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312-50V12 · Question #278

Tony wants to integrate a 128-bit symmetric block cipher with key sizes of 128, 192, or 256 bits into a software program, which involves 32 rounds of computational operations that include…

The correct answer is D. Serpent. The question asks to identify a symmetric block cipher that matches specific criteria, including a 128-bit block size, variable key lengths up to 256 bits, 32 rounds, and the use of S-boxes.

Submitted by emma.c· Mar 4, 2026Cryptography

Question

Tony wants to integrate a 128-bit symmetric block cipher with key sizes of 128, 192, or 256 bits into a software program, which involves 32 rounds of computational operations that include substitution and permutation operations on four 32-bit word blocks using 8-variable S-boxes with 4-bit entry and 4-bit exit. Which of the following algorithms includes all the above features and can be integrated by Tony into the software program?

Options

  • ACAST-128
  • BRC5
  • CTEA
  • DSerpent

How the community answered

(53 responses)
  • A
    6% (3)
  • B
    2% (1)
  • C
    4% (2)
  • D
    89% (47)

Why each option

The question asks to identify a symmetric block cipher that matches specific criteria, including a 128-bit block size, variable key lengths up to 256 bits, 32 rounds, and the use of S-boxes.

ACAST-128

CAST-128 is a 64-bit block cipher, which does not meet the requirement for a 128-bit block size.

BRC5

RC5 does not utilize S-boxes in its design; it is a rotation-based cipher, contradicting the specified requirement for S-box operations.

CTEA

TEA (Tiny Encryption Algorithm) is a 64-bit block cipher and does not employ S-boxes, failing to meet several specified criteria including the block size.

DSerpentCorrect

Serpent is a 128-bit block cipher designed to operate on four 32-bit word blocks, which supports key sizes of 128, 192, or 256 bits, and executes exactly 32 rounds of operations. Its design explicitly incorporates eight distinct 4x4 S-boxes (4-bit entry, 4-bit exit) and uses both substitution and permutation operations, fully aligning with all the detailed technical requirements specified.

Concept tested: Symmetric block cipher characteristics and identification

Topics

#Serpent algorithm#symmetric block cipher#cryptographic algorithms#key sizes

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