Re: BBC Micro's FRAK! did a better job of encryption back in 1984.
"Clever, but not immune from tape to tape"
Ah but the sea loader did have protection for that built in... Based on the crappy hardware available in consumer grade tape to tape machines... It was something to do with playing a really high pitched tone which the crappy tape decks would try to level out on the transfer resulting in the following low tones being essentially missed...
The guy that wrote the loader has a big blog about it, ages since I read it but it's very interesting.