Identifying Serpent From Its Cipher Characteristics
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?
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The test measures whether you can fingerprint a block cipher from its round count, word structure, and S-box input/output widths; the trap is picking a well-known name like RC5 or TEA based on '128-bit' alone while ignoring that neither uses S-boxes at all.
This CEH cryptography question describes a cipher's block size, key sizes, round count, and S-box structure, and asks which algorithm matches. The described traits belong to Serpent, a 32-round AES-candidate block cipher with a 128-bit block, 128/192/256-bit keys, and eight 4-bit S-boxes operating in parallel on four 32-bit words.
Learners most often pick RC5 or TEA: RC5 is chosen because it 'sounds like a symmetric block cipher,' but RC5 has a variable round count and uses only modular addition, XOR, and rotations—no S-boxes—while TEA has 64 rounds, a fixed 128-bit key, and a Feistel structure built on shifts and additions rather than substitution boxes. CAST-128 is wrong because it uses a 64-bit block and large 8x32-bit S-boxes.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
Serpent, designed by Anderson, Biham, and Knudsen as an AES candidate, operates on a 128-bit block with key options of 128, 192, and 256 bits, which exactly matches Tony's requirements. It performs 32 rounds, and each round's work is distributed across four 32-bit words processed in parallel using eight S-boxes that each take 4 input bits and produce 4 output bits—precisely the "8-variable S-boxes with 4-bit entry and 4-bit exit" stated in the question. Because every stated characteristic (block size, key sizes, round count, word structure, and S-box geometry) maps cleanly onto Serpent, this is the algorithm Tony should integrate.Why the Other Options Are Wrong
CAST-128 is a 64-bit block cipher with a variable key of 40 to 128 bits and only 12 to 16 rounds, and its S-boxes are large 8-by-32-bit tables, not 4-bit S-boxes, so it fails on block size, key sizes, round count, and S-box design simultaneously. RC5 uses a parameterized structure with a variable number of rounds (typically 12 to 20), a 64-bit block in its common form, and its round function relies solely on modular addition, XOR, and data-dependent rotations—RC5 has no substitution boxes at all. TEA uses a 128-bit block but a single fixed 128-bit key derived via 64 rounds of a Feistel network using shifts, XORs, and additions with a delta constant, so it fails on both the key-size range and the S-box requirement. The CEH textbook (page 3331 in the referenced edition) explicitly lists Serpent's properties with these exact figures, confirming the match.Community Comment Notes
The community consensus aligns with the analysis: every recorded vote selected D, and kenjeshry quoted the CEH textbook wording verbatim, noting that Serpent "involves 32 operating rounds on four 32-bit word blocks using 8 variable S-boxes with 4-bit entry and 4-bit exit." insaniunt likewise confirmed that "Serpent is a symmetric key block cipher that meets the specified criteria" for integration into Tony's software program. The lone non-answer comment expressed uncertainty about the ethical hacking strategy without challenging the cipher analysis, so it provides no counter-evidence.Exam Strategy
When a cipher-identification question lists block size, key sizes, round count, and S-box dimensions, treat the details as a fingerprint and eliminate options that miss even one trait. Memorize the headline facts for common ciphers: RC5/RC6 (no S-boxes, rotations and modular addition), TEA/XTEA (64 rounds, fixed 128-bit key, no S-boxes), Serpent (32 rounds, eight 4-bit S-boxes, 128/192/256-bit keys), and CAST-128 (64-bit block, large S-boxes).
Frequently Asked Questions
Why is RC5 wrong when it is also a symmetric block cipher?
RC5 has a variable round count, a 64-bit block in its common form, and uses only modular addition, XOR, and rotations—its round function contains no S-boxes, so it cannot match the described 4-bit substitution boxes.
How is Serpent different from TEA here?
TEA uses 64 rounds and a fixed 128-bit key with a shift/XOR/addition Feistel structure, whereas Serpent uses 32 rounds, three key sizes, and eight parallel 4-bit S-boxes as described.