Re: Making a partial C compiler

"John Eskie" <cyberheg@l115.langkaer.dk> wrote
> I want to make a C/C++ source code obfuscator in C or C++. I don't
> need a full parser since I think about limited flow obfuscation. So
> what I need is to identify all "sub parts" of code like statements,
> if/while/for etc. pieces of code.


Sound like you want to write a lexer - all you really need is


#1: to read in tokens
#2: to identify where each variable is defined
        if from an #INCLUDEd file then keep the same
        else rename to something short and useless (like a, b, c ...)
#3: rewrite the source to a new file, removing comments
        and unnecessary white space characters.


This should generate a simple obfuscator, though a more complex one
would recognise and shorten or rearrange statements -- this later
version would be a complex piece of kit.


For #2 in flex you could code ...
[ \t\n\r] { printf( " " ); }
"//"[^\n]* {}
... /* more */
"#define" { printf( "#define " ); expectingNew = TRUE; }
"unsigned" { printf( "unsigned " ); expectingNew = TRUE; }
"int" { printf( "int " ); expectingNew = TRUE; }
... /* more */
[a-zA-Z_][a-zA-Z0-9_]* { if( expectingNew )
            printf( createObfuscateSymbol( yytext ) );
            else printf( getObfuscatedSymbol( yytext ) ); }


Of course this would be slow, but optimisation is possible.


> Therefore I need some parsing code that could ease my job. I'm sure
> there are several people who wrote parsers and configuration scripts
> for C and C++ parsing because they are rather common languages but in
> my search I didn't know what to look for.


As mentioned above, only commercial quality obfuscation
would require a full compiler; a simple tokeniser would
be enough to obfuscate a C programme.


> Maybe some of you can recommend me in what direction I should go or
> point me to articles on the subject. I don't plan to spend several
> months on this subject but the alternative is to write my own parsing
> code for my needs which probably won't be better then what exists
> already.


You should be able to knock together an obfuscator as described
above in only a couple of hours (using flex), or a few days us-
ing straight C (which could be more efficient).


The only problem with the solution outlined above is the source
will not be tested. This should not be a problem when your aim
is only to obfuscate know good code as a final stage before you
release it.


> Thanks in advance.
> --John


C 2002/5/20

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