If I whisper sweet nothings to my wife in an email message, one bit
is sufficient to keep the casual voyeur without tools from viewing
the message in a readable spool area on a badly managed site. That
would be good enough for me. (I suppose base64 would do the same
thing ;-).
but as soon as the one-bit encryption is standardized, such voyeurs
will instantly have tools to make it easy for them to view such
messages. and as you point out with your base64 example, if all
you want to do is raise the bar very slightly, many existing
tools do this. rot13 is useful, but I wouldn't call it a privacy
mechanism.
my personal experience is that even causal attackers (i.e. hobbyists
doing it for fun and/or minor malice) are willing and able to spend
O(thousands) of cpu-hours on garden-variety workstations to defeat
security systems (e.g. dictionary attacks on encrypted password
files), not to mention the time spent developing the cracking tools.
Keith