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Text File | 1990-11-13 | 20.8 KB | 475 lines

The Kelvinator  - AutoLisp encryption utility.
Release 2a: The Kelvinator Strikes Back
by Kelvin R. Throop
September 2nd, 1987

(C) Copyright 1986,1987 Autodesk, Inc.

License is granted for the use of this software and documentation on a no
charge, sharing basis. You may distribute and copy this software and
documentation freely as long as it remains a complete unaltered package and
is not sold for profit.


------------------------

Following the release of AutoCAD version 2.1, independent developers began
to create complex applications employing the AutoCAD menu macro language
and the Variables and Expressions feature as an embedded programming
language.  With the release of version 2.18, which introduced the AutoLisp
programming language, AutoCAD became a fully programmable and extensible
system.

From inception, AutoCAD's open architecture has been intended to encourage
outside developers and individual users to customize AutoCAD and implement
applications for diverse markets.  Since menu macros and AutoLisp are
interpreted languages, applications must be shipped in source code form,
and Autodesk recognized the need to protect the developers of these
applications from theft of their proprietary products.

Encryption of menu macros and AutoLisp programs was implemented and made
available to authorized developers.  This encryption never purported to be
particularly secure, but it does prevent casual examination of distributed
code. Given that a standard AutoCAD must be able to decode a file encrypted
by any developer, there is no cryptological technique of which I am aware
which would be significantly more secure than the one used.  The fact that
the encryption program is made available to hundreds of developers who
cannot always prevent its further distribution makes penetration by reverse
engineering of the program or a known-plaintext attack inevitable should
somebody be so callous as to steal the work of our developers and undermine
the viability of their businesses.


A SECOND LAYER OF PROTECTION

Since encrypting files cannot provide total security, a second level of
defence was called for.  The Kelvinator is a program which translates an
AutoLisp program into a form which is essentially gibberish when examined
by a human but which should execute identically to the original program.
Thus, if the distributed program is decrypted, the resulting source code
will be little use to the looter.

Kelvination of a program consists of:

1) Deleting comments.
2) Translating variable names into gibberish.
3) Removing indentation and line breaks.

As an example of the consequences of Kelvination, the following AutoLisp
program was processed.

;
;       Display spiral
;
;       Designed and implemented by
;       Kelvin R. Throop in January 1985
;
;       (cspiral <# rotations> <base point>
;                <growth per rotation>
;                <points per circle>)
;
(defun cspiral (ntimes bpoint cfac lppass
        / ang dist tp ainc dinc circle bs cs)
        (setq cs (getvar "cmdecho"))
        (setq bs (getvar "blipmode"))
        (setvar "blipmode" 0)
        (setvar "cmdecho" 0)
        (setq circle (* 3.141596235 2))
        (setq ainc (/ circle lppass))
        (setq dinc (/ cfac lppass))
        (setq ang 0.0)
        (setq dist 0.0)
        (command "pline" bpoint)
        (repeat ntimes
           (repeat lppass
              (setq tp (polar bpoint
               (setq ang (+ ang ainc))
               (setq dist (+ dist dinc))))
              (command tp)
           )
        )
        (command)
        (setvar "blipmode" bs)
        (setvar "cmdecho" cs)
        nil
)
;
;       Interactive spiral generation
;
(defun C:SPIRAL ( / nt bp cf lp)
        (prompt "\nCentre point: ")
        (setq bp (getpoint))
        (prompt "\nNumber of rotations: ")
        (setq nt (getint))
        (prompt "\nGrowth per rotation: ")
        (setq cf (getdist bp))
        (prompt "\nPoints per rotation: ")
        (setq lp (getint))
        (cond ((null lp) (setq lp 30)))
        (cspiral nt bp cf lp)
)

The output of the Kelvinator is as follows (line breaks have been manually
inserted: there were none in the actual output file).

(DEFUN Qj(Q@ QQ Ql Q& / Q1 Q# Q0 Q$ QO
Q| Q% Q?j)(SETQ
Q?j(GETVAR"cmdecho"))(SETQ
Q%(GETVAR"blipmode"))
(SETVAR"blipmode"0)(SETVAR"cmdecho"0)(SETQ
Q|(* 3.141596235 2))(SETQ Q$(/ Q|
Q&))(SETQ QO(/ Ql Q&))(SETQ Q1 0.0)(SETQ
Q# 0.0)(COMMAND"pline"QQ)(REPEAT
Q@(REPEAT Q&(SETQ Q0(POLAR QQ(SETQ Q1(+
Q1 Q$))(SETQ Q#(+ Q# QO))))(COMMAND
Q0)))(COMMAND)(SETVAR"blipmode"Q%)
(SETVAR"cmdecho"Q?j)NIL)(DEFUN
C:SPIRAL(/ Qjj Q@j QQj
Qlj)(PROMPT"\nCentre point: ")(SETQ
Q@j(GETPOINT))(PROMPT
"\nNumber of rotations: " )(SETQ
Qjj(GETINT))(PROMPT"\nGrowth per
rotation: ")(SETQ QQj(GETDIST
Q@j))(PROMPT"\nPoints per rotation: ")
(SETQ Qlj(GETINT))(COND((NULL
Qlj)(SETQ Qlj 30)))(Qj Qjj Q@j QQj Qlj))


If you imagine several thousand lines of this in a real application instead
of the few lines generated by this simple AutoLisp program (which, by the
way, was the first AutoLisp program ever written), it should be clear that
not much can be gleaned from decrypting such ``source code''.


Kelvinating a File

To Kelvinate a file, use the command:

KELV file [options]

Input is read from the specified file.  If no file type is specified, .lsp
is assumed.  Output is written to standard output, and will usually be
redirected to a file.  Various options can be specified to control the
operation of the Kelvinator; these are discussed below.  For example, to
Kelvinate the AutoLisp application brainsim.lsp and write the output into
ubrains.lsp, use the command:

KELV brainsim >ubrains.lsp

Note that a while a file type of .lsp is assumed for the input file, an
explicit file type must be supplied when the output of the Kelvinator is
redirected.


The Definition File

When a program is Kelvinated, all of the user-defined symbols are
translated to gibberish, but AutoLisp's built-in symbols (such as CONS,
SETQ, and ENTGET) must not be modified.  In addition the application may
wish to leave some symbols unchanged (for example the C:name used to invoke
user-defined commands, or functions called by the user directly from the
keyboard). Finally, when a multiple-file program is Kelvinated, consistent
translation must be applied across all files comprising the program.  All
of these requirements are implemented via the Kelvinator definition file.

If you are simply using the Kelvinator to protect standard AutoLisp
programs, you probably won't have any need to examine or modify the
definition file. Feel free to skip this section until you need it.

When the Kelvinator is called as in the example above, it searches the
current directory for a file named.  If you wish to use a different
definition file, multiple definition files, or a different source for the
definition file, you may use the -d switch on the KELV call line to supply
a different file name.  For example:

KELV prog1 >kp1.lsp -dmydefs.def
KELV prog2 >kp2.lsp -d/usr/appdev/KELV.def
KELV prog3 >kp3.lsp -dKELV.def -dlocal.def


In the first example a locally-created definition file is used instead of
the standard KELV.def.  In the second, the Kelvinator is told to obtain its
definition file from the specified path rather than from the current
directory. In the third example, two definition files are used: the
standard definitions and a local version specifying additional symbols.
Any number of definition files may be specified when the Kelvinator is
called; they are loaded in the order named on the call line.  If a symbol
is declared more than once, whether within one definition file or across
several, the last declaration takes precedence (this allows local
definitions to override those given in the standard KELV.def).

A Kelvinator definition file is composed of four types of declarations:
Protected symbols, Pattern masks, Translation specifications, and a Symbol
code.  Each type of declaration is placed in a section of the file
introduced by a section definition statement whose first character is a
right parenthesis `` )''.  Blank lines and comment lines which begin with
two right parentheses are ignored.

Let's look at a simple definition file:


)) Sample Kelvinator definition file

)Pattern
C:?

)Protect
cons
setq
entget

)Translate
mysym z1
yoursym z2

)Protect
car

)Code
200


This file contains five sections:
one  )Pattern section, two )Protect sections, one  )Translate section, and
one )Code section.  A file may contain as many sections as you like.
Capital and lower case letters are considered identical; this is consistent
with AutoLisp's symbol syntax.

THE )PROTECT SECTION

The  )Protect section consists simply of a list of names, one per line.
Any symbol named in a  )Protect section will be left unchanged by the
Kelvinator. In this case, the four standard AutoLisp symbols  CONS,  SETQ,
ENTGET, and  CAR will be left intact when a file is Kelvinated.  The
standard KELV.def lists all AutoLisp built-in names as protected symbols.
If a new built-in name is added in a subsequent release of AutoLisp, simply
adding it to the  KELV.def file will cause it to be correctly processed by
the Kelvinator.

If your application provides functions the user invokes from the keyboard
(or from a menu), you may request the Kelvinator to leave these names
untranslated by adding them to the  )Protect section. The best way to
accomplish this is to create a second definition file for your application
and specify both  KELV.def and your file with  -d specifications when
invoking the Kelvinator.


THE )PATTERN SECTION

Sometimes an entire syntactic class of symbols should be left untranslated.
An example of this are the names used to invoke user-defined commands in
AutoLisp; all user-defined commands are functions whose names begin with
``C:''.  Each line in the pattern section specifies a pattern against which
symbols are tested. If a symbol matches the pattern up to the length of the
pattern, with the ``?'' character in the pattern matching any non-null
character in the symbol, that symbol is automatically protected (i.e., not
translated by the Kelvinator).

The standard  KELV.def file contains the pattern declaration `` ?:'', which
protects all symbols with a colon as their second character; this is
compatible with the original Kelvinator in which this was hard-coded.


THE )TRANSLATE SECTION

When the Kelvinator encounters a symbol in a program which is neither
protected by matching a pattern nor by having been named in a  )Protect
section, it searches for the symbol in the translation table.  If the
symbol is found, the value from the translation table replaces the original
symbol.  If the symbol is not in the translation table, the Kelvinator
creates a gibberish symbol and makes an entry in the translation table so
that all subsequent occurrences of the new symbol will be translated to it.

The standard definition file,  KELV.def, contains no  )Translate section,
so all nonprotected symbols are assigned new gibberish values.  If you wish
to cause a symbol to be translated to a specific value rather than have the
Kelvinator make up a gibberish value for it, simply specify the original
symbol and the value it should be translated to, separated by a single
blank character, as a line in the  )Translate section.  In the example
above, all occurrences of `` mysym'' will be translated to `` z1'', and all
occurrences of `` yoursym'' to `` z2''.

The  )Translate section is rarely used to perform explicit symbol
translation. The primary application of the translate section is to permit
convenient Kelvination of multi-file programs, as discussed below.


THE )CODE SECTION

When the Kelvinator processes a symbol not present in the  Protect or
Translate sections which does not match a pattern, a gibberish name is
generated and this and all subsequent occurrences of the symbol are
translated to the gibberish name.  The gibberish name is generated
programmatically from a symbol code which is incremented for each new
symbol encountered.  The Kelvinator normally starts this code at zero, but
can be instructed to generate symbols starting at any desired code by
specifying the starting number in a )Code section.  If more than one number
is specified, in one or more )Code section(s), the last number encountered
is used.  The  )Code section is almost never specified explicitly; it is
generated by the Kelvinator to allow automatic Kelvination of multi-file
programs.


MULTIPLE FILE PROGRAMS

If an AutoLisp program is a single file which is loaded into AutoCAD and
run as a unit, the default translation performed by the Kelvinator is
usually adequate.  If an application consists of more than one original
.lsp file and the running program loads different AutoLisp modules during
execution, special care must be taken when Kelvinating the application.
Simply running the Kelvinator on a source file translates all the variable
names in it into a standard format.  If each module of a multiple module
program were Kelvinated, this would result in two distinct problems:


1) Variables shared between modules would be given different names in
different modules, so values passed would not be received correctly in the
loaded module.

2) Variables with distinct names in the original source might be translated
into the same name in the Kelvinated output.  Thus duplication of variable
names would almost certainly occur.


SHARED VARIABLES

The first problem may be addressed in any of several ways.  One option is
to simply give all variables shared between modules names which match a
pattern that causes them to be remain untranslated.  Using the standard
pattern in KELV.def, simply using a colon as the second character of all
shared variables accomplishes this.

If you don't want to name the shared variables to match a pattern, you can
simply make a list of all the shared variables and specify that list as a
)Protect section in a definition file. None of the shared variables will be
translated, and thus they will retain their values across modules.

Finally, and by far the easiest, you can use the technique for automatic
multiple file Kelvination given below.  This will guarantee that variables
with the same name in the original source will share the same encoded name
in the Kelvinated output.


UNIQUE VARIABLE NAMES

There are two approaches to preventing inadvertent duplication of variable
names as a result of Kelvination.  The first technique requires the use of
an option on the call to the Kelvinator which permits generation of unique
names for each module.  If the Kelvinator is called with a  -P prefix
argument, the generated variable names will begin with the specified
prefix.  If no prefix is specified, the default of `` Q'' is used.  The
prefix may be any string which is a valid AutoLisp symbol, but should not
be a symbol likely to duplicate an AutoLisp-defined symbol.  If you're
Kelvinating a four module electronic application, you might use:


KELV arisia   -pZA  >parisia.lsp
KELV scapture -pZB  >pscapt.lsp
KELV pcboard  -pZC  >ppcbd.lsp
KELV simulate -pZD  >psim.lsp


Symbols in the respective modules will be given prefixes of ZA through  ZD,
and are therefore guaranteed to be unique. Remember that AutoLisp is far
more efficient when storing symbols of 6 characters or less, so don't go
overboard in choosing a variable prefix.  Two characters should be
adequate.

If you use the automatic multiple file Kelvination technique described
below, you need not assign unique prefixes; distinct names in the original
set of files will be guaranteed distinct in the Kelvinated output.


AUTOMATIC MULTI-FILE KELVINATION

The Kelvinator allows AutoLisp programs which span multiple files to be
Kelvinated in a close-to-automatic manner.  When the Kelvinator is used in
this fashion, names will be translated for the program taken as a whole,
thus preserving the uniqueness of names across modules and guaranteeing
that two names which were distinct in the original set of files will never
be translated to the same name.

Automatic multi-file Kelvination is made possible by a feature which causes
the Kelvinator to write out the translation table used to Kelvinate a file
in a form that can be used as the definition file for a subsequent run of
the Kelvinator.  If the  -W file specification is placed on a call to the
Kelvinator, after a file has been Kelvinated a complete definition file is
written to the named file.  All protected symbols and patterns used for the
Kelvination are written to the file, and  )Translate commands are given for
all symbols changed by the Kelvinator.  In addition, a  )Code section is
written to start symbol generation at the correct point in the next module.
By passing on these definition files from run to run of the Kelvinator, the
translated names assigned by the Kelvinator may be made uniform across the
input file set.

For example, here are the commands you would use to perform automatic multi-
file Kelvination on the electronic application mentioned in the last
section:


KELV         -wd1.def arisia   >parisia.lsp
KELV -dd1.def -wd2.def scapture >pscapt.lsp
KELV -dd2.def -wd3.def pcboard  >ppcbd.lsp
KELV -dd3.def -wd4.def simulate >psim.lsp


Note how we pass the output definitions from each file (which include all
of the information from the files processed before) on to the Kelvination
of the next file.  We don't specify the  -D switch for the first file since
we're starting with the standard KELV.def file for that run.  We don't need
to save the definitions from the last file, but we did in this case because
the saved definition file can be a handy tool for debugging the
application.  If a Kelvinated program crashes, the AutoLisp walkback will
contain mostly gibberish symbols.  If you retain a copy of the final
definition file from the Kelvination run, you can translate this walkback
into the original code and more easily locate the problem in the source.
This capability makes saving the definitions worthwhile even for a single
file program.


OTHER BENEFITS OF KELVINATION

Even if security is not a consideration, you might consider Kelvinating
your program.  By removing comments and white space and replacing long
descriptive variable names with short cryptic ones, Kelvination may
radically reduce the size of an AutoLisp program.

For example, Kelvination of AutoShade reduced the  ASHADE.LSP file from
13669 bytes to 4095 bytes, a reduction of more than three to one (the code
was heavily commented).  In addition, if you intend to Kelvinate the file
before shipment, you are free to use long variable names for documentation
purposes; Kelvination will reduce them to short names that AutoLisp can
store efficiently. The new Kelvinator compresses its output somewhat more
densely than the original Kelvinator---it knows quite a bit more about
AutoLisp's input parser and sails very close to the wind in eliminating
spaces wherever AutoLisp doesn't need them to delimit symbols in the input.

The only speed benefit of Kelvination is that gained while loading because
a Kelvinated file is usually smaller.  Once loaded, a Kelvinated program
will run in exactly the same time as a non-Kelvinated one (unless
shortening of variable names has increased free memory and thus reduced
garbage collections, which will yield a small edge to the Kelvinated
program).

------------------------------

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