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Serpent

AES finalist cipher by Anderson, Biham, and Knudsen with 32 rounds and 8 S-boxes. Uses substitution-permutation network with 128-bit blocks and 128/192/256-bit keys. Conservative security design.

Properties

Property	Value
Category	Block Ciphers
Sub-category	Block Cipher
Security status	Not classified
Complexity	Advanced
Inventor	Ross Anderson, Eli Biham, Lars Knudsen
Year	1998
Origin	🇬🇧 United Kingdom
Source	algorithms/block/serpent.js

Parameters

Parameter	Supported values
Key sizes	16 bytes (128 bits) to 32 bytes (256 bits) in steps of 8 bytes
Block sizes	16 bytes (128 bits)

Security

Status: not classified — treat as unverified.

Known vulnerabilities

Issue	Description	Mitigation
Performance vs AES	Slower than AES, which contributed to AES selection by NIST	AES preferred for performance-critical applications, Serpent acceptable for high-security needs

Documentation

• Serpent Algorithm Specification
• Serpent: A New Block Cipher Proposal
• NIST AES Candidate Submission

References

• Crypto++ Serpent Implementation
• libgcrypt Serpent Implementation
• Bouncy Castle Serpent Implementation

Test vectors

6 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.

Vector 1 — libgcrypt serpent_test - Serpent-128

Field	Value
key	00000000000000000000000000000000
input	d29d576fcea3a3a7ed9099f29273d78e
expected	b2288b968ae8b08648d1ce9606fd992d

Vector 2 — libgcrypt serpent_test - Serpent-192

Field	Value
key	000000000000000000000000000000000000000000000000
input	d29d576fceaba3a7ed9899f2927bd78e
expected	130e353e1037c22405e8faefb2c3c3e9

Vector 3 — libgcrypt serpent_test - Serpent-256

Field	Value
key	0000000000000000000000000000000000000000000000000000000000000000
input	d095576fcea3e3a7ed98d9f29073d78e
expected	b90ee5862de69168f2bdd5125b45472b

Vector 4 — libgcrypt serpent_test - Serpent-256, counting plaintext

Field	Value
key	0000000000000000000000000000000000000000000000000000000000000000
input	00000000010000000200000003000000
expected	2061a42782bd52ec691ec383b03ba77c

Vector 5 — Botan serpent.vec - variable key, single bit 0x80 in last byte

Field	Value
key	00000000000000000000000000000080
input	00000000000000000000000000000000
expected	ddd26b98a5ffd82c05345a9dadbfaf49

Vector 6 — Botan serpent.vec - variable key, single bit 0x01 in last byte

Field	Value
key	00000000000000000000000000000001
input	00000000000000000000000000000000
expected	f668c7091f81b2827da77dd419b708e1

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