Introduction

The block length of all practical block ciphers is very small, usually 64-256 bits, but messages commonly exceed 16 bytes. Therefore, we need a means of dividing a message into blocks which match the block length of the cipher used. There are numerous ways to achieve this, called modes of operations, and, as it turns out, not all methods are created equal.

Using a secure block cipher is *not* enough - one needs to also use a proper mode of operation. A secure block cipher ensures that each *block* is encrypted securely, while a secure mode of operation ensures that *the entire message* is encrypted securely.
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In practice, a block cipher is never used on its own - there is always a mode of operation involved. Therefore, saying that one "encrypts something with AES" is not enough - one needs to also specify the mode of operation used, for example AES-CBC or DES-CTR.

When discussing modes of operation, the message length is assumed to be a multiple of the block length. In practice, however, this is not the case and certain techniques need to be used to make all message blocks of the same length.
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