Memory requirements of binary data

I have a need to use the binary datatype (encrypting some information in

the document).
It seems that I can encode binary data in two formats (mime and hex).
My question is this: How many bits does it take to pass 8 bits of binary

data?

I've no experience with the mime type, so I can't even take a guess.
In the case of hex, since the byte is translated to two hex characters,
does it take 16 or 32 bits (because XML is unicode)?

I know it's a bit esoteric, but I need to justify the performance
characteristics of one method over another.

Thanks,

Nick