C++ question, please help. Thank you.

proj05_functions.h : https://paste.ee/p/1UBMl

Background, Foursquare Cipher A cipher is an algorithm used to send a message in such a way that only someone who understands the algorithm can understand the underlying message. The process of creating the cipher is called encoding, and recovering the original message is decoding. There are lots of interesting ciphers in the world, so much so that Simon Singh wrote a book on it, "The Code Book". We are going to look at a particular cipher Felix Delastellle (1840-1902) wrote in a book in 1902. It has a 2D graphical explanation which we will discuss, but we can easily write an algorithm to do the work with simple linear strings. Wikipedia does a pretty good job of explaining it, please see https://en.wikipedia.org/wiki/Four- square cipher and follow along Details There are 4 blocks in a foursquare cipher (shocking!). Let's label them 0,1,2,3 0 0 1 2 3 4 You'll note that blocks 0 and 2 are the same, an enumeration in two-D of the alphabet minus the letter q, thus making a 5x5 2D block. Blocks 1 and 3 are more interesting