Exclusive ORing Techniques

One of the simpler encryption techniques is based on exclusive-ORing (XORing) the plain-text data with a key value. If you do this on a file that has lots of blank space or repeated strings of the same character, it’s very possible to determine the encryption key. By simply re-XORing the repeated areas with their own value, the plain-text values cancel out, leaving the key.

You might think that with such a weakness, XORing against a key value wouldn’t continue to be used. But one advantage of XORing is its extreme speed; XOR is one of the most fundamental computer operations performed.

There are several ways to strengthen an XOR algorithm:

•  Use a longer, or ideally, an extremely long key value.
  
  
• Combine additional information, such as the offset position of the character in the file, or the value of the preceding character(s), with the plain-text character before encrypting.
  
  
• Use XOR as only one level of a multilevel encryption scheme.

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