Block Cipher

Block Cipher processes messages (plaintext) into blocks, and then encrypt or decrypt them one by block. It usually requires equal or larger than 64 bits. However, in practise, it might be used to encrypt or decrypt arbitrary size of data blocks or stream, therefore defined in Ansi x3.106-1983 modes of use (FIPS 81), there are 4 possible modes for Block Cipher to use including block and stream modes. For block mode of operation, it may need to use message padding# for short block (usually the last block).

Most Block Ciphers are based on Feistel Cipher Structure. And instead of encrypting the entire block which could be costly, the encrypted block is assembled from smaller one utilising Product Cipher.

Examples of Block Cipher are:

All variants of Data Encryption Standard (DES)#
International Data Encryption Algorithm (IDEA)#
Blowfish#
RC5#
Cast-128#

Links to this page

TNS3131 Chapter 2: Conventional Encryption and Message Confidentiality

Block Cipher
TAC3121 Chapter 5: Symmetric Cryptography

Block Cipher
RC5

RC5 is a fast and simple #Block Cipher, invented by Ron Rivest, utilises data-dependent rotations that could have a variable number of rounds of #round funnction (up to 255 rounds) and variable-length key (up to 2048 bits). It can be used to encrypt a 64-bit large data block. It is suited for both software and hardware encryption. Moreover, it can adapt to processors of different word lengths with low memory requirement and provide high security guarantee.
Product Cipher

Product Cipher is a cipher that composed of both #substitution (S-box) and #permutation (P-box). Introduced by Claude Shannon, it is the backbone for the modern Block Cipher. It provides diffusion and confusion of message and key.
Output Feedback (OFB)

Output Feedback is a stream mode of operation# of #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where the preceding encryption output of the permuted encrypted initialisation value (feedback) is used as the input to the encryption #algorithm. The feedback is indenpendent of message thus can be computed in advance. Full blocks are used. It is pretty similar to Cipher Feedback (CFB) otherwise.
International Data Encryption Algorithm (IDEA)

IDEA is a #Block Cipher that uses 128-bit key on 64 bits of data and performs 8 rounds of round function# for encryption. It is one of the underlying algorithms used by Pretty Good Privacy (PGP)#.
Electronic Codebook (ECB)

ECB is a block mode of operation# from #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where each block of plaintext bits is encoded using Substitution# independently# with the same key. The formal equation for the encryption is as follow:
Cryptography

Block Cipher#
Counter (CTR)

Counter is a stream mode of operation# of #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where each block of plaintext is XORed (\(\oplus\)) with an encrypted counter to produce a ciphertext. The counter is incremented for each successive block. Other than encrypting counter value instead of output, the #algorithm is pretty similar to Output Feedback (OFB).
Cipher Feedback (CFB)

CFB is the most common stream mode of operation# defined in #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where preceding ciphertext is used as input to the encryption #algorithm to produce pseudorandom output. The output will then be XORed (\(\oplus\)) with plaintext to produce the next unit of ciphertext. The input is processed \(s\) bits at a time. This process is almost identical to the decryption side of end with the same encryption mode.
Cipher Block Chaining (CBC)

CBC is a block mode of operation# defined in #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where the input to the encryption #algorithm is the XOR (\(\oplus\)) of the next block of plaintext and the preceding block of plaintexts. Decrypting the message can be as easy as just doing it reversely.
Cast-128

Cast-128 is a #Block Cipher that uses key size range from 40 to 128 bits and could have differing round functions from round to round.
Blowfish

Blowfish is a #Block Cipher that has high execution speed and runs in less than 5K of memory. It could have a key size up to 448 bits, and used to encrypt a 64-bit data block. Blowfish will perform 16 rounds of round function#. It is comparatively easier to implement compare to other cryptographic #algorithm.

#cryptography