S-box design (input/output)

Comments

• zibadian

  January 2008

  : Hello,
  :
  : On a article of Wikipedia
  : (http://en.wikipedia.org/wiki/Substitution_box ), we can found a
  : example of 6
• z4n9ief

  January 2008

  Thanks for your answer, but I don't very well understand it.
  
  In fact I think if a S-box use a input.length < output.length, then it's beacause this S-box use some information about the input and therefore there is a redundancy, so this kind of S-box is not interesting.
  That's the reason why I think it's impossible to do, and even if we try to, it's a bad idea.
  
• zibadian

  January 2008

  : Thanks for your answer, but I don't very well understand it.
  :
  : In fact I think if a S-box use a input.length < output.length, then
  : it's beacause this S-box use some information about the input and
  : therefore there is a redundancy, so this kind of S-box is not
  : interesting.
  : That's the reason why I think it's impossible to do, and even if we
  : try to, it's a bad idea.
  :
  Here's an example to clarify:
  Take for example this S-box: abcd
  and as input the byte 1.
  A cypher could perform the following:
  output = ((a xor 1) and (b xor 1)) or ((c xor 1) and (d xor 1))
  after the output has been determined, the S-box is rotated (becoming cadb) and is ready to process the second byte.
  As you can see the output is also 1 byte and the whole S-box (4 bytes) is used in the transformation.
  As you also might see, there's no redundancy in the S-box.
  
  Given that blowfish is considered to be a good blockcipher, I tend to disagree with your idea that having a larger S-box than the input is a bad idea.
• z4n9ief

  January 2008

  : Here's an example to clarify:
  : Take for example this S-box: abcd
  : and as input the byte 1.
  : A cypher could perform the following:
  : output = ((a xor 1) and (b xor 1)) or ((c xor 1) and (d xor 1))
  : after the output has been determined, the S-box is rotated (becoming
  : cadb) and is ready to process the second byte.
  : As you can see the output is also 1 byte and the whole S-box (4
  : bytes) is used in the transformation.
  : As you also might see, there's no redundancy in the S-box.
  :
  : Given that blowfish is considered to be a good blockcipher, I tend
  : to disagree with your idea that having a larger S-box than the input
  : is a bad idea.
  
  Thank you very much for your explication. Now I see that's possible, thanks to some calculs (unlike in the S-box example on Wikipedia).
  
  Regards.