Serpent (cipher)

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Serpent
Serpent's linear mixing stage
General
Designers	Ross Anderson, Eli Biham, Lars Knudsen
First published	1998-08-21
Derived from	Square
Certification	AES finalist
Cipher detail
Key sizes	128, 192 or 256 bits
Block sizes	128 bits
Structure	Substitution-permutation network
Rounds	32

Serpent is a symmetric key block cipher which was a finalist in the Advanced Encryption Standard (AES) contest, where it came second to Rijndael. Serpent was designed by Ross Anderson, Eli Biham, and Lars Knudsen.

Like other AES submissions, Serpent has a block size of 128 bits and supports a key size of 128, 192 or 256 bits. The cipher is a 32-round substitution-permutation network operating on a block of four 32-bit words. Each round applies one of eight 4-bit to 4-bit S-boxes 32 times in parallel. Serpent was designed so that all operations can be executed in parallel, using 32 1-bit slices. This maximizes parallelism, but also allows use of the extensive cryptanalysis work performed on DES.

Serpent was widely viewed as taking a more conservative approach to security than the other AES finalists, opting for a larger security margin: the designers deemed 16 rounds to be sufficient against known types of attack, but specified 32 rounds as insurance against future discoveries in cryptanalysis.

The Serpent cipher has not been patented. It is completely in the public domain and can be freely used by anyone. There are no restrictions or encumbrances whatsoever regarding its use. As a result, anyone is free to incorporate Serpent in their software (or hardware implementations) without paying license fees.

Rijndael vs. Serpent

Rijndael is a substitution-linear transformation network with ten, twelve, or fourteen rounds, depending on the key size, and with block sizes of 128 bits, 192 bits, or 256 bits, independently specified. Serpent is a substitution-linear transformation network which has thirty-two rounds, plus an initial and a final permutation to simplify an optimized implementation. The round function in Rijndael consists of three parts: a nonlinear layer, a linear mixing layer, and a key-mixing XOR layer. The round function in Serpent consists of key-mixing XOR, thirty-two parallel applications of the same 4x4 S-box, and a linear transformation, except in the last round, wherein another key-mixing XOR replaces the linear transformation. The nonlinear layer in Rijndael uses an 8x8 S-box whereas Serpent uses eight different 4x4 S-boxes. The 32 rounds make Serpent more secure than Rijndael; however, Rijndael with 10 rounds is faster and easier to implement for small blocks. Hence, Rijndael was selected as the winner in the AES competition. However, it is interesting to note that, in October 2005, a cache timing attack (assuming that the attacker is able to run programs on the machine computing Rijndael) against Rijndael was demonstrated by Dag Osvik, Adi Shamir, and Eran Tromer with one attack being able to obtain an entire Rijndael key in just 800 operations triggering encryptions, and taking only 65 milliseconds.

Security

The XSL attack, if effective, would weaken Serpent (though not as much as it would weaken Rijndael, which became AES). However, many cryptanalysts believe that once implementation considerations are taken into account the XSL attack would be more expensive than a brute force attack.

See also

• Tiger - hash function by the same authors.

External links

• Serpent homepage
• SCAN's entry for Serpent
• In Pellicano Case, Lessons in Wiretapping Skills NYTimes May 5, 2008

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