Could someone give me suggesstions on how i go about doing this guys?

http://www.moschino.dd.am/cipher.PNG

please guys, i would very much appreciate it, i already have these 2 pre-defined const and function

const int alphaLen = 'Z'-'A'+1; //26

//converts character to uppercase
char upper( char c ) {
if ( c>='a' && c<='z' ) //small case character
c = c + 'A' - 'a';

return c;
}