Question

We studied three different modes of operations: ECB, CBC, OFB. One mode may be better than another according to a particular criterion. For the following, you don’t have to give any explanations, just state which of the two modes is better according to that criterion:

(a) Preventing BG from building a code book of matching P, C pairs. Which is better: OFB or ECB?

(b) Being able to encrypt blocks in parallel: Which is better: CBC or ECB?

(c) Preventing BG from fooling the recipient by switching two blocks: Which is better: CBC or ECB?

(d) Being able to transmit less than a full block of data: Which is better: OFB or ECB?

(e) Recovering from a transmission error where a 0 got flipped to a 1. Which is better: CBC or ECB?

Answer 1

[A] Preventing BG from building a code book of matching P, C pairs -- > ECB is better becasue Identical plaintexts encrypted similarly.

[B] Being able to encrypt blocks in parallel: Which is better: CBC or ECB --> CBC is better because Decryption is parallelizable in CBC.

[C] Preventing BG from fooling the recipient by switching two blocks: Which is better: CBC or ECB --> CBC is better gives better security by changing the IV for each block.

[D] transmit less than a full block of data: OFB is better.

[E] Recovering from a transmission error where a 0 got flipped to a 1. : CBC is better in case of Error Propogation.