Man, I judge, but the original poster ("Huh, 'changeme' is valid Base64, who would've thunk?") is one ignorant programmer... I got explained how Base64 works when I was a pimply 12 year old while on a car trip, you don't even need pen and paper to explain it.
Interestingly "change_me!" is no longer legal, but the decoder might drop illegal characters and decode it anyway...
Interestingly "change_me!" is no longer legal, but the decoder might drop illegal characters and decode it anyway...