Re: [stella] A7800

[John Saeger <john@xxxxxxxxxxx>]

Keith W Gerdes wrote:
> 
> Here's the working version of Bruce Tomlin's CRYPT78
> converted to C.
> 
> This program validates the 120-byte checksum in 7800 carts.

Well, I've been wanting to see the magic number Z in decimal, just to
see for myself that it's really impossible to factor...

Here's how I got it.

vect78.c (based on crypt78) dumps the contents of A_ROM into a row
vector suitable for inputting into a multi-precision calculator.  I'm
using gp for now.  It looks like A_ROM has an extra byte tagged onto the
end that never gets used.  I dump that out anyway just in case.

vect78.gp is the output of vect.c.

maken78.gp is a little gp program to convert the row vector into an
integer.  It only does the first 120 elements and ignores the extra one
at the end.

n78.gp is the output of maken78.gp.  Its the decimal representation of
Z.  (Unless of course I've made some mistake.)  288 digits.

So, I'm under the impression that if we can factor this puppy, we can
reverse the 7800 encryption scheme.  So I've been having gp try to
factor it for a while...  No luck yet though ;-).

I wonder if there's a way to split up the work of doing the
factorization, along the lines of Brad's suggestion?

John

Attachment: vect78.c
Description: application/unknown-content-type-c_auto_file

vect=[9, 202, 201, 198, 180, 18, 8, 27, 96, 88, 129, 75, 134, 1, 216, 191, 217, 37, 160, 123, 220, 50, 121, 132, 59, 124, 188, 47, 226, 226, 250, 141, 10, 0, 59, 197, 236, 175, 45, 138, 205, 6, 147, 106, 165, 20, 70, 119, 196, 106, 178, 83, 54, 239, 140, 206, 12, 162, 104, 113, 211, 115, 232, 247, 109, 6, 181, 32, 239, 35, 71, 12, 81, 85, 200, 254, 244, 88, 196, 63, 32, 167, 103, 56, 176, 118, 226, 196, 216, 5, 99, 248, 60, 88, 59, 45, 34, 204, 136, 179, 113, 143, 29, 128, 10, 135, 189, 161, 89, 35, 233, 112, 226, 211, 236, 70, 104, 128, 66, 57, 234]

{
  n=0;
  for (i=1, 120,
    n=n*256;
    n=n+vect[i];
    );
  print(n);
}

372763642186038806257268716646134295445276919770501371632454989813933011717191815478227077666354717093229568663134016637150236330535225150892192538221159833191692967632985205532866327828781379842477084956795255916389772921854265394451056360909015523895044054544800868529030160209747657273