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Serpent (cipher)
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Serpent (cipher)
Serpent is a symmetric key block cipher that was a finalist in the Advanced Encryption Standard (AES) contest, in which it ranked 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 bit slices. This maximizes parallelism but also allows use of the extensive cryptanalysis work performed on DES.
Serpent took a conservative approach to security, opting for a large 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 official NIST report on AES competition classified Serpent as having a high security margin like MARS and Twofish and in contrast to the adequate security margin of RC6 and Rijndael (currently AES). In final voting, Serpent had the fewest negative votes among the finalists but ranked in second place overall because Rijndael had substantially more positive votes, the deciding factor being that Rijndael allowed for a far more efficient software implementation.[citation needed]
The Serpent cipher algorithm is in the public domain and has not been patented. The reference code is public domain software, and the optimized code is licensed under the GPL. There are no restrictions or encumbrances regarding its use. As a result, anyone is free to incorporate Serpent in their software (or in hardware implementations) without paying license fees.
The Serpent key schedule consists of 3 main stages. In the first stage the key is initialized by adding padding if necessary. This is done in order to make short keys map to long keys of 256-bits, one "1" bit is appended to the end of the short key followed by "0" bits until the short key is mapped to a long key length.
In the next phase, the "prekeys" are derived using the previously initialized key. 32-bit key parts XORed, the FRAC which is the fraction of the Golden ratio and the round index is XORed with the key parts, the result of the XOR operation is rotated to left by 11. The FRAC and round index were added to achieve an even distribution of the keys bits during the rounds.
Finally the "subkeys" are derived from the previously generated "prekeys". This results in a total of 33 128-bit "subkeys".
At the end the round key or "subkey" are placed in the "initial permutation IP" to place the key bits in the correct column.
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Serpent (cipher)
Serpent is a symmetric key block cipher that was a finalist in the Advanced Encryption Standard (AES) contest, in which it ranked 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 bit slices. This maximizes parallelism but also allows use of the extensive cryptanalysis work performed on DES.
Serpent took a conservative approach to security, opting for a large 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 official NIST report on AES competition classified Serpent as having a high security margin like MARS and Twofish and in contrast to the adequate security margin of RC6 and Rijndael (currently AES). In final voting, Serpent had the fewest negative votes among the finalists but ranked in second place overall because Rijndael had substantially more positive votes, the deciding factor being that Rijndael allowed for a far more efficient software implementation.[citation needed]
The Serpent cipher algorithm is in the public domain and has not been patented. The reference code is public domain software, and the optimized code is licensed under the GPL. There are no restrictions or encumbrances regarding its use. As a result, anyone is free to incorporate Serpent in their software (or in hardware implementations) without paying license fees.
The Serpent key schedule consists of 3 main stages. In the first stage the key is initialized by adding padding if necessary. This is done in order to make short keys map to long keys of 256-bits, one "1" bit is appended to the end of the short key followed by "0" bits until the short key is mapped to a long key length.
In the next phase, the "prekeys" are derived using the previously initialized key. 32-bit key parts XORed, the FRAC which is the fraction of the Golden ratio and the round index is XORed with the key parts, the result of the XOR operation is rotated to left by 11. The FRAC and round index were added to achieve an even distribution of the keys bits during the rounds.
Finally the "subkeys" are derived from the previously generated "prekeys". This results in a total of 33 128-bit "subkeys".
At the end the round key or "subkey" are placed in the "initial permutation IP" to place the key bits in the correct column.
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