Automatic analysis, Caesar encryption method (Menu Analysis \ Symmetric Encryption (classic) \ Ciphertext-Only)

A ciphertext-only attack is available for the Caesar encryption algorithm in the form of an automated key search.

In the analysis, a frequency distribution is calculated for the individual characters, i.e. producing a histogram. This distribution is compared with the distribution of the German language. This entails superimposing the distributions with various offsets (autocorrelation) and comparing their frequencies. The offset with the greatest similarity to the distribution of the German language is assumed to be the correct key. In the Options for automatic analysis dialog it is possible to specify which and how many intermediate results should be displayed in separate windows. If the user clicks on Cancel instead of OK when viewing one of these intermediate results, no more of these windows or subordinate windows will appear. The key which CrypTool has worked out is displayed in the Automatic analysis dialog. It can be overwritten in this dialog.

In the Examples chapter there is an example of an attack on the encryption algorithm.

Conditions which the document must satisfy for a successful attack

The key will normally be found if the document

is not too short (at least 20 characters long);

contains German text in which the character frequency distribution is normal.

The second condition means, for example, that it is not possible to calculate the key for a plaintext

qw yx as qw yx as qw yx as qw yx as qw yx as,

even though there are sufficient characters.

The two above conditions are normally satisfied if the plaintext consists of German words.

However, it may occasionally still be possible to find the key even with a very short text.