What are the different modes of operation in Block Cipher in information security?

A block cipher is a symmetric cryptographic algorithm that works on a constant-size block of information using a shared, private key. Plaintext can be used during the encryption, and the resulting encrypted text is known as ciphertext. The similar key can be used for both the encryption of the plaintext and the decryption of the ciphertext.

There are different modes of operations in block cipher which are as follows −

Electronic Code Book (ECB) Mode − This is the easiest mode. In this mode, the plain text is breaked into a block where each block is 64 bits. Thus each block is encrypted independently. The equal key can be used for the encryption of all blocks. Each block is encrypted utilizing the key and creates the block of ciphertext.

Electronic Code Book handle the cipher function separately to every block of plaintext to encrypt it (and the inverse function to every block of ciphertext to decrypt it). This can define that CBC can encrypt and decrypt several blocks in parallel (because they don't based on each other), speeding up the process.

Ciphertext Block Chaining (CBC) Mode − In CBC mode, each plaintext block is Exclusive-ORed with the prior ciphertext block before being encrypted. For the first block an initialization vector can be utilized for EX-ORing the sender, and the receiver agrees upon the pre-represented initialization vector.

CBC can be used to encipher message but because of chaining structure, parallel processing is not applicable. It cannot be used to encrypt and decrypt random access file information.

Output feedback (OFB) Mode − It is equivalent to CFB mode, with one difference. Each bit of the ciphertext is independent of the earlier bit. This can prevent the flow of error from one block to another.

The benefit of this method is that bit errors in transmission do not reproduce. The disadvantage of this method is that it is more vulnerable to a message flow alteration attack than is CFB.

Cipher Feedback (CFB) Mode − In this mode, the data is encrypted in the design of units where each unit is of 8 bits. It can be used for performing stream ciphering in applications that needed them.

Moreover, each ciphertext block is introduced as an input to the next plain text block during the encryption procedure. The CFB mode also differs from the ECB mode as each ciphertext block encryption is based on the input plain text block, the encryption key, and the prior ciphertext block.

CFB mode is generally used in applications that are created to operate on smaller information units in place of blocks.

Counter Mode − A counter is similar to the plaintext block size is used. For encryption, the counter is encrypted and thus XORed with the plaintext block to make the ciphertext block.

For decryption, similar sequence of counter values can be used with each encrypted counter XORed with ciphertext block. CTR mode is used in hardware and software effectiveness, pre-processing, protection and integrity.

Updated on: 14-Mar-2022


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