What is Block Cipher? | What is Stream Cipher? | Block vs Stream Cipher | Stream vs Block Cipher | Application of Block Cipher and Stream cipher

Introduction

Basically, cryptographic algorithm is used for transformation of plaintext into cipher text. There are basically two methods on which cryptographic algorithm is work.

Stream Cipher

In stream cipher keys and algorithms are applied to each binary digit in a data stream, one bit at a time, rather than encrypting block of data. One time pad cipher is the best example of stream cipher. Cipher feedback mode is converting block cipher into stream cipher.

 

Figure: Process of Stream Cipher

A stream cipher operates on plaintext accepting only one bit at a time. Stream cipher in which each bit of plain text message XOR with each bit of key to obtain cipher text message.

 

Figure: Example of Stream Cipher

There is a key stream generator which outputs a stream of bits: k1, k2, k3,...ki and XORed with a stream of plaintext bits p1, p2,...,pi to produce the stream of cipher text bits.

Ci = Pi ⊕ Ki

During decryption, the cipher text bits are XORed with a same key stream to recover the plain text bits.

Pi = Ci ⊕ Ki

If the key stream output is random that, it will take harder for a cryptanalyst to break it.

Advantage

High Speed transformation

Low error propagation

Disadvantage

Low diffusion

Less secure

Block Cipher

In Block cipher, plain text message divide into fixed size blocks and encrypt each block with some fixed size of key. Divide each plain text message into block of 64, 128, 256 bits and apply key size 40, 56, 64, 80, 128, 168, 192 and 256 bits which generates cipher text block same as size of plain text block. Data Encryption Standard (DES) is the best example of block cipher in which each block of 64-bit gets encrypted using 56-key bit and cipher text of 64-bit get generated.

Figure: Block diagram of block Cipher

A receiver side decrypt message with same key to generate plain text. Block cipher also uses the concept of key generator. Block cipher are used chaining mode, this is because for repeating text pattern, the same cipher block will be generated which can give clue to cryptanalyst regarding what is the original plain text. As a chaining mode, previous block is mixed with current block to avoid repeats in patterns. It is more secure.

Advantage

High Diffusion

More secure

Disadvantage

Encryption process is slow

Error propagation

Difference between Block Cipher and Stream cipher

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