The cryptanalysis of the Kryha machine

• Wheel unknown
• Superposition Method
• Friedmand's Method
• Start sector
• Cryptograms in-depth
• Hill Climbing

Introduction

This page will primarily describe the methods used to analyze the original Kryha machine, which uses a removable wheel.

For this machine, there are essentially two cases:

• The wheel used is known.
• The wheel used is unknown.

Once the wheel's structure is determined, we are in the first case, where the wheel used is known.

We will therefore begin by studying a cryptogram for which we know nothing about the wheel used.

Find the structure of the wheel

General Sacco was the first to find a method for deducing the structure of a Kyha wheel from the cryptogram. More recently, Dr. Konheim provided a purely mathematical approach to the problem.

The wheel page describes the methods of Sacco and Konheim, as well as my own method, which combines the two previous approaches.

But before using Sacco's or Konheim's methods, the key length must be known. The superposition method addresses this issue. The following paragraph discusses this method.

The superposition method

Superposition is a method that can be applied to all substitution systems. It is one of the most powerful methods in classical cryptography for attacking encryption machines. It can be used to prove that messages have been encrypted with the same key (they are in-depth). It can also be used to merge several ciphertexts. Finally, it can also be used to determine the length of a key.

The Friedman's method

In 1933, Friedman and his team achieved the feat of deciphering a message of approximately 1000 characters. The cryptogram had been generated by a Kryha machine equipped with the standard wheel. This machine and the wheel were known to Friedman.

The attack took place in two stages:

• Determine the start sector (see the following paragraph)
• Find the internal alphabet and the rest of the key (link)

Find the starting sector of a known wheel

The first step in Friedman's attack is to search for the starting sector of the cryptographic chain (link).

In-depth Messages

Regardless of the encryption method used (including the most recent ones), it will not hold up if the cryptanalyst has a set of ciphertexts encrypted with the same key. These are called in-depth messages.

The cryptanalyst can find the plaintext of the messages if he has enough in-depth messages and if these messages are long enough. He may even be able to reconstruct the encryption key if the method is not too complex. The Kryha machine is not considered a complex encryption system.

The page I have dedicated to this method is based primarily on studies conducted by Parker Hitt in the 1930s.

Hill Climbing

The methods presented above are manual. They require only a pencil and paper. For the past few decades, the Hill Climbing method has revolutionized the breaking of cipher machines and, more broadly, of cryptographic methods predating World War II.

On the page Hill Climbing, I describe the use of this method to break ciphers generated by the Kryha machine when the wheel is known.