Linux Security Cookbook [Electronic resources] نسخه متنی

SSL for Securing Mail

Most major mail clients (pine,
mutt, etc.) support

secure
POP and IMAP using the Secure Sockets Layer
(SSL) protocol (also known by its later, IETF-standards name,
Transport Layer
Security or
TLS). Most commercial mail servers and ISPs, however, do not support
SSL, which is highly annoying. But if you're lucky
enough to find a mail server that does support it, or if you run your
own server [Recipe 8.9], here's a
brief introduction to how it works.

A mail server may support SSL in two ways,
to protect your session against eavesdroppers:

The mail server listens on the

normal service
port for unsecured connections, such as 110 for POP3 or
143 for IMAP, and permits a client to "turn
on" SSL

after the fact . The

IMAP command for this is
STARTTLS; the POP command,
STLS; we will refer to this approach generically
as STARTTLS.

The mail server listens on a

separate port , such
as 995 for POP3 or 993 for IMAP, and requires that SSL be negotiated
on that port

before speaking to the mail
protocol.

STARTTLS is the more modern, preferred method (see RFC 2595 for
reasoning), but both are common. Our recipes suggest that you try
STARTTLS first, and if it's unsupported, fall back
to SSL-port.

The most critical thing to protect in email sessions is, of course,
your mail server password. The strong session
protection provided by SSL is one
approach, which protects not only the password but also all other
data in the session. Another approach is strong
authentication
, which focuses on protecting the password (or
other credential), as found in Kerberos [Recipe 4.16]
for example.^[1] These two classes of protection
are orthogonal: they can be used separately or together, as shown in
Table 8-1.

Whatever happens, you don't want your password
flying unprotected over the network, where hordes of
dsniff-wielding script kiddies can snarf it up
while barely lifting a finger. [Recipe 9.19] In most
cases, protecting the content of the email over POP or IMAP is less
critical, since it has already traversed the public network as plain
text before delivery. (If this concerns you, encrypt your mail
messages.)

Finally, as with any use of SSL, check your
certificates;
otherwise server authentication is meaningless. [Recipe 4.4]