Question at position 12 Which mode allows full parallelization for both encryption and decryption?CFB - Cipher Feedback ModeCTR - Counter ModeCBC - Cipher Block ChainingOFB - Output Feedback ModeClear my selection

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BlackTom AI · Accepted Answer

Let's examine each mode and how it affects parallelization across encryption and decryption.

Option A: CFB - Cipher Feedback Mode. In CFB, each block of plaintext is XORed with a shifted version of the previous ciphertext block after it has been produced. The dependency on the previous ciphertext means you cannot parallelize encryption (and likewise decryption) across blocks; each step must wait for the prior block result. This makes full parallel processing impossible.

Option B: CTR - Counter Mode. Here, a counter value is encrypted to produce a keystream block, which is then XORed with the plaintext to yield ciphertext (and the same keystream is XORed with ciphertext to recover plaintext). The crucial point is that keystream blocks are independent of each other and can be generated in parallel for different counter values. As a result, both encryption and decryption can be fully parallelized because you can compute multiple keystream blocks simultaneously and apply XOR without any inter-block dependencies.

Option C: CBC - Cipher Block Chaining. In CBC, each plaintext block is XORed with the previous ciphertext block before encryption. The decryption process similarly depends on the previous ciphertext. Because of this chaining, blocks must be processed sequentially, preventing full parallelization of either encryption or decryption.

Option D: OFB - Output Feedback Mode. OFB feeds back the output of the block cipher (encrypted feedback) to produce the keystream, which is then XORed with the plaintext. While the keystream is deterministic with respect to the IV and can be computed beforehand, the generation of the keystream itself depends on prior outputs, introducing dependencies that limit true full parallelization for both encryption and decryption. In practice, OFB does not offer the same straightforward, block-level parallelism as CTR.

Putting this together, among the options, CTR - Counter Mode uniquely enables full parallelization for both encryption and decryption due to its independent keystream blocks. The other modes introduce inter-block dependencies that hinder parallel processing.

Question at position 12 Which mode allows full parallelization for both encryption and decryption?CFB - Cipher Feedback ModeCTR - Counter ModeCBC - Cipher Block ChainingOFB - Output Feedback ModeClear my selection

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