[mark-r]

I remember a system that overprinted about 5 characters to black out a password after you entered it. Good times.

The NUL character was useful for inserting a delay into the serial stream.

[jacquesm]

Hehe, security by obscure it y.

And in sync links (not TTY's those were mostly async with start and stop bits, more like HDLC style stuff) those nuls were mandatory, you had to send something.