Question

Why is the Cipher Block Chaining (CBC) mode of operation considered preferable to the Electronic Code Book (ECB) mode? Is it possible to perform encryption operations in parallel on multiple blocks of plaintext in the CBC mode? How about decryption?

Answer 1

Electronic Code Book (ECB) mode is essentially the first generation of the AES(Advanced Encryption Standard) . It is the most basic form of block cipher encryption. Whereas, the Cipher Block Chaining (CBC) mode of operation is the advance form of block cipher encryption. Instead of just processing each block separately, in CBC every block will be XOR’ed with the encrypted previous block.

**The CBC code provide confidentiality and authentication, Advantage of CBC over ECB is that it changes IV results in different ciphertext for identical message. With CBC mode the effects of an attacker predicting the IV are more limited. So you might say that CBC mode is more robust against mishandling the IV. Thus, CBC adds an extra level of complexity to the encrypted data.

In CBC encryption, the input block to each forward cipher operation (except the first) depends on the resul tof the previous forward cipher operation.So, it cannot perform parallel encryption operations.

Whereas, CBC mode performs the parallel decryption as one does not need to decrypt the previous block before using it as the IV for the decryption of the current one.