Make more TLS checks trigger on the default `medium' level

* doc/emacs/misc.texi (Network Security): Update the doc to say
what's on the different levels.

* lisp/net/nsm.el (nsm-protocol-check--intermediary-sha1): Check
intermediary certificates for SHA1.
(nsm-protocol-check--3des): Check for 3DES ciphers.
(network-security-protocol-checks): Put most of the checks on
`medium'.

1 files changed, 24 insertions, 13 deletions

diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index 177cc8fa469..78f28ccfad9 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -352,19 +352,6 @@ over these connections.  Similarly, if you're sending email via
 connection to be encrypted.  If the connection isn't encrypted,
 @acronym{NSM} will warn you.
 
-@end table
-
-If @code{network-security-level} is @code{high}, the following checks
-will be made, in addition to the above:
-
-@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 which issue new certificates for third-party
-services, you may want to keep track of these changes.
-
 @item Diffie-Hellman low prime bits
 When doing the public key exchange, the number of prime bits
 should be high to ensure that the channel can't be eavesdropped on by
@@ -374,10 +361,34 @@ third parties.  If this number is too low, you will be warned.
 The @acronym{RC4} stream cipher is believed to be of low quality and
 may allow eavesdropping by third parties.
 
+@item @acronym{SHA1} in the host certificate or in intermediary certificates
+It is believed that if an intermediary certificate uses
+the @acronym{SHA1} hashing algorithm, then third parties can issue
+certificates pretending to be that issuing instance.  These
+connections are therefore vulnerable to man-in-the-middle attacks.
+
 @item @acronym{SSL1}, @acronym{SSL2} and @acronym{SSL3}
 The protocols older than @acronym{TLS1.0} are believed to be
 vulnerable to a variety of attacks, and you may want to avoid using
 these if what you're doing requires higher security.
+
+@end table
+
+If @code{network-security-level} is @code{high}, the following checks
+will be made, in addition to the above:
+
+@table @asis
+@item @acronym{3DES} cipther
+The @acronym{RC4} stream cipher is believed by some to be of low
+quality and may allow eavesdropping by third parties.
+
+@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 which 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