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Which of the following block cipher modes is best to use when needing to encrypt a file?

Which of the following block cipher modes is best to use when needing to encrypt a file?

PrepAway - Latest Free Exam Questions & Answers

A.
CBC

B.
CFB

C.
OFB

D.
ECB

2 Comments on “Which of the following block cipher modes is best to use when needing to encrypt a file?

  1. Michael McNeil says:

    Admin: No answer is showing and it should be “A”

    Cipher Block Chaining (CBC) does not reveal a pattern, because each block of text, the key, and the value based on the previous block are processed in the algorithm and applied to the next block of text. This results in more random ciphertext. Ciphertext is extracted and used from the previous block of text. This provides dependence between the blocks, in a sense chaining them together. This is where the name Cipher Block Chaining comes from, and it is this chaining effect that hides any patterns.

    The results of one block are XORed with the next block before it is ncrypted, meaning each block is used to modify the following block. This chaining effect means that a particular ciphertext block is dependent upon all blocks before it, not just the previous block.

    As an analogy, let’s say you have five buckets of marbles. Each bucket contains a specific color of marbles: red, blue, yellow, black, and green. The first bucket of red marbles (block of bits) you shake and tumble around (encrypt) to get them all mixed up. Then you take the second bucket of marbles, which are blue, and pour in the red marbles and go through the same exercise of shaking and tumbling them. You pour this bucket of marbles into your next bucket and shake them all up. This illustrates the incorporated randomness that is added when using chaining in a block encryption process.

    When we encrypt our very first block using CBC, we do not have a previous blockof ciphertext to “dump in” and use to add the necessary randomness to the encryption process. If we do not add a piece of randomness when encrypting this first block, then the bad guys could identify patterns, work backward, and uncover the key. So, we use an initialization vector (IVs were introduced previously in the “Initialization Vectors” section). The 64-bit IV is XORed with the first block of plaintext and then it goes through its encryption process. The result of that (ciphertext) is XORed with the second block of plaintext, and then the second block is encrypted. This continues for the whole message. It is the chaining that adds the necessary randomness that allows us to use CBC mode to encrypt large files. Neither the individual blocks nor the whole message will show patterns that will allow an attacker to reverse-engineer and uncover the key.

    If we choose a different IV each time we encrypt a message, even if it is the same message, the ciphertext will always be unique. This means that if you send the same message out to 50 people and encrypt each one using a different IV, the ciphertext for each message will be different. Pretty nifty.




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