* processes.texi (Network Security): Made into its own section and fleshed out

doc/lispref/ChangeLog

@@ -1,3 +1,8 @@
+2014-11-24  Lars Magne Ingebrigtsen  <larsi@gnus.org>
+
+	* processes.texi (Network Security): Made into its own section and
+	fleshed out.
+
 2014-11-23  Lars Magne Ingebrigtsen  <larsi@gnus.org>
 
 	* processes.texi (Network): Mention the new :warn-unless-encrypted

doc/lispref/elisp.texi

@@ -1299,6 +1299,7 @@ Processes
 * System Processes::        Accessing other processes running on your system.
 * Transaction Queues::      Transaction-based communication with subprocesses.
 * Network::                 Opening network connections.
+* Network Security::        Managing the network security.
 * Network Servers::         Network servers let Emacs accept net connections.
 * Datagrams::               UDP network connections.
 * Low-Level Network::       Lower-level but more general function

doc/lispref/processes.texi

@@ -52,6 +52,7 @@ Processes}.
 * System Processes::         Accessing other processes running on your system.
 * Transaction Queues::       Transaction-based communication with subprocesses.
 * Network::                  Opening network connections.
+* Network Security::         Managing the network security.
 * Network Servers::          Network servers let Emacs accept net connections.
 * Datagrams::                UDP network connections.
 * Low-Level Network::        Lower-level but more general function
@@ -2072,25 +2073,89 @@ The connection type: @samp{plain} or @samp{tls}.
 
 @end defun
 
+
+@node Network Security
+@section Network Security
 @cindex Network Security Manager
-After establishing the connection, the connection is then passed on to
-the Network Security Manager (@acronym{NSM}).  If the connection is a
-@acronym{TLS} or @acronym{STARTTLS} connection, the @acronym{NSM} will
-check whether the certificate used to establish the identity of the
-server we're connecting to can be verified.  If this can't be done,
-the @acronym{NSM} will query the user whether to proceed with the
+@cindex encryption
+@cindex SSL
+@cindex TLS
+@cindex STARTTLS
+
+After establishing a network connection, the connection is then passed
+on to the Network Security Manager (@acronym{NSM}).
+
+@vindex network-security-level
+The @code{network-security-level} variable determines the security
+level.  If this is @code{low}, no security checks are performed.
+
+If this variable is @code{medium} (which is the default), a number of
+checks will be performed.  If the @acronym{NSM} determines that the
+network connection might be unsafe, the user is made aware of this,
+and the @acronym{NSM} will ask the user what to do about the network
 connection.
 
 The user is given the choice of registering a permanent security
 exception, a temporary one, or whether to refuse the connection
 entirely.
 
-If the connection is unencrypted, but it was encrypted in previous
-sessions, the user will also be notified about this.
+Below is a list of the checks done on the @code{medium} level.
 
-@vindex network-security-level
-The @code{network-security-level} variable determines the security level.
-If this is @code{low}, no security checks are performed.
+@table @asis
+
+@item unable to verify a @acronym{TLS} certificate
+If the connection is a @acronym{TLS}, @acronym{SSL} or
+@acronym{STARTTLS} connection, the @acronym{NSM} will check whether
+the certificate used to establish the identity of the server we're
+connecting to can be verified.
+
+While an invalid certificate is often the cause for concern (there may
+be a Man-in-the-Middle hijacking your network connection and stealing
+your password), there may be valid reasons for going ahead with the
+connection anyway.
+
+For instance, the server may be using a self-signed certificate, or
+the certificate may have expired.  It's up to the user to determine
+whether it's acceptable to continue the connection.
+
+@item a self-signed certificate has changed
+If you've previously accepted a self-signed certificate, but it has
+now changed, that either means that the server has just changed the
+certificate, or this might mean that the network connection has been
+hijacked.
+
+@item previously encrypted connection now unencrypted
+If the connection is unencrypted, but it was encrypted in previous
+sessions, this might mean that there is a proxy between you and the
+server that strips away @acronym{STARTTLS} announcements, leaving the
+connection unencrypted.  This is usually very suspicious.
+
+@item talking to an unencrypted service when sending a password
+When connecting to an @acronym{IMAP} or @acronym{POP3} server, these
+should usually be encrypted, because it's common to send passwords
+over these connections.  Similarly, if you're sending email via
+@acronym{SMTP} that requires a password, you usually want that
+connection to be encrypted.  If the connection isn't encrypted, the
+@acronym{NSM} will warn you.
+
+@end table
+
+If @code{network-security-level} is @code{high}, the following checks
+will be made:
+
+@table @asis
+@item a validated certificate changes the public key
+Servers change their keys occasionally, and that is normally nothing
+to be concerned about.  However, if you are worried that your network
+connections are being hijacked by agencies who have access to pliable
+Certificate Authorities that issue new certificates for third-party
+services, you may want to keep track of these changes.
+@end table
+
+Finally, if @code{network-security-level} is @code{paranoid}, you will
+also be notified the first time the @acronym{NSM} sees any new
+certificate.  This will allow you to inspect all the certificates from
+all the connections that Emacs makes.
 
 
 @node Network Servers