reverse engineering of code

I'm writting a program to give a user a unique password based on his email address and i found this piece of code online. I'm now trying to reverse the code so that i can get back the email address from the password, pls can anyone tell me start with the decoding.

static char [] ValidChars = { '2' , '3' , '4' , '5' , '6' , '7' , '8' , '9' , 'A' , 'B' , 'C' , 'D' , 'E' , 'F' , 'G' , 'H' , 'J' , 'K' , 'L' , 'M' , 'N' , 'P' , 'Q' , 'R' , 'S' , 'T' , 'U' , 'V' , 'W' , 'X' , 'Y' , 'Z' }; // len=32 
const string  hashkey = "password" ; 
//key for HMAC function -- change! 
const int  codelength  = 6 ; // lenth of passcode 
string GetCodeForEmail ( string  address ) 
{ 
byte []  hash ;  
   using  ( HMACSHA1 sha1  = new  HMACSHA1 ( ASCIIEncoding . ASCII .GetBytes ( hashkey )))         hash  =  sha1 . ComputeHash ( UTF8Encoding . UTF8 . GetBytes ( address )); int  startpos  =  hash [ hash . Length - 1 ] % ( hash . Length -  codelength ); StringBuilder  passbuilder  = new StringBuilder (); for ( int  i  =  startpos ;  i  <  startpos  +  codelength ;  i ++)         passbuilder . Append ( ValidChars [ hash [ i ] % ValidChars . Length ]); return  passbuilder . ToString (); }

Tags: C# (C# 4.0)