1- Compare the security and performance advantages and disadvantages of each var

Question

1- Compare the security and performance advantages and disadvantages of each variant of CBC mode: a fixed IV, a counter IV, a random IV, and a nonce-generated IV.

2- Suppose you, as an attacker, observe the following 32-byte ciphertext C1 (in hex)

4D 11 30 28 13 C7 8C EB 31 21 20 B0 70 B4 C3 B4

FA 3A 20 12 02 13 1C FD C5 44 07 25 D3 96 1F 74

and the following 32-byte ciphertext C2 (also in hex)

50 15 30 30 56 E6 91 BF 22 6E 22 B0 7D E0 CB B5

F0 6E 3A 22 15 0D 19 E5 8A 40 1A 31 D1 8D 03 74

Suppose you know these ciphertexts were generated using CTR mode with the same nonce. You also know that the plaintext P1 corresponding to C1 is

49 6C 69 6B 65 49 6E 74 72 6F 64 75 63 74 69 6F

6E 74 6F 43 72 79 70 74 6F 67 72 61 70 68 79 21

What information, if any, can you infer about the plaintext P2 corresponding to C2?

Explanation / Answer

ADVANTAGES OF unique IV :per encrypted_file: The IV adds randomness_to your start of your encryption_process. When using a CBC

If you had no IV, and used_chained block -encryption with just your key, 2 files that begin-with identical_text_will produce identical first blocks. If the input files_different midway- through, then the two encrypted files-would begin to look-different beginning at that_point and through to the_end of the encrypted-file. If attacker_noticed the _similarity at the beginning, and knew what one of the files began with, he could deduce what the_other file began with. Knowing what_the plaintext file_began with and what it's_corresponding ciphertext is could-allow that person to_determine the key_and then_decrypt the_entire file.

Now add the IV - if each file used a random IV, their_first block would be_different. The above scenario has-been dissatisfied.

Fixed IV:

Now_what if_the IV were the_same for_each file then Well, we have the problem_scenario again. The first block of each_file will encrypt to the same_cipher. this is same as not using the_IV at all.

NONCE GENERATED IV:

A nonce, in the broad sense, is just "a number used only once".

A nonce is a random number_only used once and for a short time with the intention_to get replaced_by or-converted into something better. A IV is also used_as input for a cryptographic_primitive to attain-randomization of normally-deterministic primitives. Stream ciphers_are called ,stateful where-the same_key is-used for many_states and the-nonces are used to ensure different key streams. For this reason, we use-IVs for the_modes of operations-of block ciphers-but nonces for stream_cipher

Random IV:

If random IV for each key is used no 2 key-files would be same and each encrypted message will_have its own key file and any other_key file will not be_useful

This random IV ensures_that each message_encrypts differently, such that_seeing multiple messages_encrypted with-the same key doesn't give the attacker any_more information than-just_seeing ,a single-long-message. In specific, it ensures-that encrypting the same_message twice yields 2 completely different ciphertexts, which is -necessary in order_for the_encryption scheme to be semantically secure.

COUNTER IV:

THE COUNTERVALUES ARE NOT THAT MUCH AUTHORISED IF AN ATTACKER TRIES TO SWAP ORDER OF MESSage then cyphertext is not altered but only the MAC,during DECRYPTION BFIRST BLOCK OF MESSAGE WE WILL GET ,WOULD BE MESSED UP AND THE REMAINING MESSAGE WILL BE INTACT.

//WE CAN INFER THAT P2 ALSO CANTAINS THE WORDS OR TEXT: 69 ,75 ,74 ,72 ,21 WE CAN DEDUCE THIS BY MATCHING BOTH THE CYPHER TEXT AND KNOWNING THE WORD FROM PLAIN TEXT ,P1 CORRESPONDING TO C1

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