Electronic Code Book (ECB) and Cipher Block Chaining (CBC)

Encryption normally works by taking a number of text blocks, and then applies a key to these to produce cipher blocks. Typical block sizes are 128 or 256 bits. Unfortunately, the cipher blocks could end up being the same, for the same input text. Thus an intruder could try and guess the cipher text. This is known as electronic code book. For example if we use 3DES to encrypt the word “fred”, with a key of “bert12345”, we will always get:

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You will find that ever time you encrypt, you will get the same value. Thus the intruder could start to guess what your mapping of the plain text to cipher text was. For example he could send “Hello. Can you send me a quick answer … just yes or no?”, and look at the reply, and guess that the mapping of “yes” or “no” to the cipher text. The intruder then does not need to know the key, he can play it back to others.

Example

Apart from using a password to generate an encryption key, which complete decimates the key space, we have the problem of the algorithm used to process the plain text. If…