courtesy of NSA).

where

• The plaintext {x(j)} is alphanumeric text encoded into 5-bit strings;
• The key {k(j)} is a sequence of 5-bit strings; and
• The ciphertext {y (j)} is the XOR of the plaintext and key.

The German Cipher Bureau understood the limitations of Vernam–Vigenère encipherment. Even with multiple tapes, an analysis is possible. The SZ40 used ...