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@echo off
setlocal enabledelayedexpansion
cls

echo [R]egister new user
echo [L]og in with existing user
choice /C:RL /N >nul

if %errorlevel% equ 1 goto :register
if %errorlevel% equ 2 goto :login
exit /b

::------------------------------------------------------------------------------
:: Registers a user with an encrypted password.
::
:: Arguments: None
::------------------------------------------------------------------------------
:register
cls
set /p "register_user_name=Desired Username: "

if not exist accounts.txt echo(1>accounts.txt

findstr /c:"!register_user_name!" accounts.txt 1>nul 2>&1 && (
   echo That user already has an account on this host. Exiting.
   pause
   exit /b
)

:setPassword
call :getPassword first_password "Desired Password: "
call :getPassword verify_password "Re-enter Password: "
if not "%first_password%"=="%verify_password%" (
   echo Passwords do not match. Please enter them again.
   echo/
   goto setPassword
) else (
   call :getSHA512 "!date!!time!" salt
   call :getSHA512 "!salt!!first_password!" passhash
)

>>accounts.txt echo %register_user_name% !salt! !passhash!
exit /b

::------------------------------------------------------------------------------
:: Logs in with a username and password
::
:: Arguments: None
::------------------------------------------------------------------------------
:login
cls
set /p "login_user_name=Username: "
call :getPassword given_pass "Password: "

findstr /c:"!login_user_name!" accounts.txt 1>nul 2>&1 || (
   echo Invalid username or password. Exiting.
   pause
   exit /b
)

for /f "tokens=1-3" %%A in ('findstr /c:"!login_user_name!" accounts.txt 2^>nul') do (
   set "stored_salt=%%B"
   set "stored_hash=%%C"
)


call :getSHA512 "!stored_salt!!given_pass!" givenpasshash
if not !givenpasshash!==!stored_hash! (
   echo Invalid username or password. Exiting.
) else (
   echo Login successful.
)

pause
exit /b

::------------------------------------------------------------------------------
:: Returns the SHA512 value of a string that has been echoed to a text file.
::
:: Arguments: %1 - the string to encode
::            %2 - the SHA512 value of %1
::------------------------------------------------------------------------------
:getSHA512
>shafile echo %~1
for /f "delims=" %%A in ('certutil -hashfile shafile SHA512 ^| find /v "hash"') do (
   set line=%%A
   set linehash=!line: =!
)
del shafile

set "%~2=!linehash!"
set "linehash="
goto :eof

::------------------------------------------------------------------------------
:: Hides user input and returns the input as a variable.
:: http://www.dostips.com/forum/viewtopic.php?p=33538#p33538 (and other places)
::
:: Arguments: %1 - the variable to store the password in
::            %2 - the prompt to display when receiving input
::------------------------------------------------------------------------------
:getPassword
set "_password="

:: We need a backspace to handle character removal
for /f %%a in ('"prompt;$H&for %%b in (0) do rem"') do set "BS=%%a"

:: Prompt the user 
set /p "=%~2" <nul 

:keyLoop
:: Retrieve a keypress
set "key="
for /f "delims=" %%a in ('xcopy /l /w "%~f0" "%~f0" 2^>nul') do if not defined key set "key=%%a"
set "key=%key:~-1%"

:: If No keypress (enter), then exit
:: If backspace, remove character from password and console
:: Otherwise, add a character to password and go ask for next one
if defined key (
    if "%key%"=="%BS%" (
        if defined _password (
         set "_password=%_password:~0,-1%"
      )
    ) else (
        set "_password=%_password%%key%"
        set /p "="<nul
    )
    goto :keyLoop
)
echo/

:: Return password to caller
set "%~1=%_password%"
goto :eof