Turing and Reverse - Disappearing Cryptography - page 146

other decision functions are lt , le , ge and eq ,whichstandfor
less than, less than or equal to, greater than or equal to, and
equal to.
The current implementation watches for errors that might be
introduced if the two variables used to make a decision were
changed along one of the branches. It does this by pushing the
names onto the Forbidden-List and then checking to see the
list before the evaluation of each operation.
ProgramCounter and Code This machine is like most other soft-
ware programs. There will be one major procedure with the tag
main . This is the first procedure executed, and the RGM ends
when finished. Other procedures are executed as they're en-
countered and a stack is used to keep track of the position in
partially finished procedures.
The source code is available from the author.
8.4 Summary
Letting a machine run backward is just one way to create the most
complicated computer-generated mimicry. You could also create
double-level grammars or some other modified grammar-based sys-
tem.
The Disguise The text produced by these reversible machines is as
good as a computer could do. But that may not be so great.
Computers have a long way to go before they can really fool a
human. Still, static text can be quite realistic.
How Secure Is It? Assessing the security of this system is even more
complicated than understanding the context-free grammars
used in Chapter 7. Theoretically, there is no Turing machine
that can make nontrivial statements about the reversible Tur-
ing machine. In practice, there may be fairly usable algorithms
that can assemble information about the patterns in use. The
question of how to create very secure programs for this revers-
ible machine is just as open as the question of how to break
certain subclasses.
How to Use It The LISP software is available from the author. It
runs on the XLISP software available for free at many locations
throughout the Internet.