__________ mode is suitable for parallel operation. Because there is no chaining, multiple blocks can be encrypted or decrypted simultaneously. Unlike CTR mode, this mode includes a nonce as well as a counter.

Question

__________ mode is suitable for parallel operation.  Because there is no chaining, multiple blocks can be encrypted or decrypted simultaneously. Unlike CTR mode, this mode includes a nonce as well as a counter.

BlackTom AI · Accepted Answer

XTS-AES is designed for parallelizable disk/block encryption: it uses a per-sector nonce (tweak) plus a per-block counter (index) so each block can be processed independently.

1) Relevant knowledge points and concepts
- Block-cipher modes determine how plaintext blocks relate to each other. Modes with chaining (CBC, OFB) create dependencies between blocks and are sequential. Modes without chaining (CTR, XTS) allow parallel processing.
- XTS-AES (XEX-based Tweaked-codebook with ciphertext Stealing using AES) is a tweakable block-cipher mode used for storage encryption. It uses two AES keys, a sector or data-unit tweak (nonce), and a block index (counter) to create a unique “tweak” for each block.
- Basic XTS math operates in GF(2^128) with a primitive polynomial (commonly x^128 + x^7 + x^2 + x + 1), using multiplication by α (a left-shift with conditional XOR 0x87) to derive successive tweaks.

2) Step-by-step reasoning
- For a given data unit (sector) with nonce N and keys K1, K2:
  a) Compute initial tweak T0 = E_K2(N). (Encrypt the nonce with K2.)
  b) For block index i compute tweak Ti = T0 · α^i (multiply in GF(2^128) — implemented as repeated left-shifts with conditional reduction).
  c) Form modified plaintext: P'i = P_i XOR Ti.
  d) Encrypt: Ci' = E_K1(P'i).
  e) Final ciphertext: C_i = Ci' XOR Ti.
- Because Ti depends only on N and i (not on P_j or C_j for j ≠ i), each block’s encryption or decryption can be done independently and therefore in parallel.

3) Intermediate calculation example (symbolic)
- T0 = E_K2(N)
- T1 = T0 · α
- P'1 = P1 XOR T1
- C1' = E_K1(P'1)
- C1 = C1' XOR T1

4) Why the correct answer is correct
- XTS explicitly uses both a nonce (data-unit/sector number) and a counter (block index) to compute per-block tweaks; it produces independent per-block transformations, enabling parallel encryption/decryption. This matches the description.

5) Why other options are incorrect
- OFB is a feedback mode: keystream generation depends on previous output, so it is sequential and not parallel-friendly.
- 3DES is a cipher (or refers to encryption algorithm), not a block mode that inherently provides nonce+counter tweak behavior for parallel block independence as described.
- S-AES (Simplified AES) is an educational toy cipher, not a standard mode designed for parallel disk encryption with tweaks.

6) Conclusion
The described mode is XTS-AES; this matches the provided correct answer.

__________ mode is suitable for parallel operation. Because there is no chaining, multiple blocks can be encrypted or decrypted simultaneously. Unlike CTR mode, this mode includes a nonce as well as a counter. Single choice

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