Re: [zooko@zooko.com: [Revctrl] colliding md5 hashes of human-meaningful documents]

[Morten Welinder <mwelinder@gmail.com>]

I looked at this.

If you can find just *one* colliding pair that does not contain a
few select bytes like 0x22 (quote), 0x5c (backslash) and
perhaps 0x0a then it is trivial to make a colliding pair of
C programs.

Call the initial pair (A,B) and make the programs

/* Common prefix that happens to have integer block size
   ending just before the A below. */
const char junk[] = "AB";
/* "AA" in program 1, "AB" in program 2.  */
...
int
main ()
{
  int hsize = sizeof (junk) / 2;
  if (memcmp (junk, junk + hsize, hsize))
    return do_program_2 ();
  else
    return do_program_1 ();
}