1. Briefly describe the criteria and properties for evaluating and constructing

Question

1. Briefly describe the criteria and properties for evaluating and constructing modes of operation for block ciphers

2. With the aid of a diagram, explain the mechanism of operation of the CBC encryption and decryption process.

3. With the aid of a diagram, demonstrate CBC’s self-healing property.

4. Compare and contrast ECB, CBC and CTR/CM with regard to the possibility (or impossibility) to parallelize the encryption and decryption process. You must provide explanation to back up your answer.

5. Describe the advantages of the counter mode (CM) relative to the other modes of operation.

6. What problems might the counter mode face if the counter used is not a nonce?

7. Under what circumstances would it be advisable or inadvisable to use the ECB mode of operation?  

Explanation / Answer

1. The fixed size data blocks are processed by the block cipher. Why because the size of the info or a message usually larger than the block size. So in order to break or divide the message in a way of series of sequential message blocks the cipher operate the blocks at a time.

The cipher blocks are having many types of mode to process the data blocks those are:
- Electronic Code Book (ECB) Mode
- Cipher Block Chaining (CBC) Mode
- Cipher Feedback (CFB) Mode
- Output Feedback (OFB) Mode
- Counter (CTR) Mode

- Electronic Code Book (ECB) Mode :

The ECB mode is a very straightforward why because it always process the data blocks in such a way that the processing of a series of sequentially listed message blocks. That means it will processed in a sequential way like in straight forward line.

- Cipher Block Chaining (CBC) Mode

The cipertext are generated and CBC mode operates it and provide the message dependence and makes them as non deterministic system.

- Cipher Feedback (CFB) Mode

The encryption process in order to encrypt the next plaintext block these mode will make sure that each ciphertext block gets ‘fed back’.

- Output Feedback (OFB) Mode

In this mode the output blocks from the underlying block cipher will get back to it like acknownledgment or feedback.

So by using the feedback it will provide string of bits to feed the encryption algorithm.

- Counter (CTR) Mode

These mode wont get the feedback from the output blocks and so we can consider it as a counter-based version of CFB mode without the feedback.

Also we must know that these mode can access both sender and receiver data blocks to a reliable counter.

Note: According to the chegg rules i have answered only one question. So for remaining question answer please post them as saperate posts. Thanking for your understandings.

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