CTR and OFB are both modes used for block algorithms. Which of the following does not describe a true difference between them?
A.
OFB uses IV values, and CTR uses values that are incremented.
B.
CTR uses chaining in its encryption process, and OFB does not.
C.
CTR encryption and decryption processes can happen in parallel, and OFB processes cannot.
D.
CTR has better performance than OFB.
No answer given.
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I believe the answer is C.
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Admin: There is no answer, the correct answer is “B”.
Counter Mode Counter Mode (CTR) is very similar to OFB mode, but instead of using a randomly unique IV value to generate the keystream values, this mode uses an IV counter that increments for each plaintext block that needs to be encrypted. The unique counter ensures that each block is XORed with a unique keystream value.
The other difference is that there is no chaining involved, which means no ciphertext is brought forward to encrypt the next block. Since there is no chaining, the encryption of the individual blocks can happen in parallel, which increases the performance. The main reason CTR would be used instead of the other modes is performance.
This mode has been around for quite some time and is used in encrypting ATM cells for virtual circuits, in IPSec, and is now integrated in the new wireless security standard, 802.11i. A developer would choose to use this mode in these situations because individual ATM cells or packets going through an IPSec tunnel or over radio frequencies may not arrive at the destination in order. Since chaining is not involved, the destination can decrypt and begin processing the packets without having to wait for the full message to arrive and then decrypt all the data.
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