Consider this (which is correct, unless I managed to include any brainfarts in my reasoning) – in the mapping between Base64 and ASCII, there are no unknown inputs. There’s no XOR or anything like that going on. It’s a pure mapping of byte values to byte values, just with an alternative representation. So there’s really no secret sauce in there, not even any obfuscation going on. Unlike AES or even a simple Vigenere cipher.