Question

I am stripping the padding from the end of of a base64 encoded string. I have found that I can successfully decode the string without adding the padding back on first (in php at least). Is this a result of the specification of base64 or implementation dependent?

And a related question, can a = sign exist in a base64 encoded string other than as padding, i.e. If it is not implementation dependent, is it safe just to strip the trailing equals signs or is it possible that the last “real” character could have been one?

Answer 1

While padding isn’t necessary to decode a base64 string, there are some implementations that will throw an error if the padding has been stripped away.

The RFC states:

Implementations MUST include appropriate pad characters at the end of
   encoded data unless the specification referring to this document
   explicitly states otherwise.

The = character will only ever be used for padding.