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I thought of implementing DES modes of operation as a project...But my guide asked to change or to modify the DES algorithm in such a way to overcome drawbacks of DES...So is there any way to do changes?IF so how?and could you please give me idea on how to bring changes in any of 5 modes of DES.please help me... Thank you

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Hi there, Welcome to IT Security Stackexchange. Unfortunately this site doesn't exist to do your homework for you. Please feel free to come back with more specific questions that can be reasonably answered. –  Terry Chia Feb 9 '13 at 6:43
    
Sounds to me like it's more open-ended than it is for a specific assignment. Figured I'd give it a punt over here. –  Jeff Ferland Feb 9 '13 at 8:25
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Isn't the assignment finding out what to change? I think you should read up on the algorithms first and make sure you understand them as best as you can. Then come back when you have more specific questions. If we do your homework for you, you won't be learning anything :) –  Lucas Kauffman Feb 9 '13 at 8:41
    
First you need to figure out what drawbacks you mean? The small key? The small block-size? The bad software performance? –  CodesInChaos Feb 9 '13 at 8:45
    
seems like the question might actually be about modes of operation instead of DES? like ECB, CBC, etc? –  Peter Elliott Feb 9 '13 at 15:23
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1 Answer

up vote 7 down vote accepted

Main drawbacks of DES are:

  • Small key space (56 bits).
  • Small blocks (64 bits).
  • Terrible performance in software, due to all the bit-juggling.
  • Relative weaknesses with regards to linear and differential cryptanalysis.

Changing any of these will imply heavy changes, not little tweaking. Doing that while maintaining or increasing security is no mere feat... and it has been done. It was called the AES competition and involved dozens of cryptographers devoting several years of thinking time to the subject. Therefore, to fulfill your guide instruction to the letter, the most reasonable way is to ditch DES altogether, and use AES.

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Thank you @thomas Pornin....my doubt is ,can we bring any changes in the design of DES?like can we replace XOR with some other operations? and can we choose our own tables for initial and final permutations? –  Nandy Feb 10 '13 at 10:19
    
We can do any change we want; just about anything will keep on encrypting and decrypting. The hard part is to make a secure algorithm. –  Thomas Pornin Feb 10 '13 at 13:11
    
Sir,are you sure that both encrypting and decrypting works well when XOR is replaced with OR in both Function F and in each round? –  Nandy Feb 10 '13 at 14:10
    
@Nandy: The OR operation is not really invertible, so this is a kinda bad idea (at least for some uses of the operation). –  Paŭlo Ebermann Feb 10 '13 at 19:24
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