Example illustrating the Vigenère encryption algorithm

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

First of all, to acquaint ourselves with the Vigenère encryption algorithm we will open a document, encrypt it and then decrypt it again. We will then try to get the computer to work out the key with which a plaintext is encrypted. To this end we open a document which contains a part of the CrypTool Help text ”Introduction to CrypTool”. The file name of the document to be opened is CrypTool-en.txt. This file is opened via the menu selection File \ Open.

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We now want to encrypt this document using the Vigenère encryption algorithm. To do this, we select Crypt/Decrypt \ Classical \ Vigenère, following which this dialog box appears. As key, we enter CRYPTOOL.

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Clicking on the Encrypt button opens a new window that contains the encrypted text.

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The plaintext version of this encrypted document can now be obtained simply by selecting Crypt/Decrypt \ Classical \ Vigenère. In the dialog box which then appears we enter the key with which the document was encrypted (CRYPTOOL). This time we do not want the text to be encrypted, but instead to be decrypted. Therefore the Decrypt field must be selected. This is done by clicking on the field itself or on the radial button to the left of the field.

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Clicking on the Decrypt button instructs CrypTool to decrypt the text. The plaintext appears immediately.

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We now wish to find the key with which the document was encrypted. Restore the window containing the encrypted document (via a mouse click) and select Analysis \ ciphertext only \ Vigenère, following which the text will automatically be analyzed. First of all a new, autocorrelation window opens.

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It is possible to work out the key length from the regular peaks in the autocorrelation. It is correctly calculated at 8 characters.

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When the Continue button is clicked, another dialog box opens to display the key that has been computed.

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Clicking on Decrypt closes this dialog box and opens another window which contains the unencrypted text.

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As you can see, the distinction between upper and lower case and the formatting (blank characters, line breaks etc.) have been retained. Special characters such as umlauts, hyphens and slashes were kept on in the encrypted document. Normally in a classical encryption algorithm all formatting is removed and only the letters are encrypted, but to make the text easier for you to read and compare, CrypTool retains the formatting in the encrypted text (and hence also in the decrypted text). By disabling the formatting options accessed via Options \ Text Options, it is possible to disable retention of formatting and, by checking the relevant boxes again, to reinstate formatting.

In the example used to illustrate the Caesar encryption algorithm a text is encrypted and decrypted both with and without retention of the formatting.