Aw: Re: [OT] CLI for noobies: The keys to GnuPG

[Albrecht Dreß]

> In this way, it is possible that your email to your mom would be
> encrypted while traveling to her mailbox.  You wouldn't specify that
> you want the message encrypted; it just happens (if her ISP's server
> supports it).  If the message goes via a relay, though, it might not
> be encrypted after that point.

Unfortunately, this is not true.  Let's have a look at a ascii sketch how the mail to your mom travels through the internet:

 -----------
| your Mac  |
 -----------
     e
     e    protocol: SMTP (RFC 2821/2822)
     e
 -----------
| your ISP  |    <<< an attacker may look here (i.e. your ISP's root)
 -----------
     u
the BAD internet (gateways, mail relays, whatever; again SMTP)
     u
     u           <<< an attacker may look here;
     u           <<< anybody with root privileges on any machine between
     u
 -----------
| mom's ISP |    <<< an attacker may look here (i.e. Mom's ISP's root)
 -----------
     u/e  
     u/e  protocol POP3 (RFC 1939) or IMAP (RFC 3501)
     u/e         <<< an attacker may look here (unlikely, wiretapping)
     u/e
 -----------
| Mom's Mac |
 -----------

As you have guessed, "e" means encrypted and "u" unencrypted. You send your message to your ISP's MTA, using SMTP. As you explained, this connection is ssl encrypted. However, as the message itself is not encrypted anyone with appropriate privileges on this machine can read or modify the message.

If you and your mom have different ISP's, your ISP transfers the message to the next ISP. This transfer again uses SMTP, but it will NEVER be transport layer encrypted. Why should it? You ISP relays mail to a different ISP, and the latter one will decide upon the recipient's address whether  to accept or not. There may be multiple machines (gateways, relays) in the internet involved, and both ISP may actually have a "farm" of machines. Again, anybody with the right privileges on any of these machines can read or modify the message.

Your Mom's ISP stores messages locally, and she will retreive them later using POP3 or IMAP. If she dials in with this particular provider, this last step is more or less safe as it would require wiretapping. The situation is different, though, if she dials in using a *different* provider than her mail provider (e.g. phone company, mail at yahoo). Now the pop3/imap process again goes Mac -> dial-in ISP -> BAD internet -> mail ISP, which is vulnerable. The same applies for you, btw, if the dial-in provider is not the mail provder, and here is the point where transport layer encryption, which is also possible with POP3 and IMAP, makes sense.

Conclusion: if you want to be safe, you have no choice but encrypting your messages on your Mac, and your Mom decrypts them on her.

Cheers, Albrecht.