I don't know if I failed to notice the final note in the problem description or if it was added later, but I have now able to fix the problem in my code. Thank you! This was a very useful kata for me.

The kata here uses the non-padding version (without = at the end). (Reference: https://en.wikipedia.org/wiki/Base64#Implementations_and_history and the examples above that heading)

I too got an error on one of the random tests:

Testing for 'mg4ttVSojCi7yeO7' to 'bWc0dHRWU29qQ2k3eWVPNw'
'bWc0dHRWU29qQ2k3eWVPNw==' should equal 'bWc0dHRWU29qQ2k3eWVPNw'

But my code seems to be consistent with the padding requirements in the Wikipedia article. Here's some of my debug output:

mg4 ==> bWc0
ttV ==> dHRW
Soj ==> U29q
Ci7 ==> Q2k3
yeO ==> eWVP
7 ==> Nw==
bWc0dHRWU29qQ2k3eWVPNw==

I'm disappointed not to be able to complete this kata since I learned quite a bit from doing it. If I'm right that this is a spurious error, please fix it. Thanks!