The Compress (Zip) function is used to compress data. When the Zip function is selected, the content of the current document is compressed and the result is presented in a new window. The original document can be restored at a later time by selecting the UnZip function.
There are a number of data compression algorithms available. CrypTool uses the GnuZip algorithm.
Before the results are presented in a new window, the rate of compression is shown. The rate of compression states the percentage by which it has been possible to compress (i.e. reduce) a document. If the rate of compression is 10%, then the compressed document is 10% shorter than the uncompressed version. With short documents or documents that have already been compressed it is possible for the compressed document to actually be a few characters longer. This is expressed as a negative rate of compression. Thus, if the rate of compression is -10%, then the compressed document is 10% longer than the uncompressed version.
As well as their smaller size, a second characteristic of compressed files is that if they contain text written in natural language, the redundancy of the data is reduced. Normally this will make an attack more difficult. For this reason the PGP (Pretty Good Privacy) encryption program, for example, compresses the data prior to actually encrypting it. The packets containing encrypted text are thus generally significantly smaller as a result while at the same time they are more secure.
The Examples chapter contains an example in which the key can be found in a Ciphertext-Only attack if the document has been encrypted using the Exclusive-OR, but not if the document has been previously compressed.
Remark 1:
In order to recognize the document as compressed the keyword "CTZ" is attached at the beginning.
Remark 2:
If you save documents compressed with CrypTool in a file, you cannot decrypt them using tools like unzip or Winzip, even if you are deleting the three magic characters at the beginning.
This is because CrypTool process no meta information required for ZIP archives.