Block Cipher modes of Operation

A block cipher processes the data blocks of fixed size. Usually, the size of a message is larger than the block size. Hence, the long message is divided into a series of sequential message blocks, and the cipher operates on these blocks one at a time. There are five Block Cipher Modes Of Operations which are listed below.

Electronic Code Book (ECB) –?

ECB is the easiest block cipher mode of functioning, because of direct encryption of each block of input plaintext and output is in form of blocks of encrypted ciphertext. Generally, if a message is larger than?the block size lets say b?bits, it can be broken down into a bunch of blocks and the procedure is repeated further until the last block. ?Decryption is the reverse process by using decryption algorithm as it is shown clearly in the image below.

Cipher Block Chaining (CBC) –?

CBC is the advancement mode of ECB since ECB compromises some security requirements like there is a direct connection between the cipher text and the plain text which makes it easier for the attackers to decrypt the encoded message. In CBC, connection between the plain text and cipher text is broken down by providing the previous cipher block as input to the next encryption algorithm after XOR with the original plaintext block. As there is no previous blocks are available for the first block, use a Initial Vector(IV) and continue the process.

Cipher Feedback Mode (CFB) –?

The cipher is given as feedback to the next block of encryption in this mode. First, an initial vector IV is used for first encryption and output bits are divided as a set of?s and b-s?bits. The left-hand side?s bits are selected and are applied an XOR operation with plaintext bits. The result is given as input to a shift register and the process continues. Both encryption and decryption processes use the same encryption algorithm.?

Output Feedback Mode (OFB) –?

The OFB follows nearly the same process as the CFB except that it sends the encrypted output as feedback instead of the actual cipher which is XOR output. In this output feedback mode, all bits of the block are sent instead of sending selected?s?bits. The OFB of block cipher holds great resistance towards bit transmission errors. It also decreases the dependency or relationship of the cipher on the plaintext.?

Counter Mode (CTR) –?

The CTR is a simple counter-based block cipher implementation. Every time a counter-initiated value is encrypted and given as input to XOR with plaintext which results in ciphertext block. The CTR mode is independent of feedback use and thus can be implemented in parallel.?Fastest mode compared to CBC, CFB and OFB due to parallel execution while in the mentioned modes each block is dependent on the previous blocks.

Note:

Only ECB and CBC use decryption algorithm for decrypting the cipher text. Remaining three uses same encryption algorithm for both encryption and decryption.