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why DES uses exactly 16 rounds?

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Why DES uses 16 rounds why not more or less then 16. what is the significant of using 16.

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From Schneier's description of DES in Chapter 12 of Applied Cryptography (12.3): “DES with any number of rounds fewer than 16 could be broken with a known-plaintext attack more efficiently than by a brute-force attack.” This explains the "Why not less than 16". As for the "why not more than 16", that is a tradeoff for speed of execution (more rounds = less speed).

up vote 3 down vote

Applied Cryptography mentioned this. With 17 or 18 rounds a differential attack is about as costly as brute-force. And 19 rounds or more makes differential attack impossible since it requires more than 2^64 chosen plaintexts, which is impossible since the DES block size is 64 bits.


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why DES uses exactly 16 rounds?

up vote 14 down vote

Why DES uses 16 rounds why not more or less then 16. what is the significant of using 16.


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up vote 9 down vote

From Schneier's description of DES in Chapter 12 of Applied Cryptography (12.3): “DES with any number of rounds fewer than 16 could be broken with a known-plaintext attack more efficiently than by a brute-force attack.” This explains the "Why not less than 16". As for the "why not more than 16", that is a tradeoff for speed of execution (more rounds = less speed).

edit

yes, but with 16 rounds you have to use brute force, since other type of attacks aren't faster than brute force at 16 rounds, while they are faster with less rounds. Regarding the "possibility to break 16 rounds", see wikipedia - DES Challenges; in 1992 it took 22 hours and 15 minutes to break DES. - tech Aug 14 '13 at 9:06

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