CBC Mode | Cipher Block Chaining Mode | Mode of Block Cipher | Application of CBC Mode

 

Introduction

Cryptographic algorithm works on main two techniques: block and stream ciphers.

In a stream cipher, the plaintext is encrypted one bit at a time. In a block cipher, the plaintext is broken into blocks of a fixed length and the bits in each block are encrypted together. One of the main issues with block ciphers is that they only allow you to encrypt messages the fixed size as their block length.

If plaintext, which has a block size 64 bits easily encrypt. But encrypt a 65-bit message, you need a way to define how the second block should be encrypted.

The solution to this is called block cipher modes of operation. Need of block cipher mode is basic building block for providing data security. In block cipher rather than encrypting one bit at a time, block of bits is encrypted at a time.

There are 5 modes of operation for block cipher that may be used in a wide variety of applications like symmetric key cryptographic algorithm. These modes define how data encrypted and decrypted.

Cipher Block Chaining Mode (CBC)

To overcome the problem of ECB mode, the cipher block chaining mode is used. In this mode, first block of plaintext is XORed with an Initialization Vector (IV) which is then encrypted using key k & produces cipher text block 1.

In next step each block of plaintext XORed with previous block of cipher text. The procedure is continuing till all plaintext block gets encrypted shown in figure. Plaintext block are represented by using P1, P2, P3,...,Pn and corresponding cipher text blocks are represented by using C1, C2, C3,...,Cn. Initialization vector doesn’t have special meaning it is simply used to make input message more complicated or unique.

Figure: Encryption Process of CBC Mode

In this mode decryption process, cipher text block 1 get decrypted using same key used earlier during encryption process & the output of this step is then XOR with IV and produces Plaintext. In next step the cipher text block 2 is decrypted and its output is XOR with cipher text block 1 which results plaintext block 2. Repeat the process for all cipher text block in order to produce original plaintext blocks as shown in figure.

Figure: Decryption Process of CBC Mode

Application of CBC Mode

CBC mode is applicable whenever large amounts of data need to be sent securely. (e.g., email, FTP, web etc..)

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