Base 64 and URL-encoding

Base64 turns 3 bytes into 4, and adds 0-3 trailing equal signs as
padding characters, which would expland to %3D in URL encoding.
Except for the equal signs, Base64 output is [A-Za-z0-9/+]

URL Encoding turns "special" bytes into the three-character sequence
%nn where nn are two hex digits representing the byte value.  Characters
that are converted are those outside "printable ascii", whitespace, and
[,=+] since those have special meaning in URL query strings.

Which one to use depends on what the data is likely to be.
        /r$