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Topic Started: Jun 28 2006, 05:28 PM (328 Views)
loki Jun 28 2006, 05:28 PM Post #1
Advanced Member
[ * * * ]
 I have been doing research into differnet mixing algorthyms, I have found two different algorythms and have done moderate testing on them. Tweaked and managled them.

The one presented here was apparently a modified version of the psuedo hadamard transformation from twofish.

I thought it was cool and took a look at it. Apart from poor coding, it looks interesting. I tweaked it for speed and it still follows the same basic idea as the original. It appears to be good to me, but what do I know? was wondering if anyone could find an exploit in the code that will make it insecure to use?

the original code is as follows
Code:
 

unsigned char pht(unsigned char a, unsigned int b)
{
  long t;
  t = a + b;
  t = (2 * a) + (2 * b);
  t = t ^ 0xa9;
  t = t ^ a;
  return t;
}


I have modified it to this.
Code:
 

static unsigned long pht(unsigned long a, unsigned long b) {
  register unsigned long t;
  t   = a ^ b  ;
  t <<= 1      ;
  t  ^= 0x74bul;
  return (t ^ a);
}


I always here about systems failing becuase of a simple algorthym, this looks good to me.

How can I improve opon it, or is it just a silly mixer ?

The only thought I can think of to improve this is, replacing 0x74b with the xor of a nd b.

Any inputs or ideas or suggestions?
c(x) = 3x3 + x2 + x + 2; Find the inverse
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insecure Jul 27 2006, 04:44 AM Post #2
Elite member
[ * * * * * ]
 Here's that code again, annotated:

Code:
 

unsigned char pht(unsigned char a, unsigned int b)
{
 long t;
 t = a + b; /* This line is redundant because of the following line */
 t = (2 * a) + (2 * b); /* simpler: t = 2 * (a + b); */
 t = t ^ 0xa9; /* assumes you don't care about high bits of big chars */
 t = t ^ a;
 return t;
}


0xA9 is 11111001 in binary. It's hard to see how this is a useful mask.

Naturally, the doubling will ensure that the low bit of t is clear. XORing this with 1 (which is done by ^ 0xA9) will set it. And XORing it with a again will either clear it (if the low bit of a is set) or set it (if the low bit of a is clear). Thus, the low bit of the result is simply the inverse of the low bit of one of the inputs.

Whether that's exploitable, I don't know. It depends how the function is being used, I guess.
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