Ensure a base64 encoded string never includes a nonalphanumerical character - base64

Question

Is there anyway to ensure that a base64 encoded string never includes a non-alphanumerical character?
For example, if I have a long string that I encode, is there something I can prepend or append to it that will ensure that when encoded with base64 will only include letters and numbers in the encoded string? Something like this:
String: 192.168.1.1
Encoded: MTkyLjE2OC4xLjE= <- I want to 'get rid' of the equal sign.
I tried appending } at the end of the string (new string is now 192.168.1.1}), and this worked (new encoded string: MTkyLjE2OC4xLjF9), but is there a method of ensuring every combination works?
Is this possible?

Answer 1

You can just rtrim() the equals signs away, which is what most people do.
but as for your question: when length of string / 3 is a whole number. So:
$pad = strlen($str) % 3; if($pad) { $str .= str_repeat(' ', $pad); }
but yeah, the parser will add the equals signs back in automatically just like that, to a multiple of 4, when you pass the string back in - so you dont need to keep them.

Answer 2

It's about the length. The = sign is padding to make the output a multiple of 4 base64 characters. 3 characters translate to 4 base64 characters, so you just need to make your input string a multiple of 3 characters in length somehow. In your case:
192.168.1.1 - 11 characters long, base64 ends with =
192.168.1.1$ - 12 characters long, base64 doesn't end with =
Choose a padding character you can easily remove.
The other alternative is to strip the = from the output, then make sure you append = signs to make a multiple of 4 characters before you try to base64 decode...