Update: Avoid the generic 'char' and switch to the explicit 'unsigned char'.

Update: Avoid the generic 'char' and switch to the explicit 'unsigned char'.

Operations against unsigned are generally safer than the signed equivalents.
Using the generic 'char' may be different between compilers and expecations cannot be guaranteed.
This could result in obscure security issues.

Changing this to 'unsigned char' guarantees consistent behavior.

There are problems with some standard libc functions that expect char.
Explicitly cast to 'char' in these cases.