base64 Incorrect Padding

[Max Erickson]

MRAB <python at mrabarnett.plus.com> wrote:

> Brandon Fredericks wrote:
>> I did a search within this group, but couldn't find any
>> information on this.
>> 
>> I am sending base64 encoded data as the content over http using
>> urllib2 urlopen. When I receive the data and attempt to decode
>> it, I get an "Incorrect Padding" error. Is there a simple way to
>> fix this? A better way to send and receive the data possibly
>> (using base64 of course)?
> 
> Base-64 encodes 3 bytes (echo in the range 0-255) to 4 bytes
> (each in a more restricted range). The length of the output is
> _always_ a multiple of 4 (ignoring whitespace); the method adds
> extra padding bytes (ASCII '=') to ensure that this is the case
> if the length of the input isn't a multiple of 3.
> 
> So, if decoding raises an "Incorrect Padding" error then strip
> out any whitespace and append extra ASCII '=' to make its length
> a multiple of 3, then try decoding again. (Or you could just
> repeatedly add one pad character and retry, up to 3 times.)

The length of the encoded string should be a multiple of 4 (as you 
state in the second sentence of your post), not a multiple of 3.


max