Computer Security and Cryptography

6.13 SZ40 CRYPTANALYSIS PROBLEMS

There are several possible versions of the cryptanalysis problem, of which the following is the most challenging:

Problem # 1

Given:    Ciphertext y

Determine:        The pin-wheels (active pins and initial positions) and plaintext x.

          The SZ40 keys consists of

• The set of active pins of the 12 wheels (501 bits);
• The starting positions of the 12 wheels (56 bits).

Some keys may be changed daily (or with each message), others less frequently. Thus, if the active pins are fixed for a month and each day the starting positions are changed, the cryptanalysis is simpler:

Problem #2

Given:	Ciphertext y and the active pins on the χ, ψ, μ, and π wheels
Determine:	The initial positions of the pin-wheels and the plaintext x:

If one plaintext message can be determined by statistical methods or cribbing, the key k(0), k(1), … might be determined. Statistical and algebraic methods can be used to recover (part of) the pin-wheel settings (active pins and initial positions). This would permit the decipherment of all messages used with the same pin-wheel settings. A discussion of one such attack is given in Carter [1997].

The attack at Bletchley Park by GCHQ used the Colossus, a digital processor designed by Alan Turing to carry out the cryptanalysis.