Serpent Encryption

Overview

About the Advanced Encryption Standard (AES)

The DES algorithm has become obsolete and is in need of replacement. To this end the National Institute of Standards and Technology (NIST) has been holding a competition to develop the Advanced Encryption Standard (AES) as a replacement for DES. Triple DES has been endorsed by NIST as a temporary standard to be used until the AES is finished sometime in 2001.

NIST has been working very closely with industry and the cryptographic community during the development of the Advanced Encryption Standard. The overall goal is to develop a Federal Information Processing Standard (FIPS) that specifies an encryption algorithm (or algorithms) capable of protecting sensitive government information well into the next century. The algorithm(s) is expected to be used by the U.S. Government and, on a voluntary basis, by the private sector.

On January 2, 1997, NIST announced the initiation of the AES development effort and made a formal call for algorithms on September 12 of that year. The call stipulated that the AES would specify an unclassified, publicly disclosed encryption algorithm(s), available royalty-free, worldwide. In addition, the algorithm(s) must implement symmetric key cryptography as a block cipher and (at a minimum) support block sizes of 128 bits and key sizes of 128, 192, and 256 bits.

On August 20, 1998, NIST announced a group of fifteen AES candidate algorithms at the First AES Candidate Conference (AES1). These algorithms had been submitted by members of the cryptographic community from around the world. At that conference and in a simultaneously published Federal Register notice, NIST solicited public comments on the candidates. A Second AES Candidate Conference (AES2) was held in March 1999 to discuss the results of the analysis conducted by the global cryptographic community on the candidate algorithms. The public comment period on the initial review of the algorithms closed on April 15, 1999. Using the analyses and comments received, NIST selected five algorithms from the original fifteen submissions.

The AES finalist candidate algorithms are MARS, RC6, Rijndael, Serpent, and Twofish. Four of the algorithms (MARS, Rijndael, Serpent, and Twofish) are supported by Private Encryptor. NIST has developed a Round 1 Report describing the selection of the finalists.

These finalist algorithms received further analysis during a second, more in-depth review period prior to the selection of the final algorithm(s) for the AES. The comment period on the remaining algorithms ended on May 15, 2000. Comments and analysis were actively sought by NIST on any aspect of the candidate algorithms, including, but not limited to, the following topics: cryptanalysis, intellectual property, comparative analyses of all of the AES finalists, and overall recommendations and implementation issues. An informal AES discussion forum was also provided by NIST for interested parties to discuss the AES finalists and relevant AES issues.

Near the end of Round 2, NIST sponsored the Third AES Candidate Conference (AES3), which was an open, public forum for discussion of the analyses of the AES finalists. AES3 was held April 13-14, 2000 in New York. Submitters of the AES finalists were invited to attend and engage in discussions regarding comments on their algorithms.

At the time this document is being written, the Round 2 public analysis period is just ending. Over the next few months NIST intends to study all available information from the Round 2 analysis and make a selection for the AES from among one or more of the finalists. Currently, NIST anticipates that it will announce the AES selection by late summer or early fall of 2000. No date has yet been set for this announcement. Following the announcement, NIST intends to publish a Round 2 Report that will summarize information from Round 2 and explain the algorithm selection.

Shortly thereafter, a draft Federal Information Processing Standard (FIPS) for the AES will be published for public review and comment. Following the comment period, the standard will be revised by NIST in response to those comments. A review and approval process will then follow. If all steps of the AES development process proceed as planned, it is anticipated that the standard will be completed by the summer of 2001.

About Serpent

Serpent was designed by Ross Anderson, Eli Biham and Lars Knudsen as a candidate for the Advanced Encryption Standard. It has been selected as one of the five finalists in the AES competition. Serpent is faster than DES and more secure than Triple DES. It provides users with a very high level of assurance that no shortcut attack will be found. To achieve this, the algorithm's designers limited themselves to well understood cryptography mechanisms, so that they could rely on the wide experience and proven techniques of block cipher cryptanalysis. The algorithm uses twice as many rounds as are necessary to block all currently known shortcut attacks. This means that Serpent should be safe against as yet unknown attacks that may be capable of breaking the standard 16 rounds used in many types of encryption today. However, the fact that Serpent uses so many rounds means that it is the slowest of the five AES finalists. But this shouldn't be an issue because it still outperforms Triple DES. The algorithm's designers maintain that Serpent has a service life of at least a century.

Serpent was released at the 5th International Workshop on Fast Software Encryption. This iteration of the algorithm was called Serpent 0 and used the original DES S-boxes. After comments, the key schedule was changed slightly and the S-boxes were changed; this new iteration of Serpent is called Serpent 1. This is the version used by Private Encryptor.

In Depth

Serpent is a 128-bit block cipher, meaning that data is encrypted and decrypted in 128-bit chunks. The key length can vary, but for the purposes of the AES it is defined to be either 128, 192, or 256 bits. This block size and variable key length is standard among all AES candidates and was one of the major design requirements specified by NIST. The Serpent algorithm uses 32 rounds, or iterations of the main algorithm.

Private Encryptor's implementation of Serpent uses a 256 bit key. We decided to use the largest possible key size to ensure that the user always enjoys the best possible security. Our design philosophy is that security always comes before speed. If a shorter key is provided by the user, the Serpent algorithm itself pads the key to make it 256 bits long.

Like DES, Serpent includes an initial and final permutation of no cryptographic significance; these permutations are used to optimize the data before encryption. The detailed description of the actual algorithm is contained in the official Serpent paper submitted for the AES by the algorithm's designers. The paper is rather technical and a certain degree of mathematical proficiency is required of the reader in order to understand it.

Download the Serpent algorithm specification: Serpent.pdf

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