Question

What are the two modes of encryption that allow random access to the ciphertext blocks. Specifically, if a hacker has the information about i-th cyphertext block Ci and it can find the plaintext Pi without having any other cyphertext blocks.

Answer 1

Dear Student,

Answer :

The 2 Modes Are :

1. Cipher Block Chaining Mode ( CBC ) =

• In CBC mode, the current plaintext block is added to the previous ciphertext block, and then the result is encrypted with the key.
• Decryption is the reverse process, which involves decrypting the current ciphertext and then adding the previous ciphertext block to the result.
• Advantage of CBC over ECB is that changing IV results in different ciphertext for identical message.
• On the drawback side, the error in transmission will get propagated to few further block during decryption due to chaining effect.
• CBC mode forms the basis for a well-known data origin authentication mechanism.
• Thus, it has an advantage for those applications that require both symmetric encryption and data origin authentication.

2. Cipher Feedback Mode ( CFB ) =

• CFB mode differs significantly from ECB mode, the ciphertext corresponding to a given plaintext block depends not just on that plaintext block and the key, but also on the previous ciphertext block.
• In other words, the ciphertext block is dependent of message.
• CFB has a very strange feature. In this mode, user decrypts the ciphertext using only the encryption process of the block cipher.
• The decryption algorithm of the underlying block cipher is never used.
• CFB mode is converting a block cipher into a type of stream cipher.
• The encryption algorithm is used as a key-stream generator to produce key-stream that is placed in the bottom register. This key stream is then XORed with the plaintext as in case of stream cipher.
• After converting a block cipher into a stream cipher, CFB mode provides some of the advantageous properties of a stream cipher while retaining the advantageous properties of a block cipher.

Hope This Helps.

All The Best