Using ASCII Extended Table

Hey guys, I'm messing around a little bit with making my own sort of encryption. The string thats entered is changed into its ascii decimal equivalent, put to an algorithm, then the new ascii character is displayed. That all works, but now I'm trying to create a decrypt function and when I enter those extended ASCII characters to be decrypted, it gives me "warning C4566: character represented by universal-character-name", and all that are displayed are ?'s. Anyone know how to fix this so I can use extended ASCII characters?

That error should be generated if UNICODE characters are assigned to a 'char' type, but extended ASCII should be alright. You should probably post your code.