exercise $()$ :

Encrypting Text with a Substitution Cipher

Write a C program substitution.c which reads characters from its input and writes the characters to its output encrypted with a Substitution cipher.

A Substitution cipher maps each letter to another letter.

The mapping will be given to your program via the command line as a command line argument $($no spaces betweenany characters$).$ This is all on one line. This input will contain $26$ characters: an ordering of the letters $\text{'}$a$\text{'}..\text{'}$z$\text{'}.$

Characters other than letters should not be encrypted.

Your program should stop only at the end of input.

Your program should contain at least one function other than main.

Examples

dcc substitution.c $-$o substitution

$./$substitution qwertyuiopasdfghjklzxcvbnm

Enter text:

I was scared of dentists and the dark

O vql leqktr gy rtfzolzl qfr zit rqka

I was scared of pretty girls and starting conversations

O vql leqktr gy hktzzn uoksl qfr lzqkzofu egfctklqzogfl

$./$substitution abcdefghijklmnopqrstuvwxyz

Enter text:

The identity cipher!!!

The identity cipher!!!

$./$substitution bcdefghijklmnopqrstuvwxyza

Enter text:

The Caesar cipher is a subset of the substitution cipher!

Uif Dbftbs djqifs jt b tvctfu pg uif tvctujuvujpo djqifs!

Your program will only be tested with an appropriate and valid mapping input for marking $-$ but a good programmer would check the input is present and appropriate.