Dec 182017
Proof you can modify a file w/o changing its CRC!.
File PROVECRC.ZIP from The Programmer’s Corner in
Category System Diagnostics
Proof you can modify a file w/o changing its CRC!.
File Name File Size Zip Size Zip Type
PROVECRC.EXE 12422 7888 deflated
READ.ME 4707 1712 deflated

Download File PROVECRC.ZIP Here

Contents of the READ.ME file

Gilmore Systems
PO Box 3831
Beverly Hills, CA 90212-0831

Voice: (213) 275-8006 BBS: (213) 276-5263

* *
* CRC checking is NOT reliable for detecting file changes! *
* *

CRC checking as a means of detecting changes to a file, for purposes of
detecting the presence of a computer virus for example, is an unreliable
check which can be easily evaded.

There are many programs which claim to check your files for changes
based on their CRC (Cyclic Redunancy Check) value. These programs all
assume that if the CRC of a file changes, it means that the contents of
the file have changed - a correct assumption. However, they also assume
that if the CRC hasn't changed, neither has the contents of the file -
an incorrect and risky assumption!

CRC checking is fine for telecommunications - which is what it was
designed for in the first place. Sending and receiving sequential bytes
of a file with your modem and CRC checking work very nicely. However,
when some resident (TSR) virus is in your system, it's got all day to
alter your files and figure out how to leave CRC's unchanged. It can
bypass so called "vaccine" programs.

The deviants that write computer viruses are inserting code that
attempts to evade all forms of detection which can fool both vaccine and
detection programs.

In a short period of time, we've developed a program which proves that
we can alter the contents of a file, yet keep the original CRC value the
same. This means that the altered file will go undetected as being
changed by a CRC checking program.

Our program, PROVECRC.EXE will not alter your original file. It will
leave your original file intact but will make an altered copy of the
file under a different name. If you run a CRC checking program on both
the original and the altered copy, you'll find the CRC to be identical
for both files. However, running the DOS COMP program will show that
both files are indeed different!

We're very concerned about computer viruses here, and have developed
some protection against these things. For instance, our File Integrity
Checking system not only does more than any other CRC checker written,
it performs our unique MCRC (Modified CRC) which is absolutely
foolproof! Our MCRC check picks up alterations that CRC doesn't.

PROVECRC.EXE will prove that a file can be altered and keep the original
CRC. If some virus-writing programmer hacks away at our File Integrity
Checking system to figure out our unique MCRC algorithm and writes viral
code to evade that check ... no problem! If MCRC checking can't detect
the change, CRC checking will. Likewise, if CRC checking can't detect
the change, MCRC checking will. It's impossible to write code that can
alter a file and keep both CRC and MCRC the same!

Download a shareware version of FICHECK3.ARC from our BBS and try it.
If you like it, register it and we'll send you our registered user
version which performs dual CRC/MCRC checking in a single pass (the
shareware versions are separate CRC and MCRC programs), and update your
access to full priveleges on our BBS for 6 months. By the way, our BBS
is dedicated to the topic of computer viruses.

For instructions on how to use the PROVECRC.EXE program, simply run the
program without any parameters - instructions will be printed on your
screen. You'll need a CRC checking program to prove to yourself that
the CRC values of original and altered copy are identical. You'll also
need the DOS COMP program (comes with DOS) to prove that the files are
indeed different. Version 3 of our File Integrity Checking system will
also prove that the files are different based on our MCRC check!

- Chuck Gilmore

Gilmore Systems
PO Box 3831
Beverly Hills, CA 90212-0831

Voice: (213) 275-8006 BBS: (213) 276-5263


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