Could someone explain this code for me please

Question

I found this Caesar cipher encryption code on the web and I'm trying to understand how it works

#include<stdio.h>

int main()
{
char message[100], ch;
int i, key;

printf("Enter a message to encrypt: ");
gets(message);
printf("Enter key: ");
scanf("%d", &key);

for(i = 0; message[i] != '\0'; ++i){
    ch = message[i];

    if(ch >= 'a' && ch <= 'z'){
        ch = ch + key;

        if(ch > 'z'){
            ch = ch - 'z' + 'a' - 1;
        }

        message[i] = ch;
    }
    else if(ch >= 'A' && ch <= 'Z'){
        ch = ch + key;

        if(ch > 'Z'){
            ch = ch - 'Z' + 'A' - 1;
        }

        message[i] = ch;
    }
}

printf("Encrypted message: %s", message);

return 0;
}

• Meaning of if(ch >= 'a' && ch <= 'z') Like does c include the alphabet as an array or something or how does it know that the letter is b or others ?

• Adding an int to a char in ch = ch + key;

• this math thing ch = ch - 'Z' + 'A' - 1;

And thanks very very much

Answer 1

The codes adds a certain value (key) to each inputed character and "rotates" the character in the range a-z if small caps and A-Z if big caps.

In C each single character has an implicit ascii/int value and the compare operator is used to decide if the inputted character is in the set of characters a-z (which are aligned after each other) or if it is in the set of A-Z which also follow behind each other.

The rest of the code deals with the wraparound if the inputted character plus the key "overshoots" f.e. z or Z and loops it around by substracting the value of 'z' and adding the value of 'a' -1 so that the resulting char is again one in the selected range of a-z or A-Z