Suppose you want to send a letter to your brother. And let’s
suppose it’s got some, oh, maybe potentially embarrassing
financial information – he owes you some money and you’re having
trouble paying the bills. Obviously, that’s not the sort of
thing you want to put on a postcard; you’d put that in an
envelope. (Your brother is notorious about checking his email).
You want him to know that the letter is actually from you, so
you sign it – you have a distinct signature that is very hard to
forge. And, on top of that, you want him to know that nobody
else read the letter, so you also sign across the fold of the
envelope, so it can’t just be put in a new envelope. So, you’ve
done the basic security – it’s authenticated (with your
signature), it’s not readable by third parties (because of the
envelope) and it’s tamper-evident (because you signed the
envelope, too). It’s not the most secure communication possible,
but you’ve clearly done due diligence. So what if I told you
people were doing that almost 4000 years