How to Code a Substitution Cipher in C?

In this video your task is to do a substitution cipher in any language you want (I've picked C). This is not too complicated if you think about the problem as a mapping problem, but feel free to think about it any way you want!
It's not commented, but everything is explained in the video!
If you have comments or request leave them down below and have a nice day!

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[meighn]

nice code. But what should i do if i have a text file with space, sentence symbols and big alphabet?

[evanconnect]

Has anybody found code substitution for rolling keyfobs?