Just want to extract 18Bit aut of a String

I'm Working on a Programm that is abel to do DES, it's for a Maturity-Project.
I have a Text like "Hallo, I'm a programmer", now I change it into binary. If I do that with Asc() for every Letter in the String, it results in some numbers, like 134. I than have to Calculate them into binary and save them in a String. Then I take the pices of allways 18Bit and want to XOR them with something else.

Now there's the problem, If I make everything with strings, I have to XOR "by hand", means one Mid(string,i,1) after the other. And I use mutch memory, one byte for every letter in the string, where just stand a 1 or a 0, too mutch memory if you ask me. And to work with that string in arry it's also very very difficult.

Does someone have a better solution for me how to do?

I'm thinking of a binary Datatype. Byte and Boolean don't work they're too short.