Write a C++ program that will go through the first step of encrypting an input file and writing the encrypted contents to an output file. For each letter read in from the input file, add 3 letters to the value of the input letter. If the input letter is an A then the encrypted letter would be a D. If the input letter from the input file is an X then the encrypted letter would be an A. All spaces, digits, punctuation, and special characters do not need to be encrypted at this level. Simply write the space, punctuation, or special character to the output file as is. If a letter is uppercase in the input file, it should remain as an uppercase character in the output file. If a letter is lowercase in the input file, it should remain as a lowercase character in the output file.