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block cipher

By TechTarget Contributor

What is a block cipher?

A block cipher is a method of encrypting data in blocks to produce ciphertext using a cryptographic key and algorithm. The block cipher processes fixed-size blocks simultaneously, as opposed to a stream cipher, which encrypts data one bit at a time. Most modern block ciphers are designed to encrypt data in fixed-size blocks of either 64 or 128 bits.

How does a block cipher work?

A block cipher uses a symmetric key and algorithm to encrypt and decrypt a block of data. A block cipher requires an initialization vector (IV) that is added to the input plaintext in order to increase the keyspace of the cipher and make it more difficult to use brute force to break the key. The IV is derived from a random number generator, which is combined with text in the first block and the key to ensure all subsequent blocks result in ciphertext that does not match that of the first encryption block.

The block size of a block cipher refers to the number of bits that are processed together. Data Encryption Standard (DES) and Advanced Encryption Standard (AES) are both symmetric block ciphers.

The DES block cipher was originally designed by IBM in 1975 and consisted of 64-bit blocks and a 56-bit key. This cipher is not considered secure anymore, due to the short key size, and was replaced in 1998 by AES. AES uses a 128-bit block size and a 128-, 192- or 256-bit key size.

What are the different modes of operation in block cipher?

Block ciphers only encrypt messages that are the same size as their block length, so each block of plaintext with more or less blocks needs to be encrypted separately. The following block cipher modes of operation define how these blocks are encrypted:

Authenticated encryption with additional data modes

The following modes provide message encryption and can supply additional data -- including sequence number or header -- that is not included in the ciphertext:

Learn how cloud providers are tackling multi-cloud key challenges using key management as a service.

13 May 2021

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