Re: tripple des

[Timothy Newsham <newsham@wiliki.eng.hawaii.edu>]

> > 
> > Crypto question:
> > why was the following chosen for tripple DES :
> >    EN(DE(EN(data,k1),k2),k3);   
> 
> In fact, "triple" DES goes three times through the engine, but only uses
> two keys:
> 
>      EN(DE(EN(data,k1),k2),k1)
> 

hmm...  I am using d3des which I had assumed uses 'tripple-DES'.
at any rate,  I used the Ddes() function, printed its output,
then used the des() function 3 times and prined its output.  They
matched up which suggests that d3des uses the method I posted at
the top.  


> My understanding is that this was chosen for hardware implementations
> because it is equivalent to single DES when k1 = k2.  This is important,
> of course, when some people you want to talk to are still using single DES
> and the hardware is hard to reconfigure.
interesting.  Wouldnt the first scheme do the same?  for k1=k2, and
k3 = any key ?
(or k2=k3 and k1 = any key)

> Joe
> --
> Joe Thomas <jthomas@access.digex.net>           Say no to the Wiretap Chip!


-- main.c, compares Ddes output and des output ---------
#include "d3des.h"
unsigned long enkey[96],dekey[96];
unsigned long e1[32],e2[32],e3[32];
unsigned long d1[32],d2[32],d3[32];

main()
{
  char *a,b[100],*k,*k1,*k2,*k3;

  strcpy(b,"this is a test");
  k="testing123423456789212345678";
  k1="testing1";
  k2="23423456";
  k3="78921234";
  deskey(k1,0); cpkey(e1);
  deskey(k2,1); cpkey(d2);
  deskey(k3,0); cpkey(e3);
  des3key(k,0); cp3key(enkey);    /* set up long keys , encrypt */
  des3key(k,1); cp3key(dekey);    /* decrypt */
  use3key(enkey); Ddes(b,b);      /* encrypt b */
  write(1,b,16);
  use3key(dekey); Ddes(b,b);      /* decrypt b */
  write(1,b,16);

  usekey(e1); des(b,b);
  usekey(d2); des(b,b);
  usekey(e3); des(b,b);
  write(1,b,16);
}