I can't make this coder in applescript work

Question

I'm making a script that makes a text file, then replaces some letters with others. To make it unreadable. Im doing this because what i then could use another script in reverse to make it like the text i first submitted. The problem is: When i run the script it does give me the same text as i submit first back without coding it. Please help me
Here is my script.
set my_password to display dialog ¬
  "Please enter your password:" with title ¬
  "Password" with icon caution ¬
  default answer ¬
  "" buttons {"Cancel", "OK"} default button 2 ¬
  giving up after 295 ¬
  with hidden answer
if text returned of my_password is "mm" then
  display dialog "Access Granted!" buttons ["OK"] default button 1
else
  quit
end if
display dialog "MM C0ding prototype version 1.00" default answer "" buttons {"Code"} default button 1
set dialogText to {text returned of result}
do shell script "echo " & dialogText & " > ~/desktop/foo.txt"
set file_path to POSIX path of " ~/Desktop/foo.txt"
set search_string to "a"
set replacement_text to "f"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "b"
set replacement_text to "g"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "c"
set replacement_text to "h"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "d"
set replacement_text to "i"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "e"
set replacement_text to "j"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "f"
set replacement_text to "0"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "g"
set replacement_text to "1"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "h"
set replacement_text to "2"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "i"
set replacement_text to "3"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "j"
set replacement_text to "4"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "k"
set replacement_text to "p"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "l"
set replacement_text to "q"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "m"
set replacement_text to "r"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "n"
set replacement_text to "s"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "o"
set replacement_text to "t"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "p"
set replacement_text to "5"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "q"
set replacement_text to "6"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "r"
set replacement_text to "7"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "s"
set replacement_text to "8"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "t"
set replacement_text to "9"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "u"
set replacement_text to "x"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "v"
set replacement_text to "y"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "w"
set replacement_text to "z"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "x"
set replacement_text to "U"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "y"
set replacement_text to "V"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set search_string to "z"
set replacement_text to "W"
do shell script "perl -pi -e 's/" & search_string & "/" & replacement_text & "/g' " & quoted form of (POSIX path of file_path)
set milefile to ("Users:madsludvigtimmfagerlund:Desktop:foo.txt")
set finalcode to (read file milefile)
display dialog "Your code is:
" & finalcode buttons {"OK"}

Answer 1

Use code such as:
set answer to text returned of (display dialog " Which password do you need?" default answer "")
if answer contains "hotmail account" then set button_pressed to button returned of (display dialog " Password: 1234" buttons {"Ok", "Back"})
if button_pressed is "back" then
set answer to text returned of (display dialog " Which password do you need?" default answer "")
end if
or:
repeat
set answer to text returned of (display dialog " Which password do you need?" default answer "")
if answer contains "hotmail account" then display dialog " Password: 1234" buttons {"Run again", "Cancel"}
end repeat
(43497)