Merge remote-tracking branch 'origin/netsec'

3 files changed, 928 insertions, 314 deletions

diff --git a/lisp/net/gnutls.el b/lisp/net/gnutls.el
index 61480f35877..da7665089ec 100644
--- a/lisp/net/gnutls.el
+++ b/lisp/net/gnutls.el
@@ -113,16 +113,14 @@ Security'."
     "/etc/ssl/cert.pem"                      ; macOS
     )
   "List of CA bundle location filenames or a function returning said list.
+If a file path contains glob wildcards, they will be expanded.
 The files may be in PEM or DER format, as per the GnuTLS documentation.
 The files may not exist, in which case they will be ignored."
   :group 'gnutls
   :type '(choice (function :tag "Function to produce list of bundle filenames")
                  (repeat (file :tag "Bundle filename"))))
 
-;;;###autoload
-(defcustom gnutls-min-prime-bits 256
-  ;; Several mail servers send fewer bits than the GnuTLS default.
-  ;; Currently, 256 appears to be a reasonable choice (Bug#11267).
+(defcustom gnutls-min-prime-bits nil
   "Minimum number of prime bits accepted by GnuTLS for key exchange.
 During a Diffie-Hellman handshake, if the server sends a prime
 number with fewer than this number of bits, the handshake is
@@ -138,9 +136,22 @@ network security is handled at a higher level via
 `open-network-stream' and the Network Security Manager.  See Info
 node `(emacs) Network Security'."
   :type '(choice (const :tag "Use default value" nil)
-                 (integer :tag "Number of bits" 512))
+                 (integer :tag "Number of bits" 2048))
   :group 'gnutls)
 
+(defcustom gnutls-crlfiles
+  '(
+    "/etc/grid-security/certificates/*.crl.pem"
+    )
+  "List of CRL file paths or a function returning said list.
+If a file path contains glob wildcards, they will be expanded.
+The files may be in PEM or DER format, as per the GnuTLS documentation.
+The files may not exist, in which case they will be ignored."
+  :group 'gnutls
+  :type '(choice (function :tag "Function to produce list of CRL filenames")
+                 (repeat (file :tag "CRL filename")))
+  :version "27.1")
+
 (defun open-gnutls-stream (name buffer host service &optional parameters)
   "Open a SSL/TLS connection for a service to a host.
 Returns a subprocess-object to represent the connection.
@@ -304,6 +315,7 @@ here's a recent version of the list.
 It must be omitted, a number, or nil; if omitted or nil it
 defaults to GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT."
   (let* ((trustfiles (or trustfiles (gnutls-trustfiles)))
+         (crlfiles (or crlfiles (gnutls-crlfiles)))
          (maybe-dumbfw (if (memq 'ClientHello\ Padding (gnutls-available-p))
                            ":%DUMBFW"
                          ""))
@@ -345,13 +357,18 @@ defaults to GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT."
                 :verify-error ,verify-error
                 :callbacks nil)))
 
+(defun gnutls--get-files (files)
+  (cl-loop for f in files
+           if f do (setq f (if (functionp f) (funcall f) f))
+           append (cl-delete-if-not #'file-exists-p (file-expand-wildcards f t))))
+
 (defun gnutls-trustfiles ()
   "Return a list of usable trustfiles."
-  (delq nil
-        (mapcar (lambda (f) (and f (file-exists-p f) f))
-                (if (functionp gnutls-trustfiles)
-                    (funcall gnutls-trustfiles)
-                  gnutls-trustfiles))))
+  (gnutls--get-files gnutls-trustfiles))
+
+(defun gnutls-crlfiles ()
+  "Return a list of usable CRL files."
+  (gnutls--get-files gnutls-crlfiles))
 
 (declare-function gnutls-error-string "gnutls.c" (error))
 
diff --git a/lisp/net/net-utils.el b/lisp/net/net-utils.el
index dcc7e01b6b4..4f68e5db61d 100644
--- a/lisp/net/net-utils.el
+++ b/lisp/net/net-utils.el
@@ -43,6 +43,10 @@
 ;; still use them for queries).  Actually the trend these
 ;; days is for /sbin to be a symlink to /usr/sbin, but we still need to
 ;; search both for older systems.
+
+(require 'subr-x)
+(require 'cl-lib)
+
 (defun net-utils--executable-find-sbin (command)
   "Return absolute name of COMMAND if found in an sbin directory."
   (let ((exec-path '("/sbin" "/usr/sbin" "/usr/local/sbin")))
@@ -514,7 +518,11 @@ Optional argument NAME-SERVER says which server to use for
 DNS resolution.
 Interactively, prompt for NAME-SERVER if invoked with prefix argument.
 
-This command uses `nslookup-program' for looking up the DNS information."
+This command uses `nslookup-program' for looking up the DNS information.
+
+See also: `nslookup-host-ipv4', `nslookup-host-ipv6' for
+non-interactive versions of this function more suitable for use
+in Lisp code."
   (interactive
    (list (read-from-minibuffer "Lookup host: " (net-utils-machine-at-point))
          (if current-prefix-arg (read-from-minibuffer "Name server: "))))
@@ -531,6 +539,71 @@ This command uses `nslookup-program' for looking up the DNS information."
      options)))
 
 ;;;###autoload
+(defun nslookup-host-ipv4 (host &optional name-server format)
+  "Return the IPv4 address for HOST (name or IP address).
+Optional argument NAME-SERVER says which server to use for DNS
+resolution.
+
+If FORMAT is `string', returns the IP address as a
+string (default).  If FORMAT is `vector', returns a 4-integer
+vector of octets.
+
+This command uses `nslookup-program' to look up DNS records."
+  (let* ((args `(,nslookup-program "-type=A" ,host ,name-server))
+         (output (shell-command-to-string
+                  (string-join (cl-remove nil args) " ")))
+         (ip (or (and (string-match
+                       "Name:.*\nAddress: *\\(\\([0-9]\\{1,3\\}\\.?\\)\\{4\\}\\)"
+                       output)
+                      (match-string 1 output))
+                 host)))
+    (cond ((memq format '(string nil))
+           ip)
+          ((eq format 'vector)
+           (apply #'vector (mapcar #'string-to-number (split-string ip "\\."))))
+          (t (error "Invalid format: %s" format)))))
+
+(defun ipv6-expand (ipv6-vector)
+  (let ((len (length ipv6-vector)))
+    (if (< len 8)
+        (let* ((pivot (cl-position 0 ipv6-vector))
+               (head (cl-subseq ipv6-vector 0 pivot))
+               (tail (cl-subseq ipv6-vector (1+ pivot) len)))
+          (vconcat head (make-vector (- 8 (1- len)) 0) tail))
+      ipv6-vector)))
+
+;;;###autoload
+(defun nslookup-host-ipv6 (host &optional name-server format)
+  "Return the IPv6 address for HOST (name or IP address).
+Optional argument NAME-SERVER says which server to use for DNS
+resolution.
+
+If FORMAT is `string', returns the IP address as a
+string (default).  If FORMAT is `vector', returns a 8-integer
+vector of hextets.
+
+This command uses `nslookup-program' to look up DNS records."
+  (let* ((args `(,nslookup-program "-type=AAAA" ,host ,name-server))
+         (output (shell-command-to-string
+                  (string-join (cl-remove nil args) " ")))
+         (hextet "[0-9a-fA-F]\\{1,4\\}")
+         (ip-regex (concat "\\(\\(" hextet "[:]\\)\\{1,6\\}\\([:]?\\(" hextet "\\)\\{1,6\\}\\)\\)"))
+         (ip (or (and (string-match
+                       (if (eq system-type 'windows-nt)
+                           (concat "Name:.*\nAddress: *" ip-regex)
+                         (concat "has AAAA address " ip-regex))
+                       output)
+                      (match-string 1 output))
+                 host)))
+    (cond ((memq format '(string nil))
+           ip)
+          ((eq format 'vector)
+           (ipv6-expand (apply #'vector
+                               (cl-loop for hextet in (split-string ip "[:]")
+                                        collect (string-to-number hextet 16)))))
+          (t (error "Invalid format: %s" format)))))
+
+;;;###autoload
 (defun nslookup ()
   "Run `nslookup-program'."
   (interactive)
diff --git a/lisp/net/nsm.el b/lisp/net/nsm.el
index dbfa2101f0c..da1fbf930a1 100644
--- a/lisp/net/nsm.el
+++ b/lisp/net/nsm.el
@@ -26,7 +26,9 @@
 
 (require 'cl-lib)
 (require 'rmc)                       ; read-multiple-choice
-(eval-when-compile (require 'subr-x))
+(require 'subr-x)
+(require 'seq)
+(require 'map)
 
 (defvar nsm-permanent-host-settings nil)
 (defvar nsm-temporary-host-settings nil)
@@ -44,20 +46,42 @@ connection should be handled.
 
 The following values are possible:
 
-`low': Absolutely no checks are performed.
-`medium': This is the default level, should be reasonable for most usage.
-`high': This warns about additional things that many people would
-not find useful.
-`paranoid': On this level, the user is queried for most new connections.
+`low': Check for problems known before Edward Snowden.
+`medium': Default.  Suitable for most circumstances.
+`high': Warns about additional issues not enabled in `medium' due to
+compatibility concerns.
 
 See the Emacs manual for a description of all things that are
 checked and warned against."
   :version "25.1"
   :group 'nsm
   :type '(choice (const :tag "Low" low)
-		 (const :tag "Medium" medium)
-		 (const :tag "High" high)
-		 (const :tag "Paranoid" paranoid)))
+                 (const :tag "Medium" medium)
+                 (const :tag "High" high)))
+
+;; Backward compatibility
+(when (eq network-security-level 'paranoid)
+  (setq network-security-level 'high))
+
+(defcustom nsm-trust-local-network nil
+  "Disable warnings when visiting trusted hosts on local networks.
+
+The default suite of TLS checks in NSM is designed to follow the
+most current security best practices.  Under some situations,
+such as attempting to connect to an email server that do not
+follow these practices inside a school or corporate network, NSM
+may produce warnings for such occasions.  Setting this option to
+a non-nil value, or a zero-argument function that returns non-nil
+tells NSM to skip checking for potential TLS vulnerabilities when
+connecting to hosts on a local network.
+
+Make sure you know what you are doing before enabling this
+option."
+  :version "27.1"
+  :group 'nsm
+  :type '(choice (const :tag "On" t)
+                 (const :tag "Off" nil)
+                 (function :tag "Custom function")))
 
 (defcustom nsm-settings-file (expand-file-name "network-security.data"
 						 user-emacs-directory)
@@ -98,241 +122,666 @@ to keep track of the TLS status of STARTTLS servers.
 
 If WARN-UNENCRYPTED, query the user if the connection is
 unencrypted."
-  (if (eq network-security-level 'low)
-      process
-    (let* ((status (gnutls-peer-status process))
-	   (id (nsm-id host port))
-	   (settings (nsm-host-settings id)))
-      (cond
-       ((not (process-live-p process))
-	nil)
-       ((not status)
-	;; This is a non-TLS connection.
-	(nsm-check-plain-connection process host port settings
-				    warn-unencrypted))
-       (t
-	(let ((process
-	       (nsm-check-tls-connection process host port status settings)))
-	  (when (and process save-fingerprint
-		     (null (nsm-host-settings id)))
-	    (nsm-save-host host port status 'fingerprint 'always))
-	  process))))))
+  (let* ((status (gnutls-peer-status process))
+         (id (nsm-id host port))
+         (settings (nsm-host-settings id)))
+    (cond
+     ((not (process-live-p process))
+      nil)
+     ((not status)
+      ;; This is a non-TLS connection.
+      (nsm-check-plain-connection process host port settings
+                                  warn-unencrypted))
+     (t
+      (let ((process
+             (nsm-check-tls-connection process host port status settings)))
+        (when (and process save-fingerprint
+                   (null (nsm-host-settings id)))
+          (nsm-save-host host port status 'fingerprint nil 'always))
+        process)))))
+
+(defcustom nsm-tls-checks
+  '(;; Pre-Snowden Known Weaknesses
+    (nsm-tls-check-version                . low)
+    (nsm-tls-check-compression            . low)
+    (nsm-tls-check-renegotiation-info-ext . low)
+    (nsm-tls-check-verify-cert            . low)
+    (nsm-tls-check-same-cert              . low)
+    (nsm-tls-check-null-suite             . low)
+    (nsm-tls-check-export-kx              . low)
+    (nsm-tls-check-anon-kx                . low)
+    (nsm-tls-check-md5-sig                . low)
+    (nsm-tls-check-rc4-cipher             . low)
+    ;; Post-Snowden Apocalypse
+    (nsm-tls-check-dhe-prime-kx           . medium)
+    (nsm-tls-check-sha1-sig               . medium)
+    (nsm-tls-check-ecdsa-cbc-cipher       . medium)
+    ;; Towards TLS 1.3
+    (nsm-tls-check-dhe-kx                 . high)
+    (nsm-tls-check-rsa-kx                 . high)
+    (nsm-tls-check-3des-cipher            . high)
+    (nsm-tls-check-cbc-cipher             . high))
+  "This variable specifies what TLS connection checks to perform.
+It's an alist where the key is the name of the check, and the
+value is the minimum security level the check should begin.
+
+Each check function is called with the parameters HOST PORT
+STATUS SETTINGS.  HOST is the host domain, PORT is a TCP port
+number, STATUS is the peer status returned by
+`gnutls-peer-status', and SETTINGS is the persistent and session
+settings for the host HOST.  Please refer to the contents of
+`nsm-setting-file' for details.  If a problem is found, the check
+function is required to return an error message, and nil
+otherwise.
+
+See also: `nsm-check-tls-connection', `nsm-save-host-names',
+`nsm-settings-file'"
+  :version "27.1"
+  :group 'nsm
+  :type '(repeat (cons (function :tag "Check function")
+                       (choice :tag "Level"
+                               :value medium
+                               (const :tag "Low" low)
+                               (const :tag "Medium" medium)
+                               (const :tag "High" high)))))
+
+(defun nsm-save-fingerprint-maybe (host port status &rest _)
+  "Saves the certificate's fingerprint.
+
+In order to detect man-in-the-middle attacks, when
+`network-security-level' is `high', this function will save the
+fingerprint of the certificate for check functions to check."
+  (when (>= (nsm-level network-security-level) (nsm-level 'high))
+    ;; Save the host fingerprint so that we can check it the
+    ;; next time we connect.
+    (nsm-save-host host port status 'fingerprint nil 'always)))
+
+(defvar nsm-tls-post-check-functions '(nsm-save-fingerprint-maybe)
+  "Functions to run after checking a TLS session.
+
+Each function will be run with the parameters HOST PORT STATUS
+SETTINGS and RESULTS.  The parameters HOST PORT STATUS and
+SETTINGS are the same as those supplied to each check function.
+RESULTS is an alist where the keys are the checks run and the
+values the results of the checks.")
+
+(defun nsm-network-same-subnet (local-ip mask ip)
+  "Returns t if IP is in the same subnet as LOCAL-IP/MASK.
+LOCAL-IP, MASK, and IP are specified as vectors of integers, and
+are expected to have the same length.  Works for both IPv4 and
+IPv6 addresses."
+  (let ((matches t)
+        (length (length local-ip)))
+    (unless (memq length '(4 5 8 9))
+      (error "Unexpected length of IP address %S" local-ip))
+    (dotimes (i length)
+      (setq matches (and matches
+                         (=
+                          (logand (aref local-ip i)
+                                  (aref mask i))
+                          (logand (aref ip i)
+                                  (aref mask i))))))
+    matches))
+
+(defun nsm-should-check (host)
+  "Determines whether NSM should check for TLS problems for HOST.
+
+If `nsm-trust-local-network' is or returns non-nil, and if the
+host address is a localhost address, or in the same subnet as one
+of the local interfaces, this function returns nil.  Non-nil
+otherwise."
+  (let ((addresses (network-lookup-address-info host))
+        (network-interface-list (network-interface-list))
+        (off-net t))
+    (when
+     (or (and (functionp nsm-trust-local-network)
+              (funcall nsm-trust-local-network))
+         nsm-trust-local-network)
+     (mapc
+      (lambda (address)
+        (mapc
+         (lambda (iface)
+           (let ((info (network-interface-info (car iface))))
+             (when
+                 (nsm-network-same-subnet (substring (car info) 0 -1)
+                                          (substring (car (cddr info)) 0 -1)
+                                          address)
+               (setq off-net nil))))
+         network-interface-list))
+      addresses))
+     off-net))
 
 (defun nsm-check-tls-connection (process host port status settings)
-  (when-let ((process
-              (nsm-check-certificate process host port status settings)))
-    ;; Do further protocol-level checks.
-    (nsm-check-protocol process host port status settings)))
+  "Check TLS connection against potential security problems.
+
+This function runs each test defined in `nsm-tls-checks' in the
+order specified against the TLS connection's peer status STATUS
+for the host HOST and port PORT.
+
+If one or more problems are found, this function will collect all
+the error messages returned by the check functions, and confirm
+with the user in interactive mode whether to continue with the
+TLS session.
+
+If the user declines to continue, or problem(s) are found under
+non-interactive mode, the process PROCESS will be deleted, thus
+terminating the connection.
+
+This function returns the process PROCESS if no problems are
+found, and nil otherwise.
+
+See also: `nsm-tls-checks' and `nsm-noninteractive'"
+  (when (nsm-should-check host)
+    (let* ((results
+            (cl-loop for check in nsm-tls-checks
+                     for type = (intern (format ":%s"
+                                                (string-remove-prefix
+                                                 "nsm-tls-check-"
+                                                 (symbol-name (car check))))
+                                        obarray)
+                     ;; Skip the check if the user has already said that this
+                     ;; host is OK for this type of "error".
+                     for result = (and (not (memq type (plist-get settings :conditions)))
+                                       (>= (nsm-level network-security-level)
+                                           (nsm-level (cdr check)))
+                                       (funcall (car check) host port status settings))
+                     when result
+                     collect (cons type result)))
+           (problems (nconc (plist-get status :warnings) (map-keys results))))
+      (when (and results
+                 (not (seq-set-equal-p (plist-get settings :conditions) problems))
+                 (not (nsm-query host port status
+                                 'conditions
+                                 problems
+                                 (format-message
+		                  "The TLS connection to %s:%s is insecure\nfor the following reason%s:\n\n%s"
+		                  host port
+		                  (if (> (length problems) 1)
+			              "s" "")
+		                  (concat "* " (string-join
+                                                (split-string
+                                                 (string-join
+                                                  (map-values results)
+                                                  "\n")
+                                                 "\n")
+                                                "\n* ")))))
+                 (delete-process process)
+                 (setq process nil)))
+      (run-hook-with-args 'nsm-tls-post-check-functions
+                          host port status settings results)))
+  process)
+
+
+
+;; Certificate checks
 
 (declare-function gnutls-peer-status-warning-describe "gnutls.c"
-		  (status-symbol))
+                  (status-symbol))
+
+(defun nsm-tls-check-verify-cert (host port status settings)
+  "Check for warnings from the certificate verification status.
 
-(defun nsm-check-certificate (process host port status settings)
+This is the most basic security check for a TLS connection.  If
+ certificate verification fails, it means the server's identity
+ cannot be verified by the credentials received.
+
+Think very carefully before removing this check from
+`nsm-tls-checks'."
   (let ((warnings (plist-get status :warnings)))
-    (cond
+    (and warnings
+         (not (nsm-warnings-ok-p status settings))
+         (mapconcat #'gnutls-peer-status-warning-describe warnings "\n"))))
 
-     ;; The certificate validated, but perhaps we want to do
-     ;; certificate pinning.
-     ((null warnings)
-      (cond
-       ((< (nsm-level network-security-level) (nsm-level 'high))
-	process)
-       ;; The certificate is fine, but if we're paranoid, we might
-       ;; want to check whether it's changed anyway.
-       ((and (>= (nsm-level network-security-level) (nsm-level 'high))
-	     (not (nsm-fingerprint-ok-p host port status settings)))
-	(delete-process process)
-	nil)
-       ;; We haven't seen this before, and we're paranoid.
-       ((and (eq network-security-level 'paranoid)
-	     (null settings)
-	     (not (nsm-new-fingerprint-ok-p host port status)))
-	(delete-process process)
-	nil)
-       (t
-	process)))
-
-     ;; The certificate did not validate.
-     ((not (equal network-security-level 'low))
-      ;; We always want to pin the certificate of invalid connections
-      ;; to track man-in-the-middle or the like.
-      (if (not (nsm-fingerprint-ok-p host port status settings))
-	  (progn
-	    (delete-process process)
-	    nil)
-	;; We have a warning, so query the user.
-	(if (and (not (nsm-warnings-ok-p status settings))
-		 (not (nsm-query
-		       host port status 'conditions
-		       "The TLS connection to %s:%s is insecure for the following reason%s:\n\n%s"
-		       host port
-		       (if (> (length warnings) 1)
-			   "s" "")
-		       (mapconcat #'gnutls-peer-status-warning-describe
-                                  warnings
-                                  "\n"))))
-	    (progn
-	      (delete-process process)
-	      nil)
-	  process))))))
-
-(defvar network-security-protocol-checks
-  '((diffie-hellman-prime-bits medium 1024)
-    (rc4 medium)
-    (signature-sha1 medium)
-    (intermediate-sha1 medium)
-    (3des high)
-    (ssl medium))
-  "This variable specifies what TLS connection checks to perform.
-It's an alist where the first element is the name of the check,
-the second is the security level where the check kicks in, and the
-optional third element is a parameter supplied to the check.
-
-An element like `(rc4 medium)' will result in the function
-`nsm-protocol-check--rc4' being called with the parameters
-HOST PORT STATUS OPTIONAL-PARAMETER.")
-
-(defun nsm-check-protocol (process host port status settings)
-  (cl-loop for check in network-security-protocol-checks
-           for type = (intern (format ":%s" (car check)) obarray)
-           while process
-           ;; Skip the check if the user has already said that this
-           ;; host is OK for this type of "error".
-           when (and (not (memq type (plist-get settings :conditions)))
-                     (>= (nsm-level network-security-level)
-                         (nsm-level (cadr check))))
-           do (let ((result
-                     (funcall (intern (format "nsm-protocol-check--%s"
-                                              (car check))
-                                      obarray)
-                              host port status (nth 2 check))))
-                (unless result
-                  (delete-process process)
-                  (setq process nil))))
-  ;; If a test failed we return nil, otherwise the process object.
-  process)
+(defun nsm-tls-check-same-cert (host port status settings)
+  "Check for certificate fingerprint mismatch.
 
-(defun nsm--encryption (status)
-  (format "%s-%s-%s"
-          (plist-get status :key-exchange)
-	  (plist-get status :cipher)
-	  (plist-get status :mac)))
+If the fingerprints saved do not match the fingerprint of the
+certificate presented, the TLS session may be under a
+man-in-the-middle attack."
+  (and (not (nsm-fingerprint-ok-p status settings))
+       (format-message
+        "fingerprint has changed")))
+
+;; Key exchange checks
+
+(defun nsm-tls-check-rsa-kx (host port status &optional settings)
+  "Check for static RSA key exchange.
 
-(defun nsm-protocol-check--diffie-hellman-prime-bits (host port status bits)
+Static RSA key exchange methods do not offer perfect forward
+secrecy, therefore, the security of a TLS session is only as
+secure as the server's private key.  Due to TLS' use of RSA key
+exchange to create a session key (the key negotiated between the
+client and the server to encrypt traffic), if the server's
+private key had been compromised, the attacker will be able to
+decrypt any past TLS session recorded, as opposed to just one TLS
+session if the key exchange was conducted via a key exchange
+method that offers perfect forward secrecy, such as ephemeral
+Diffie-Hellman key exchange.
+
+By default, this check is only enabled when
+`network-security-level' is set to `high' for compatibility
+reasons.
+
+Reference:
+
+Sheffer, Holz, Saint-Andre (May 2015).  \"Recommendations for Secure
+Use of Transport Layer Security (TLS) and Datagram Transport Layer
+Security (DTLS)\", \"(4.1.  General Guidelines)\"
+`https://tools.ietf.org/html/rfc7525\#section-4.1'"
+  (let ((kx (plist-get status :key-exchange)))
+    (and (string-match "^\\bRSA\\b" kx)
+         (format-message
+          "RSA key exchange method (%s) does not offer perfect forward secrecy"
+          kx))))
+
+(defun nsm-tls-check-dhe-prime-kx (host port status &optional settings)
+  "Check for the key strength of DH key exchange based on integer factorization.
+
+This check is a response to Logjam[1].  Logjam is an attack that
+allows an attacker with sufficient resource, and positioned
+between the user and the server, to downgrade vulnerable TLS
+connections to insecure 512-bit export grade crypotography.
+
+The Logjam paper suggests using 1024-bit prime on the client to
+mitigate some effects of this attack, and upgrade to 2048-bit as
+soon as server configurations allow.  According to SSLLabs' SSL
+Pulse tracker, only about 75% of server support 2048-bit key
+exchange in June 2018[2].  To provide a balance between
+compatibility and security, this function only checks for a
+minimum key strength of 1024-bit.
+
+See also: `nsm-tls-check-dhe-kx'
+
+Reference:
+
+[1]: Adrian et al (2014).  \"Imperfect Forward Secrecy: How
+Diffie-Hellman Fails in Practice\", `https://weakdh.org/'
+[2]: SSL Pulse (June 03, 2018).  \"Key Exchange Strength\",
+`https://www.ssllabs.com/ssl-pulse/'"
   (let ((prime-bits (plist-get status :diffie-hellman-prime-bits)))
-    (or (not prime-bits)
-        (>= prime-bits bits)
-	(nsm-query
-	 host port status :diffie-hellman-prime-bits
-	 "The Diffie-Hellman prime bits (%s) used for this connection to %s:%s is less than what is considered safe (%s)."
-	 prime-bits host port bits))))
-
-(defun nsm-protocol-check--3des (host port status _)
-  (or (not (string-match "\\b3DES\\b" (plist-get status :cipher)))
-      (nsm-query
-       host port status :rc4
-       "The connection to %s:%s uses the 3DES cipher (%s), which is believed to be unsafe."
-       host port (plist-get status :cipher))))
-
-(defun nsm-protocol-check--rc4 (host port status _)
-  (or (not (string-match "\\bRC4\\b" (nsm--encryption status)))
-      (nsm-query
-       host port status :rc4
-       "The connection to %s:%s uses the RC4 algorithm (%s), which is believed to be unsafe."
-       host port (nsm--encryption status))))
-
-(defun nsm-protocol-check--signature-sha1 (host port status _)
-  (let ((signature-algorithm
-         (plist-get (plist-get status :certificate) :signature-algorithm)))
-    (or (not (string-match "\\bSHA1\\b" signature-algorithm))
-        (nsm-query
-         host port status :signature-sha1
-         "The certificate used to verify the connection to %s:%s uses the SHA1 algorithm (%s), which is believed to be unsafe."
-         host port signature-algorithm))))
-
-(defun nsm-protocol-check--intermediate-sha1 (host port status _)
-  ;; Skip the first certificate, because that's the host certificate.
-  (cl-loop for certificate in (cdr (plist-get status :certificates))
+    (if (and (string-match "^\\bDHE\\b" (plist-get status :key-exchange))
+             (< prime-bits 1024))
+        (format-message
+         "Diffie-Hellman key strength (%s bits) too weak (%s bits)"
+         prime-bits 1024))))
+
+(defun nsm-tls-check-dhe-kx (host port status &optional settings)
+  "Check for existence of DH key exchange based on integer factorization.
+
+In the years since the discovery of Logjam, it was discovered
+that there were rampant use of small subgroup prime or composite
+number for DHE by many servers, and thus allowed themselves to be
+vulnerable to backdoors[1].  Given the difficulty in validating
+Diffie-Hellman parameters, major browser vendors had started to
+remove DHE since 2016[2].  Emacs stops short of banning DHE and
+terminating connection, but prompts the user instead.
+
+References:
+
+[1]: Dorey, Fong, and Essex (2016).  \"Indiscreet Logs: Persistent
+Diffie-Hellman Backdoors in TLS.\",
+`https://eprint.iacr.org/2016/999.pdf'
+[2]: Chrome Platform Status (2017).  \"Remove DHE-based ciphers\",
+`https://www.chromestatus.com/feature/5128908798164992'"
+  (let ((kx (plist-get status :key-exchange)))
+    (when (string-match "^\\bDHE\\b" kx)
+      (format-message
+       "unable to verify Diffie-Hellman key exchange method (%s) parameters"
+       kx))))
+
+(defun nsm-tls-check-export-kx (host port status &optional settings)
+  "Check for RSA-EXPORT key exchange.
+
+EXPORT cipher suites are a family of 40-bit and 56-bit effective
+security algorithms legally exportable by the United States in
+the early 90s[1].  They can be broken in seconds on 2018 hardware.
+
+Prior to 3.2.0, GnuTLS had only supported RSA-EXPORT key
+exchange.  Since 3.2.0, RSA-EXPORT had been removed, therefore,
+this check has no effect on GnuTLS >= 3.2.0.
+
+Reference:
+
+[1]: Schneier, Bruce (1996). Applied Cryptography (Second ed.). John
+Wiley & Sons. ISBN 0-471-11709-9.
+[2]: N. Mavrogiannopoulos, FSF (Apr 2015).  \"GnuTLS NEWS -- History
+of user-visible changes.\" Version 3.4.0,
+`https://gitlab.com/gnutls/gnutls/blob/master/NEWS'"
+  (when (< libgnutls-version 30200)
+    (let ((kx (plist-get status :key-exchange)))
+      (and (string-match "\\bEXPORT\\b" kx)
+           (format-message
+            "EXPORT level key exchange (%s) is insecure"
+            kx)))))
+
+(defun nsm-tls-check-anon-kx (host port status &optional settings)
+  "Check for anonymous key exchange.
+
+Anonymous key exchange exposes the connection to
+man-in-the-middle attacks.
+
+Reference:
+
+GnuTLS authors (2018). \"GnuTLS Manual 4.3.3 Anonymous
+authentication\",
+`https://www.gnutls.org/manual/gnutls.html\#Anonymous-authentication'"
+  (let ((kx (plist-get status :key-exchange)))
+    (and (string-match "\\bANON\\b" kx)
+         (format-message
+          "anonymous key exchange method (%s) can be unsafe"
+          kx))))
+
+;; Cipher checks
+
+(defun nsm-tls-check-cbc-cipher (host port status &optional settings)
+  "Check for CBC mode ciphers.
+
+CBC mode cipher in TLS versions earlier than 1.3 are problematic
+because of MAC-then-encrypt.  This construction is vulnerable to
+padding oracle attacks[1].
+
+Since GnuTLS 3.4.0, the TLS encrypt-then-MAC extension[2] has
+been enabled by default[3]. If encrypt-then-MAC is negotiated,
+this check has no effect.
+
+Reference:
+
+[1]: Sullivan (Feb 2016).  \"Padding oracles and the decline of
+CBC-mode cipher suites\",
+`https://blog.cloudflare.com/padding-oracles-and-the-decline-of-cbc-mode-ciphersuites/'
+[2]: P. Gutmann (Sept 2014).  \"Encrypt-then-MAC for Transport Layer
+Security (TLS) and Datagram Transport Layer Security (DTLS)\",
+`https://tools.ietf.org/html/rfc7366'
+[3]: N. Mavrogiannopoulos (Nov 2015).  \"An overview of GnuTLS
+3.4.x\",
+`https://nikmav.blogspot.com/2015/11/an-overview-of-gnutls-34x.html'"
+  (when (not (plist-get status :encrypt-then-mac))
+    (let ((cipher (plist-get status :cipher)))
+      (and (string-match "\\bCBC\\b" cipher)
+           (format-message
+            "CBC mode cipher (%s) can be insecure"
+            cipher)))))
+
+(defun nsm-tls-check-ecdsa-cbc-cipher (host port status &optional settings)
+  "Check for CBC mode cipher usage under ECDSA key exchange.
+
+CBC mode cipher in TLS versions earlier than 1.3 are problematic
+because of MAC-then-encrypt.  This construction is vulnerable to
+padding oracle attacks[1].
+
+Due to current widespread use of CBC mode ciphers by servers,
+this function only checks for CBC mode cipher usage in
+combination with ECDSA key exchange, which is virtually
+non-existent[2].
+
+Since GnuTLS 3.4.0, the TLS encrypt-then-MAC extension[3] has
+been enabled by default[4]. If encrypt-then-MAC is negotiated,
+this check has no effect.
+
+References:
+
+[1]: Sullivan (Feb 2016).  \"Padding oracles and the decline of
+CBC-mode cipher suites\",
+`https://blog.cloudflare.com/padding-oracles-and-the-decline-of-cbc-mode-ciphersuites/'
+[2]: Chrome Platform Status (2017). \"Remove CBC-mode ECDSA ciphers in
+TLS\", `https://www.chromestatus.com/feature/5740978103123968'
+[3]: P. Gutmann (Sept 2014).  \"Encrypt-then-MAC for Transport Layer
+Security (TLS) and Datagram Transport Layer Security (DTLS)\",
+`https://tools.ietf.org/html/rfc7366'
+[4]: N. Mavrogiannopoulos (Nov 2015).  \"An overview of GnuTLS
+3.4.x\",
+`https://nikmav.blogspot.com/2015/11/an-overview-of-gnutls-34x.html'"
+  (when (not (plist-get status :encrypt-then-mac))
+    (let ((kx (plist-get status :key-exchange))
+          (cipher (plist-get status :cipher)))
+      (and (string-match "\\bECDSA\\b" kx)
+           (string-match "\\bCBC\\b" cipher)
+           (format-message
+            "CBC mode cipher (%s) can be insecure"
+            cipher)))))
+
+(defun nsm-tls-check-3des-cipher (host port status &optional settings)
+  "Check for 3DES ciphers.
+
+Due to its use of 64-bit block size, it is known that a
+ciphertext collision is highly likely when 2^32 blocks are
+encrypted with the same key bundle under 3-key 3DES.  Practical
+birthday attacks of this kind have been demostrated by Sweet32[1].
+As such, NIST is in the process of disallowing its use in TLS[2].
+
+[1]: Bhargavan, Leurent (2016).  \"On the Practical (In-)Security of
+64-bit Block Ciphers — Collision Attacks on HTTP over TLS and
+OpenVPN\", `https://sweet32.info/'
+[2]: NIST Information Technology Laboratory (Jul 2017).  \"Update to
+Current Use and Deprecation of TDEA\",
+`https://csrc.nist.gov/News/2017/Update-to-Current-Use-and-Deprecation-of-TDEA'"
+  (let ((cipher (plist-get status :cipher)))
+    (and (string-match "\\b3DES\\b" cipher)
+         (format-message
+          "3DES cipher (%s) is weak"
+          cipher))))
+
+(defun nsm-tls-check-rc4-cipher (host port status &optional settings)
+  "Check for RC4 ciphers.
+
+RC4 cipher has been prohibited by RFC 7465[1].
+
+Since GnuTLS 3.4.0, RC4 is not enabled by default[2], but can be
+enabled if requested.  This check is mainly provided to secure
+Emacs built with older version of GnuTLS.
+
+Reference:
+
+[1]: Popov A (Feb 2015).  \"Prohibiting RC4 Cipher Suites\",
+`https://tools.ietf.org/html/rfc7465'
+[2]: N. Mavrogiannopoulos (Nov 2015).  \"An overview of GnuTLS
+3.4.x\",
+`https://nikmav.blogspot.com/2015/11/an-overview-of-gnutls-34x.html'"
+  (let ((cipher (plist-get status :cipher)))
+    (and (string-match "\\bARCFOUR\\b" cipher)
+         (format-message
+          "RC4 cipher (%s) is insecure"
+          cipher))))
+
+;; Signature checks
+
+(defun nsm-tls-check-sha1-sig (host port status &optional settings)
+  "Check for SHA1 signatures on certificates.
+
+The first SHA1 collision was found in 2017[1], as a precaution
+against the events following the discovery of cheap collisions in
+MD5, major browsers[2][3][4][5] have removed the use of SHA1
+signatures in certificates.
+
+References:
+
+[1]: Stevens M, Karpman P et al (2017).  \"The first collision for
+full SHA-1\", `https://shattered.io/static/shattered.pdf'
+[2]: Chromium Security Education TLS/SSL.  \"Deprecated and Removed
+Features (SHA-1 Certificate Signatures)\",
+`https://www.chromium.org/Home/chromium-security/education/tls\#TOC-SHA-1-Certificate-Signatures'
+[3]: Jones J.C (2017).  \"The end of SHA-1 on the Public Web\",
+`https://blog.mozilla.org/security/2017/02/23/the-end-of-sha-1-on-the-public-web/'
+[4]: Apple Support (2017).  \"Move to SHA-256 signed certificates to
+avoid connection failures\",
+`https://support.apple.com/en-gb/HT207459'
+[5]: Microsoft Security Advisory 4010323 (2017).  \"Deprecation of
+SHA-1 for SSL/TLS Certificates in Microsoft Edge and Internet Explorer
+11\",
+`https://docs.microsoft.com/en-us/security-updates/securityadvisories/2017/4010323'"
+  (cl-loop for certificate in (plist-get status :certificates)
+           for algo = (plist-get certificate :signature-algorithm)
+           ;; Don't check root certificates -- root is always trusted.
+           if (and (not (equal (plist-get certificate :issuer)
+                               (plist-get certificate :subject)))
+                   (string-match "\\bSHA1\\b" algo))
+           return (format-message
+                   "SHA1 signature (%s) is prone to collisions"
+                   algo)
+           end))
+
+(defun nsm-tls-check-md5-sig (host port status &optional settings)
+  "Check for MD5 signatures on certificates.
+
+In 2008, a group of researchers were able to forge an
+intermediate CA certificate that appeared to be legitimate when
+checked by MD5[1].  RFC 6151[2] has recommended against the usage
+of MD5 for digital signatures, which includes TLS certificate
+signatures.
+
+Since GnuTLS 3.3.0, MD5 has been disabled by default, but can be
+enabled if requested.
+
+References:
+
+[1]: Sotirov A, Stevens M et al (2008).  \"MD5 considered harmful today
+- Creating a rogue CA certificate\",
+`http://www.win.tue.nl/hashclash/rogue-ca/'
+[2]: Turner S, Chen L (2011).  \"Updated Security Considerations for
+the MD5 Message-Digest and the HMAC-MD5 Algorithms\",
+`https://tools.ietf.org/html/rfc6151'"
+  (cl-loop for certificate in (plist-get status :certificates)
            for algo = (plist-get certificate :signature-algorithm)
-           ;; Don't check root certificates -- SHA1 isn't dangerous
-           ;; there.
-           when (and (not (equal (plist-get certificate :issuer)
-                                 (plist-get certificate :subject)))
-                     (string-match "\\bSHA1\\b" algo)
-                     (not (nsm-query
-                           host port status :intermediate-sha1
-                           "An intermediate certificate used to verify the connection to %s:%s uses the SHA1 algorithm (%s), which is believed to be unsafe."
-                           host port algo)))
-           do (cl-return nil)
-           finally (cl-return t)))
-
-(defun nsm-protocol-check--ssl (host port status _)
+           ;; Don't check root certificates -- root is always trusted.
+           if (and (not (equal (plist-get certificate :issuer)
+                               (plist-get certificate :subject)))
+                   (string-match "\\bMD5\\b" algo))
+           return (format-message
+                   "MD5 signature (%s) is very prone to collisions"
+                   algo)
+           end))
+
+;; Extension checks
+
+(defun nsm-tls-check-renegotiation-info-ext (host port status &optional settings)
+  "Check for renegotiation_info TLS extension status.
+
+If this TLS extension is not used, the connection established is
+vulnerable to an attack in which an impersonator can extract
+sensitive information such as HTTP session ID cookies or login
+passwords.
+
+Reference:
+
+E. Rescorla, M. Ray, S. Dispensa, N. Oskov (Feb 2010).  \"Transport
+Layer Security (TLS) Renegotiation Indication Extension\",
+`https://tools.ietf.org/html/rfc5746'"
+  (let ((unsafe-renegotiation (not (plist-get status :safe-renegotiation))))
+    (and unsafe-renegotiation
+         (format-message
+          "safe renegotiation is not supported, connection not protected from impersonators"))))
+
+;; Compression checks
+
+(defun nsm-tls-check-compression (host port status &optional settings)
+  "Check for TLS compression.
+
+TLS compression attacks such as CRIME would allow an attacker to
+decrypt ciphertext.  As a result, RFC 7525 has recommended its
+disablement.
+
+Reference:
+
+Sheffer, Holz, Saint-Andre (May 2015).  \"Recommendations for Secure
+Use of Transport Layer Security (TLS) and Datagram Transport Layer
+Security (DTLS)\", `https://tools.ietf.org/html/rfc7525'"
+  (let ((compression (plist-get status :compression)))
+    (and (string-match "^\\bDEFLATE\\b" compression)
+         (format-message
+          "compression method (%s) may lead to leakage of sensitive information"
+          compression))))
+
+;; Protocol version checks
+
+(defun nsm-tls-check-version (host port status &optional settings)
+  "Check for SSL/TLS protocol version.
+
+This function guards against the usage of SSL3.0, which has been
+deprecated by RFC7568[1], and TLS 1.0, which has been deprecated
+by PCI DSS[2].
+
+References:
+
+[1]: Barnes, Thomson, Pironti, Langley (2015).  \"Deprecating Secure
+Sockets Layer Version 3.0\", `https://tools.ietf.org/html/rfc7568'
+[2]: PCI Security Standards Council (2016).  \"Migrating from SSL and
+Early TLS\"
+`https://www.pcisecuritystandards.org/documents/Migrating-from-SSL-Early-TLS-Info-Supp-v1_1.pdf'"
   (let ((protocol (plist-get status :protocol)))
-    (or (not protocol)
-	(not (string-match "SSL" protocol))
-	(nsm-query
-	 host port status :ssl
-	 "The connection to %s:%s uses the %s protocol, which is believed to be unsafe."
-	 host port protocol))))
+    (and protocol
+         (or (string-match "SSL" protocol)
+             (and (string-match "TLS1.\\([0-9]+\\)" protocol)
+                  (< (string-to-number (match-string 1 protocol)) 1)))
+         (format-message
+          "%s protocol is deprecated by standard bodies"
+          protocol))))
+
+;; Full suite checks
+
+(defun nsm-tls-check-null-suite (host port status &optional settings)
+  "Check for NULL cipher suites.
+
+This function checks for NULL key exchange, cipher and message
+authentication code key derivation function.  As the name
+suggests, a NULL assigned for any of the above disables an
+integral part of the security properties that makes up the TLS
+protocol."
+  (let ((suite (nsm-cipher-suite status)))
+    (and (string-match "\\bNULL\\b" suite)
+         (format-message
+          "NULL cipher suite (%s) violates authenticity, integrity, or confidentiality guarantees"
+          suite))))
+
+
 
 (defun nsm-fingerprint (status)
   (plist-get (plist-get status :certificate) :public-key-id))
 
-(defun nsm-fingerprint-ok-p (host port status settings)
-  (let ((did-query nil))
-    (if (and settings
-	     (not (eq (plist-get settings :fingerprint) :none))
-	     (not (equal (nsm-fingerprint status)
-			 (plist-get settings :fingerprint)))
-	     (not
-	      (setq did-query
-		    (nsm-query
-		     host port status 'fingerprint
-		     "The fingerprint for the connection to %s:%s has changed from %s to %s"
-		     host port
-		     (plist-get settings :fingerprint)
-		     (nsm-fingerprint status)))))
-	;; Not OK.
-	nil
-      (when did-query
-	;; Remove any exceptions that have been set on the previous
-	;; certificate.
-	(plist-put settings :conditions nil))
-      t)))
-
-(defun nsm-new-fingerprint-ok-p (host port status)
-  (nsm-query
-   host port status 'fingerprint
-   "The fingerprint for the connection to %s:%s is new: %s"
-   host port
-   (nsm-fingerprint status)))
+(defun nsm-fingerprint-ok-p (status settings)
+  (let ((saved-fingerprints (plist-get settings :fingerprints)))
+    ;; Haven't seen this host before or not pinning cert
+    (or (null saved-fingerprints)
+        ;; Plain connection allowed
+        (memq :none saved-fingerprints)
+        ;; We are pinning certs, and we have seen this host
+        ;; before, but the credientials for this host differs
+        ;; from the last times we saw it
+        (member (nsm-fingerprint status) saved-fingerprints))))
+
+(set-advertised-calling-convention
+ 'nsm-fingerprint-ok-p '(status settings) "27.1")
 
 (defun nsm-check-plain-connection (process host port settings warn-unencrypted)
-  ;; If this connection used to be TLS, but is now plain, then it's
-  ;; possible that we're being Man-In-The-Middled by a proxy that's
-  ;; stripping out STARTTLS announcements.
-  (cond
-   ((and (plist-get settings :fingerprint)
-	 (not (eq (plist-get settings :fingerprint) :none))
-	 (not
-	  (nsm-query
-	   host port nil 'conditions
-	   "The connection to %s:%s used to be an encrypted connection, but is now unencrypted.  This might mean that there's a man-in-the-middle tapping this connection."
-	   host port)))
-    (delete-process process)
-    nil)
-   ((and warn-unencrypted
-	 (not (memq :unencrypted (plist-get settings :conditions)))
-	 (not (nsm-query
-	       host port nil 'conditions
-	       "The connection to %s:%s is unencrypted."
-	       host port)))
-    (delete-process process)
-    nil)
-   (t
-    process)))
-
-(defun nsm-query (host port status what message &rest args)
+  (if (nsm-should-check host)
+      ;; If this connection used to be TLS, but is now plain, then it's
+      ;; possible that we're being Man-In-The-Middled by a proxy that's
+      ;; stripping out STARTTLS announcements.
+      (let ((fingerprints (plist-get settings :fingerprints)))
+        (cond
+         ((and fingerprints
+	       (not (memq :none fingerprints))
+	       (not
+	        (nsm-query
+	         host port nil 'conditions '(:unencrypted)
+                 (format-message
+	          "The connection to %s:%s used to be an encrypted connection, but is now unencrypted.  This might mean that there's a man-in-the-middle tapping this connection."
+	          host port))))
+          (delete-process process)
+          nil)
+         ((and warn-unencrypted
+	       (not (memq :unencrypted (plist-get settings :conditions)))
+	       (not (nsm-query
+	             host port nil 'conditions '(:unencrypted)
+                     (format-message
+	              "The connection to %s:%s is unencrypted."
+	              host port))))
+          (delete-process process)
+          nil)
+         (t
+          process)))
+    process))
+
+(defun nsm-query (host port status what problems message)
   ;; If there is no user to answer queries, then say `no' to everything.
   (if (or noninteractive
 	  nsm-noninteractive)
@@ -340,9 +789,7 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
     (let ((response
 	   (condition-case nil
                (intern
-                (car (split-string
-                      (nsm-query-user message args
-                                      (nsm-format-certificate status))))
+                (car (split-string (nsm-query-user message status)))
                 obarray)
 	     ;; Make sure we manage to close the process if the user hits
 	     ;; `C-g'.
@@ -356,46 +803,111 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
                      "Accepting certificate for %s:%s this session only"
                    "Permanently accepting certificate for %s:%s")
                  host port)
-	(nsm-save-host host port status what response)
-	t))))
-
-(defun nsm-query-user (message args cert)
-  (catch 'return
-    (while t
-      (let ((buffer (get-buffer-create "*Network Security Manager*")))
-        (save-window-excursion
-          ;; First format the certificate and warnings.
-          (with-help-window buffer
-            (with-current-buffer buffer
-              (erase-buffer)
-              (when (> (length cert) 0)
-                (insert cert "\n"))
-              (let ((start (point)))
-                (insert (apply #'format-message message args))
-                (goto-char start)
-                ;; Fill the first line of the message, which usually
-                ;; contains lots of explanatory text.
-                (fill-region (point) (line-end-position)))))
-          ;; Then ask the user what to do about it.
-          (pcase (unwind-protect
-                     (cadr
-                      (read-multiple-choice
-                       "Continue connecting?"
-                       '((?a "always" "Accept this certificate this session and for all future sessions.")
-                         (?s "session only" "Accept this certificate this session only.")
-                         (?n "no" "Refuse to use this certificate, and close the connection.")
-                         (?r "reshow" "Reshow certificate information."))))
-                   (kill-buffer buffer))
-            ("reshow")
-            (val (throw 'return val))))))))
-
-(defun nsm-save-host (host port status what permanency)
+        (nsm-save-host host port status what problems response)
+        t))))
+
+(set-advertised-calling-convention
+ 'nsm-query '(host port status what problems message) "27.1")
+
+(declare-function gnutls-format-certificate "gnutls.c" (cert))
+
+(defun nsm-query-user (message status)
+  (let ((buffer (get-buffer-create "*Network Security Manager*"))
+        (cert-buffer (get-buffer-create "*Certificate Details*"))
+        (certs (plist-get status :certificates)))
+    (save-window-excursion
+      ;; First format the certificate and warnings.
+      (with-current-buffer-window
+       buffer nil nil
+       (insert (nsm-format-certificate status))
+       (insert message)
+       (goto-char (point-min))
+       ;; Fill the first line of the message, which usually
+       ;; contains lots of explanatory text.
+       (fill-region (point) (line-end-position)))
+      ;; Then ask the user what to do about it.
+      (unwind-protect
+          (let* ((accept-choices '((?a "always" "Accept this certificate this session and for all future sessions.")
+                                   (?s "session only" "Accept this certificate this session only.")
+                                   (?n "no" "Refuse to use this certificate, and close the connection.")
+                                   (?d "details" "See certificate details")))
+                 (details-choices '((?b "backward page" "See previous page")
+                                    (?f "forward page" "See next page")
+                                    (?n "next" "Next certificate")
+                                    (?p "previous" "Previous certificate")
+                                    (?q "quit" "Quit details view")))
+                 (answer (read-multiple-choice "Continue connecting?"
+                                               accept-choices))
+                 (show-details (char-equal (car answer) ?d))
+                 (pems (cl-loop for cert in certs
+                                collect (gnutls-format-certificate
+                                         (plist-get cert :pem))))
+                 (cert-index 0))
+            (while show-details
+              (unless (get-buffer-window cert-buffer)
+                (set-window-buffer (get-buffer-window buffer) cert-buffer)
+                (with-current-buffer cert-buffer
+                  (read-only-mode -1)
+                  (insert (nth cert-index pems))
+                  (goto-char (point-min))
+                  (read-only-mode)))
+
+              (setq answer (read-multiple-choice "Viewing certificate:" details-choices))
+
+              (cond
+               ((char-equal (car answer) ?q)
+                (setq show-details (not show-details))
+                (set-window-buffer (get-buffer-window cert-buffer) buffer)
+                (setq show-details (char-equal
+                                    (car (setq answer
+                                               (read-multiple-choice
+                                                "Continue connecting?"
+                                                accept-choices)))
+                                    ?d)))
+
+               ((char-equal (car answer) ?b)
+                (with-selected-window (get-buffer-window cert-buffer)
+                  (with-current-buffer cert-buffer
+                    (ignore-errors (scroll-down)))))
+
+               ((char-equal (car answer) ?f)
+                (with-selected-window (get-buffer-window cert-buffer)
+                  (with-current-buffer cert-buffer
+                    (ignore-errors (scroll-up)))))
+
+               ((char-equal (car answer) ?n)
+                (with-current-buffer cert-buffer
+                  (read-only-mode -1)
+                  (erase-buffer)
+                  (setq cert-index (mod (1+ cert-index) (length pems)))
+                  (insert (nth cert-index pems))
+                  (goto-char (point-min))
+                  (read-only-mode)))
+
+               ((char-equal (car answer) ?p)
+                (with-current-buffer cert-buffer
+                  (read-only-mode -1)
+                  (erase-buffer)
+                  (setq cert-index (mod (1- cert-index) (length pems)))
+                  (insert (nth cert-index pems))
+                  (goto-char (point-min))
+                  (read-only-mode)))))
+            (cadr answer))
+        (kill-buffer cert-buffer)
+        (kill-buffer buffer)))))
+
+(set-advertised-calling-convention 'nsm-query-user '(message status) "27.1")
+
+(defun nsm-save-host (host port status what problems permanency)
   (let* ((id (nsm-id host port))
-	 (saved
-	  (list :id id
-		:fingerprint (or (nsm-fingerprint status)
-				 ;; Plain connection.
-				 :none))))
+         (saved-fingerprints (plist-get (nsm-host-settings id) :fingerprints))
+         (fingerprints (cl-delete-duplicates
+                        (append saved-fingerprints
+                                (list (or (nsm-fingerprint status)
+                                          ;; Plain connection.
+                                          :none)))
+                        :test #'string=))
+         (saved (list :id id :fingerprints fingerprints)))
     (when (or (eq what 'conditions)
 	      nsm-save-host-names)
       (nconc saved (list :host (format "%s:%s" host port))))
@@ -403,20 +915,19 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
     ;; of the certificate/unencrypted connection.
     (cond
      ((eq what 'conditions)
-      (cond
-       ((not status)
-	(nconc saved '(:conditions (:unencrypted))))
-       ((plist-get status :warnings)
-	(nconc saved
-	       (list :conditions (plist-get status :warnings))))))
-     ((not (eq what 'fingerprint))
+      (plist-put saved :conditions problems))
+     ;; Make sure the conditions are not erased when we save a
+     ;; fingerprint
+     ((eq what 'fingerprint)
       ;; Store additional protocol settings.
       (let ((settings (nsm-host-settings id)))
-	(when settings
-	  (setq saved settings))
-	(if (plist-get saved :conditions)
-	    (nconc (plist-get saved :conditions) (list what))
-	  (nconc saved (list :conditions (list what)))))))
+        (when settings
+          (setq saved settings))
+        (if (plist-get saved :conditions)
+            (plist-put saved :conditions
+                       (cl-delete-duplicates
+                        (nconc (plist-get saved :conditions) problems)))
+          (plist-put saved :conditions problems)))))
     (if (eq permanency 'always)
 	(progn
 	  (nsm-remove-temporary-setting id)
@@ -426,6 +937,11 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
       (nsm-remove-temporary-setting id)
       (push saved nsm-temporary-host-settings))))
 
+(set-advertised-calling-convention
+ 'nsm-save-host
+ '(host port status what problems permanency)
+ "27.1")
+
 (defun nsm-write-settings ()
   (with-temp-file nsm-settings-file
     (insert "(\n")
@@ -483,44 +999,58 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
   (let ((cert (plist-get status :certificate)))
     (when cert
       (with-temp-buffer
-	(insert
-	 "Certificate information\n"
-	 "Issued by:"
-	 (nsm-certificate-part (plist-get cert :issuer) "CN" t) "\n"
-	 "Issued to:"
+        (insert
+	 (propertize "Certificate information" 'face 'underline) "\n"
+	 "  Issued by:"
+         (nsm-certificate-part (plist-get cert :issuer) "CN" t) "\n"
+	 "  Issued to:"
 	 (or (nsm-certificate-part (plist-get cert :subject) "O")
 	     (nsm-certificate-part (plist-get cert :subject) "OU" t))
-	 "\n"
-	 "Hostname:"
+         "\n"
+	 "  Hostname:"
 	 (nsm-certificate-part (plist-get cert :subject) "CN" t) "\n")
 	(when (and (plist-get cert :public-key-algorithm)
 		   (plist-get cert :signature-algorithm))
 	  (insert
-	   "Public key:" (plist-get cert :public-key-algorithm)
+	   "  Public key:" (plist-get cert :public-key-algorithm)
 	   ", signature: " (plist-get cert :signature-algorithm) "\n"))
-	(when (and (plist-get status :key-exchange)
+        (when (and (plist-get status :key-exchange)
 		   (plist-get status :cipher)
 		   (plist-get status :mac)
 		   (plist-get status :protocol))
 	  (insert
-	   "Protocol:" (plist-get status :protocol)
+	   "  Session:" (plist-get status :protocol)
 	   ", key: " (plist-get status :key-exchange)
 	   ", cipher: " (plist-get status :cipher)
 	   ", mac: " (plist-get status :mac) "\n"))
-	(when (plist-get cert :certificate-security-level)
+        (when (plist-get cert :certificate-security-level)
 	  (insert
-	   "Security level:"
+	   "  Security level:"
 	   (propertize (plist-get cert :certificate-security-level)
 		       'face 'bold)
 	   "\n"))
 	(insert
-	 "Valid:From " (plist-get cert :valid-from)
-	 " to " (plist-get cert :valid-to) "\n\n")
-	(goto-char (point-min))
+	 "  Valid:From " (plist-get cert :valid-from)
+	 " to " (plist-get cert :valid-to) "\n")
+        (insert "\n")
+        (goto-char (point-min))
 	(while (re-search-forward "^[^:]+:" nil t)
-	  (insert (make-string (- 20 (current-column)) ? )))
+	  (insert (make-string (- 22 (current-column)) ? )))
 	(buffer-string)))))
 
+(defun nsm-level (symbol)
+  "Return a numerical level for SYMBOL for easier comparison."
+  (cond
+   ((eq symbol 'low) 0)
+   ((eq symbol 'medium) 1)
+   (t 2)))
+
+(defun nsm-cipher-suite (status)
+  (format "%s-%s-%s"
+          (plist-get status :key-exchange)
+          (plist-get status :cipher)
+          (plist-get status :mac)))
+
 (defun nsm-certificate-part (string part &optional full)
   (let ((part (cadr (assoc part (nsm-parse-subject string)))))
     (cond
@@ -552,13 +1082,7 @@ HOST PORT STATUS OPTIONAL-PARAMETER.")
 	     elem)))
        (nreverse result)))))
 
-(defun nsm-level (symbol)
-  "Return a numerical level for SYMBOL for easier comparison."
-  (cond
-   ((eq symbol 'low) 0)
-   ((eq symbol 'medium) 1)
-   ((eq symbol 'high) 2)
-   (t 3)))
+(define-obsolete-function-alias 'nsm--encryption #'nsm-cipher-suite "27.1")
 
 (provide 'nsm)