Remove trailing "=" when base64 encoding

The = is padding. <!------------>

Wikipedia says

An additional pad character is allocated which may be used to force the encoded output into an integer multiple of 4 characters (or equivalently when the unencoded binary text is not a multiple of 3 bytes) ; these padding characters must then be discarded when decoding but still allow the calculation of the effective length of the unencoded text, when its input binary length would not be a multiple of 3 bytes (the last non-pad character is normally encoded so that the last 6-bit block it represents will be zero-padded on its least significant bits, at most two pad characters may occur at the end of the encoded stream).

If you control the other end, you could remove it when in transport, then re-insert it (by checking the string length) before decoding.
Note that the data will not be valid Base64 in transport.

Also, Another user pointed out (relevant to PHP users):

Note that in PHP base64_decode will accept strings without padding, hence if you remove it to process it later in PHP it's not necessary to add it back. – Mahn Oct 16 '14 at 16:33

So if your destination is PHP, you can safely strip the padding and decode without fancy calculations.

= is added for padding. The lenght of a base64 string should be multiple of 4, so 1 or 2 = are added as necessary.

Read: No, you shouldn't remove it.

I wrote part of Apache's commons-codec-1.4.jar Base64 decoder, and in that logic we are fine without padding characters. End-of-file and End-of-stream are just as good indicators that the Base64 message is finished as any number of '=' characters!

The URL-Safe variant we introduced in commons-codec-1.4 omits the padding characters on purpose to keep things smaller!

http://commons.apache.org/codec/apidocs/src-html/org/apache/commons/codec/binary/Base64.html#line.478

I guess a safer answer is, "depends on your decoder implementation," but logically it is not hard to write a decoder that doesn't need padding.

In JavaScript you could do something like this:

// if this is your Base64 encoded string
var str = 'VGhpcyBpcyBhbiBhd2Vzb21lIHNjcmlwdA=='; 

// make URL friendly:
str = str.replace(/\+/g, '-').replace(/\//g, '_').replace(/\=+$/, '');

// reverse to original encoding
if (str.length % 4 != 0){
  str += ('===').slice(0, 4 - (str.length % 4));
}
str = str.replace(/-/g, '+').replace(/_/g, '/');

See also this Fiddle: http://jsfiddle.net/7bjaT/66/