This is a subject near and dear to my heart because I did something similar in my implementation of the classic Colossal Cave Adventure game as a Windows batch file.
Within the game script I selectively encrypt display text, variable names, and comments. The code to decode the encrypted text is embedded directly within the same script! I write the source code for the game normally, and use braces to denote what portion is to be encrypted. A function within the game is able to generated the encrypted form of itself!
I used a simple symmetric rotation cipher, so really it is more obfuscation than encryption. But that is all that is needed for both the game, and your situation.
I've extracted a simplified version of the routines and provide them below.
The first script is a standalone script that selectively encrypts text within a source file and writes the result to stdout. Simply redirect the output to a new file to get the encrypted version of the file.
selectiveROT13.bat
@echo off
:selectiveROT13 InFile
::
:: Selectively applies the simple "rotate alphabet 13 places" cipher
:: to the contents of file InFile. Only text between curly braces
:: is affected. The affected content can span multiple lines.
::
:: Writes the results to stdout.
:: Percent completion is continuously written to stderr.
::
setlocal enableDelayedExpansion
set "upper=ABCDEFGHIJKLMNOPQRSTUVWXYZ"
set "lower=abcdefghijklmnopqrstuvwxyz"
for /l %%A in (0 1 25) do (
set /a "B=(%%A+13)%%26"
for /f %%B in ("!B!") do (
set "upper!upper:~%%A,1!=!upper:~%%B,1!"
set "lower!lower:~%%A,1!=!lower:~%%B,1!"
)
)
setlocal disableDelayedExpansion
>&2 cls
set "active="
for /f %%N in ('type %1^|find /c /v ""') do set /a "lnCnt=%%N, pct=-1"
for /f "skip=2 tokens=1,* delims=[]" %%a in ('find /v /n "" %1') do (
set "ln=%%b"
setlocal enableDelayedExpansion
set "str=A!ln!"
set "len=0"
for /L %%A in (12,-1,0) do (
set /a "len|=1<<%%A"
for %%B in (!len!) do if "!str:~%%B,1!"=="" set /a "len&=~1<<%%A"
)
set /a len-=1
set rtn=
for /l %%n in (0,1,!len!) do (
set "c=!ln:~%%n,1!"
if "!c!" equ "{" set "active=1"
if "!c!" equ "}" set "active="
if defined active if defined upper!c! for /f %%c in ("!c!") do (
if "!upper:%%c=%%c!" equ "!upper!" (
set "c=!upper%%c!"
) else (
set "c=!lower%%c!"
)
)
set "rtn=!rtn!!c!"
)
echo(!rtn!
for %%A in ("!active!") do (
endlocal
set "active=%%~A"
)
)
exit /b 0
Below is your joke program with a simplified version of the code to decode encrypted text. My original code worked with string variables, but this version works with string literals. The source script is written normally, without encryption. Braces indicate which code is to be encrypted. Besides your joke, I've included documentation and examples to demonstrate some of the features.
joke_src.bat
@echo off
setlocal enableDelayedExpansion
call :init
:: Disable delayed expansion to protect ! within string literals
setlocal disableDelayedExpansion
:: Curly braces are used to denote text that should be encrypted.
:: Encryption can span multiple lines
:: {
:::Line1
:::Line2
:::Line3
:: }
:: I defined a simple SHOW macro that expands to CALL :SHOW
:: Use the %show% macro to display encrypted text.
:: The braces can be hidden by using the undefined %{% & %}% variables
%show% %{%"Quote literals ("") must be doubled ("""") in the source"%}%
:: Here I use a FOR loop to show all encrypted lines within this script
:: that begin with :::
echo(
for /f "delims=: tokens=*" %%A in ('findstr /b ":::" "%~f0"') do %show% "%%A"
echo(
echo And now it is time for a little joke.
echo(
echo Your mother is so fat...
pause
%show% %{%"the recursive function computing her mass causes a stack overflow!"%}%
exit /b
:show Str
::{
:: Applies the simple "rotate alphabet 13 places" cipher to string Str
:: and writes the result to stdout. Consecutive quotes ("") are converted
:: into a single quote (").
::}
setlocal disableDelayedExpansion
set "str=%~1"
setlocal enableDelayedExpansion
set "str=!str:""="!^"
if defined {obfuscated} (
set "len=0"
set "str2=.!str!"
for /L %%A in (12,-1,0) do (
set /a "len|=1<<%%A"
for %%B in (!len!) do if "!str2:~%%B,1!"=="" set /a "len&=~1<<%%A"
)
set /a len-=1
set rtn=
for /l %%n in (0,1,!len!) do (
set "c=!str:~%%n,1!"
if defined {upper}!c! for /f %%c in ("!c!") do (
if "!{upper}:%%c=%%c!" equ "!{upper}!" (
set "c=!{upper}%%c!"
) else (
set "c=!{lower}%%c!"
)
)
set "rtn=!rtn!!c!"
)
) else set "rtn=!str!"
echo(!rtn!
exit /b 0
:init
set "}="
set "{="}
set "{upper}=ABCDEFGHIJKLMNOPQRSTUVWXYZ"
set "{lower}=abcdefghijklmnopqrstuvwxyz"
for /l %%A in (0 1 25) do (
set /a "B=(%%A+13)%%26"
for /f %%B in ("!B!") do (
set "{upper}!{upper}:~%%A,1!=!{upper}:~%%B,1!"
set "{lower}!{lower}:~%%A,1!=!{lower}:~%%B,1!"
)
)
set "{obfuscated}="
set "{obfuscationTest}={A}"
if "!{obfuscationTest}:A=!" equ "!{obfuscationTest}!" set {obfuscated}=1
set "show=call :show"
exit /b
The following command will generate the encrypted version of the script:
selectiveROT13 joke_src.bat >joke.bat
Below is the encrypted form. This is what you would send to your friend. (Without the extra documentation and examples of course)
joke.bat
@echo off
setlocal enableDelayedExpansion
call :init
:: Disable delayed expansion to protect ! within string literals
setlocal disableDelayedExpansion
:: Curly braces are used to denote text that should be encrypted.
:: Encryption can span multiple lines
:: {
:::Yvar1
:::Yvar2
:::Yvar3
:: }
:: I defined a simple SHOW macro that expands to CALL :SHOW
:: Use the %show% macro to display encrypted text.
:: The braces can be hidden by using the undefined %{% & %}% variables
%show% %{%"Dhbgr yvgrenyf ("") zhfg or qbhoyrq ("""") va gur fbhepr"%}%
:: Here I use a FOR loop to show all encrypted lines within this script
:: that begin with :::
echo(
for /f "delims=: tokens=*" %%A in ('findstr /b ":::" "%~f0"') do %show% "%%A"
echo(
echo And now it is time for a little joke.
echo(
echo Your mother is so fat...
pause
%show% %{%"gur erphefvir shapgvba pbzchgvat ure znff pnhfrf n fgnpx biresybj!"%}%
exit /b
:show Str
::{
:: Nccyvrf gur fvzcyr "ebgngr nycunorg 13 cynprf" pvcure gb fgevat Fge
:: naq jevgrf gur erfhyg gb fgqbhg. Pbafrphgvir dhbgrf ("") ner pbairegrq
:: vagb n fvatyr dhbgr (").
::}
setlocal disableDelayedExpansion
set "str=%~1"
setlocal enableDelayedExpansion
set "str=!str:""="!^"
if defined {boshfpngrq} (
set "len=0"
set "str2=.!str!"
for /L %%A in (12,-1,0) do (
set /a "len|=1<<%%A"
for %%B in (!len!) do if "!str2:~%%B,1!"=="" set /a "len&=~1<<%%A"
)
set /a len-=1
set rtn=
for /l %%n in (0,1,!len!) do (
set "c=!str:~%%n,1!"
if defined {hccre}!c! for /f %%c in ("!c!") do (
if "!{hccre}:%%c=%%c!" equ "!{hccre}!" (
set "c=!{hccre}%%c!"
) else (
set "c=!{ybjre}%%c!"
)
)
set "rtn=!rtn!!c!"
)
) else set "rtn=!str!"
echo(!rtn!
exit /b 0
:init
set "}="
set "{="}
set "{hccre}=ABCDEFGHIJKLMNOPQRSTUVWXYZ"
set "{ybjre}=abcdefghijklmnopqrstuvwxyz"
for /l %%A in (0 1 25) do (
set /a "B=(%%A+13)%%26"
for /f %%B in ("!B!") do (
set "{hccre}!{hccre}:~%%A,1!=!{hccre}:~%%B,1!"
set "{ybjre}!{ybjre}:~%%A,1!=!{ybjre}:~%%B,1!"
)
)
set "{boshfpngrq}="
set "{boshfpngvbaGrfg}={N}"
if "!{boshfpngvbaGrfg}:A=!" equ "!{boshfpngvbaGrfg}!" set {boshfpngrq}=1
set "show=call :show"
exit /b
The beauty of this system is that both joke.bat and joke_src.bat generate the exact same output:
Quote literals (") must be doubled ("") in the source
Line1
Line2
Line3
And now it is time for a little joke.
Your mother is so fat...
Press any key to continue . . .
the recursive function computing her mass causes a stack overflow!
Another nice feature is that selectiveROT13.bat can be applied to joke.bat to regenerate the original un-encrypted source.
