Must be in C

The substitution cipher is a cryptographic method for encrypting text such that it becomes unreadable to a party without access to the cryptographic key.

The encryption and decryption operations are simple substitutions of one letter with another using a 1-1 map.

As an example, a substitution key is nothing more than a permutation of the alphabet. As an example, a substitution cipher is described by the following rule

Plaintext: abcdefghijklmnopqrstuvwxyz (the entire alphabet)

Substitution key: xrhnkesdfgowqjvlzaibptmycu (a permutation of the alphabet)

During encryption, a becomes x, b becomes r, c becomes h, and so on. During decryption, the opposite rule is followed.

That is, to recover the original text, x becomes a, r becomes b, h becomes c, and so on.

Write a C program that decrypts a file named encrypted.txt and places the decryption output to a file called decrypted.txt. T

he file "encrypted.txt is encrypted with the 1-1 substitution map given in file substitution.txt.

Only alphabet letters (uppercase/lowercase) must be decrypted. The remaining characters (question marks, periods, etc.) must remain intact.