b64(a) + b64(b) ≠ b64(a + b) 

Evolution is a 

Previously each use site would have to provide the logic. But should
better to do it only once!

Previous interface was (imoh) unused. The importance of the update and
the previously broken design warrent a break in interface.

Alternativly the frontend would need to always send the empty string.

Also simplify & unify with recurring.