Caesar Cipher Encryption giving wrong output

Question

Whenever I input string such as Dizzy with key 10 for example the output is partially wrong. I am having something like this ===>ns��� while I should have nsjji.

Serial.print("KEY: ");
Serial.println(k);

if ((choice[0]=='e') || (choice[0]=='E')){

    int i;
    char ch;                                        

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

        ch=str[i];

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

            ch=ch+k;

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

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

            ch=ch+k;

            if (ch > 'Z'){
                ch=ch-'Z'+'A'-1;
            } 
            str[i]=ch;
        } 
    }

    Serial.print("encrypt: ");
    Serial.println(str);

Answer 1

The problem is that z has character value 122. Then you add 10 and you get 132. A char in C is -128 to 127 (signed), you probably want it to be 0-255 and then it has to be unsigned.

So when you get above the maximum of 127 you get the problems..

Change

char ch;

To this instead:

unsigned char ch;