Example illustrating encryption using monoalphabetic substitution

This section provides an example illustrating encryption using monoalphabetic substitution. To make it easier to follow the steps that need to be performed with CrypTool, the example is illustrated with a number of screenshots.

We will encrypt a fairly long German document and then, using the automatic analysis function for monoalphabetic substitution, we will work out the key and the plaintext version in a ciphertext-only attack.

The text runs as follows:

DIE DEUTSCHE BANK IST VON INFORMATIONEN ABHAENGIG.

INFORMATIONEN ENTSCHEIDEN UEBER UNSEREN ERFOLG UND

DEN UNSERER KUNDEN. VON GROESSTER WICHTIGKEIT IST

NEBEN DER GENAUIGKEIT UND VERFUEGBARKEIT IN DEN

MEISTEN FAELLEN AUCH DIE VERTRAULICHKEIT VON

INFORMATIONEN. JEDER ANGESTELLTE MUSS SICH DAHER

DER NOTWENDIGKEIT DER INFORMATIONSSICHERHEIT

BEWUSST SEIN UND ENTSPRECHEND HANDELN.

This text is contained in the file probetext-de.txt. To open this document in CrypTool, select File \ Open.

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The document is now encrypted with the key CRYPTOOL. The dialog box in which the key is entered is accessed via Crypt/Decrypt \ Classical \ Substitution.

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Enter CRYPTOOL into the appropriate data input field and click on the Encrypt button, following which this dialog box will appear.

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This box explains how the substitutions have been derived from the key CRYPTOOL. For encryption, the following substitutions have been made:

A
-> C
B
-> R
C
-> Y
D
-> P
E
-> T
  ...  
Y
-> B
Z
-> A

How the key is generated from the keyword

If monoalphabetic substitution is used with a key, a common way to use it, is that the letters of the key are attached to first letters of the cleartext alphabet. Then the remaining letters of the remaining letters of the target alphabet are attached to the remaining cleartext letters from the back of the alphabet.

Now click on OK, and a new window opens containing the encrypted text.

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Let us now forget the key and carry out an analysis of the encrypted text. To do this, we select Analysis \ Manual Analysis \ Substitution, following which this dialog box appears.

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Here we specify that we are expecting the plaintext to be in German and that an extended analysis should be performed. Confirmation of this dialog box opens the Manual Postprocessing window containing the analysis results.

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The bottom section of the window presents the plaintext identified so far. The upper case letters denote substitutions recognised by the automatic analysis, while the lower case letters are taken directly from the encrypted text.

Manual post-processing is now necessary. We could, for example, make the following suppositions from looking at the plaintext:

After making these corrections, the Manual Post-processing window looks like this:

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The remaining substitutions are now easy to guess and can be entered into the dialog box: R is replaced by B, V is by K, F by V, L by G, U by L, E by W, W by J and M by P.

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This is the correct solution, giving the original plaintext. To crack the encryption, apart from the encrypted document the only knowledge required was that it was a German text and a small amount of instinctive feeling as to the first substitutions required. When the OK button is clicked, the plaintext is presented in another window.

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