Block Cipher Modes of Operation

Block Cipher Modes Of Operation

In cryptography, modes of operation is the procedure of enabling the repeated and secure use of a block cipher under a single key. A block cipher by itself allows encryption only of a single data block of the cipher's block length. When targeting a variable-length message, the data must first be partitioned into separate cipher blocks. Typically, the last block must also be extended to match the cipher's block length using a suitable padding scheme. A mode of operation describes the process of encrypting each of these blocks, and generally uses randomization based on an additional input value, often called an initialization vector, to allow doing so safely.

Modes of operation have primarily been defined for encryption and authentication. Historically, encryption modes have been studied extensively in regard to their error propagation properties under various scenarios of data modification. Later development regarded integrity protection as an entirely separate cryptographic goal from encryption. Some modern modes of operation combine encryption and authentication in an efficient way, and are known as authenticated encryption modes.

While modes of operation are commonly associated with symmetric encryption, they may also be applied to public-key encryption primitives such as RSA in principle (though in practice public-key encryption of longer messages is generally realized using hybrid encryption).