I\'VE ALREADY DONE THE EXPERIMENT, I JUST NEED HELP WITH THE LAST 3 QUESTIONS. C

Question

I'VE ALREADY DONE THE EXPERIMENT, I JUST NEED HELP WITH THE LAST 3 QUESTIONS.

Create a text file that is at least 64 bytes long.

Encrypt the file using the AES-128 cipher.

Unfortunately, a single bit of the 30th byte in the encrypted file got corrupted. You can achieve this corruption using a hex editor.

Decrypt the corrupted file (encrypted) using the correct key and IV.

1. How much information can you recover by decrypting the corrupted file, if the encryption mode is ECB, CBC, CFB, or OFB, respectively?

2. Please explain why.

3. What are the implication of these differences?

Explanation / Answer

1.

3. ECB mode:- There are several reasons why the ECB mode of operation is considered unsuitable for implementation. During the process of encryption, the identical blocks are encrypted into identical cipher blocks, which allows for easy recognition of a repeated message. The attacker can also change the order of the encrypted blocks, without giving it away to the receiver. Although this mode of operation is intended for encryption of the data within the size of a single block and is considered as an erroneous one and abandoned, and therefore it should not be used.

CBC mode:- In this mode of operation, by means of the initialization vector – IV, when encrypting identical blocks of  plaintext, produces different outputs for every block. If there is an error in a block, it will propagate through all the following blocks. In order to achieve security, the IV must be a random non-predictable value, and the key must be changed before encrypting 2(n+1)/2 blocks with the size of n bits. The CBC mode of operation is vulnerable to PA and CCA attacks.

CFB mode:- This mode of operation allows stream encryption. The encryption operation is the same as the decryption. An error in a block affects the other blocks.This mode of operation is secure in the case of CPA.

OFB mode:- It also enables stream encryption, and the operation of encryption is used for decryption as well. An error in a block does not affect the other blocks. The security of this mode of operation is achieved if the encryption key is changed for every 2n/2 blocks of encryption,where n is the number of bits in a block.It does not offer security from CCA attacks and therefore, this mode of operations should be avoided.

Taking into account the positive and negative sides of the analyzed modes of operation of the AES algorithm, it can be implied that, for its proper and secure implementation, the CTR mode of operation should be implemented.

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