This is a Caesar Cipher, a project from the Harvard's CS50x course (2020), entirely written in C.
It is a command line program that encripts messages by "rotating" each letter by K positions.
At the time the user executes the program, they should decide, by providing a command-line argument, on what the key should be in the secret message they’ll provide at runtime.
Example: suppose you wanna encript the word Hello using a key, K, of 3, the encripted message will be Khoor.
Usage:
$ ./caesar 1
plaintext: hello
ciphertext: ifmmp