1.3.3 Limitations of Block Ciphers

One obvious problem with the most straightforward application of a block cipher is that it's deterministic.  If you just apply the transform to each block independently, it's easy to implement; this is known as electronic code book (EBC) mode.  BUT: If you see the same ciphertext $c$ in two different places in the message stream, you know that the two input blocks were the same!  This leaks a huge amount of information to a sophisticated attacker, and is considered unacceptable.

One answer to this is to make the encryption slightly less deterministic by XORing in the ciphertext of the previous block into the current one before performing the encryption.  This is cipher block chaining (CBC) mode, the standard mode of operation.  (There are at least two more modes that I know nothing about.)  CBC has the undesirable side effect of requiring encryption to be done serially, but attacks can be parallelized.  Nevertheless, it's the most commonly used mode.