Embedding signatures into executables

http://slashdot.org/article.pl?sid=04/08/12/2051219&tid=172

If you embed a signature of the file into the file, doesn't this by definition change the file's signature?

1. Set the swappable instructions in the program to their bitwise equivalent of 0.
2. Calculate a signature based on that number.
3. Swap the instructions to encode that number.

To decode.

1. Find swappable instructions.
2. Determine what bit setting they're at.
3. Set their bit setting to 0.
4. Recalc signature based on the new bit setting.
5. Compare to the bit setting you just retrieved.

I would still recommend publishing a separate public key, however,
and include an encrypted signature in the program. As you say, it can
always be changed and re-encoded.

On the other hand, this might be useful on a server, by
encoding a public key and checker on a CD-R and checking all your
programs periodically against the CD-R key. You could encode signatures
in each program and be able to upgrade programs from a central encoding
server without having to write a new cd each time.

================================
Say you have an executable:

1337PROGRAM

Your signature checking routine then does this:

1_3_3_7_P_R_O_G_R_A_M

and computes the hash

deadbabeca

And then sends:

1d3e3a7dPbRaObGeRcAaM

To reverse, we extract the hash (deadbabeca) and the "original" executable.

Then we compute the hash (of 1_3_3_7...) and check if it matches...

In summary, we embedded a checksum, but we removed it before we checked it. Simple, really.