So, lets look at how CBC works first. The following picture shows the encryption when using CBC (in this case, using AES as the cipher).
Basically, Cipher-Block-Chaining means that previous to putting the cleartext data block into the cipher itself (AES, DES, Triple-DES, …) it is XORed with the previous cipher block. This works fine for all but the first cleartext block, as – of course – there is no previous cipher block. So, the encrypting entity chooses a random value of block size (8bytes for DES, 16bytes for AES) to use in the first XOR. This value is the so-called Initialization Vector or IV. The following picture depicts the decryption using
