Allow exceptions for expired certificates

The browsers allow users to make an exception for expired certificates,
so we should probably also.

common/rfb/CSecurityTLS.cxx

@@ -309,7 +309,8 @@ void CSecurityTLS::checkSession()
 {
   const unsigned allowed_errors = GNUTLS_CERT_INVALID |
 				  GNUTLS_CERT_SIGNER_NOT_FOUND |
-				  GNUTLS_CERT_SIGNER_NOT_CA;
+				  GNUTLS_CERT_SIGNER_NOT_CA |
+				  GNUTLS_CERT_EXPIRED;
   unsigned int status;
   const gnutls_datum_t *cert_list;
   unsigned int cert_list_size = 0;
@@ -381,6 +382,8 @@ void CSecurityTLS::checkSession()
     vlog.debug("server cert signer not found");
   if (status & GNUTLS_CERT_SIGNER_NOT_CA)
     vlog.debug("server cert signer not CA");
+  if (status & GNUTLS_CERT_EXPIRED)
+    vlog.debug("server certificate has expired");
 
   if (status & GNUTLS_CERT_INSECURE_ALGORITHM)
     throw AuthFailureException("The server certificate uses an insecure algorithm");
@@ -435,33 +438,65 @@ void CSecurityTLS::checkSession()
     vlog.debug("Server host not previously known");
     vlog.debug("%s", info.data);
 
-    text.format("This certificate has been signed by an unknown "
-                "authority:\n\n%s\n\nSomeone could be trying to "
-                "impersonate the site and you should not "
-                "continue.\n\nDo you want to make an exception "
-                "for this server?", info.data);
+    if (status & (GNUTLS_CERT_SIGNER_NOT_FOUND |
+                  GNUTLS_CERT_SIGNER_NOT_CA)) {
+      text.format("This certificate has been signed by an unknown "
+                  "authority:\n\n%s\n\nSomeone could be trying to "
+                  "impersonate the site and you should not "
+                  "continue.\n\nDo you want to make an exception "
+                  "for this server?", info.data);
+
+      if (!msg->showMsgBox(UserMsgBox::M_YESNO,
+                           "Unknown certificate issuer",
+                           text.buf))
+        throw AuthFailureException("Unknown certificate issuer");
+    }
+
+    if (status & GNUTLS_CERT_EXPIRED) {
+      text.format("This certificate has expired:\n\n%s\n\nSomeone "
+                  "could be trying to impersonate the site and you "
+                  "should not continue.\n\nDo you want to make an "
+                  "exception for this server?", info.data);
 
-    if (!msg->showMsgBox(UserMsgBox::M_YESNO,
-                         "Unknown certificate issuer",
-                         text.buf))
-      throw AuthFailureException("Unknown certificate issuer");
+      if (!msg->showMsgBox(UserMsgBox::M_YESNO,
+                           "Expired certificate",
+                           text.buf))
+        throw AuthFailureException("Expired certificate");
+    }
   } else if (err == GNUTLS_E_CERTIFICATE_KEY_MISMATCH) {
     CharArray text;
 
     vlog.debug("Server host key mismatch");
     vlog.debug("%s", info.data);
 
-    text.format("This host is previously known with a different "
-                "certificate, and the new certificate has been "
-                "signed by an unknown authority:\n\n%s\n\nSomeone "
-                "could be trying to impersonate the site and you "
-                "should not continue.\n\nDo you want to make an "
-                "exception for this server?", info.data);
+    if (status & (GNUTLS_CERT_SIGNER_NOT_FOUND |
+                  GNUTLS_CERT_SIGNER_NOT_CA)) {
+      text.format("This host is previously known with a different "
+                  "certificate, and the new certificate has been "
+                  "signed by an unknown authority:\n\n%s\n\nSomeone "
+                  "could be trying to impersonate the site and you "
+                  "should not continue.\n\nDo you want to make an "
+                  "exception for this server?", info.data);
+
+      if (!msg->showMsgBox(UserMsgBox::M_YESNO,
+                           "Unexpected server certificate",
+                           text.buf))
+        throw AuthFailureException("Unexpected server certificate");
+    }
 
-    if (!msg->showMsgBox(UserMsgBox::M_YESNO,
-                         "Unexpected server certificate",
-                         text.buf))
-      throw AuthFailureException("Unexpected server certificate");
+    if (status & GNUTLS_CERT_EXPIRED) {
+      text.format("This host is previously known with a different "
+                  "certificate, and the new certificate has expired:"
+                  "\n\n%s\n\nSomeone could be trying to impersonate "
+                  "the site and you should not continue.\n\nDo you "
+                  "want to make an exception for this server?",
+                  info.data);
+
+      if (!msg->showMsgBox(UserMsgBox::M_YESNO,
+                           "Unexpected server certificate",
+                           text.buf))
+        throw AuthFailureException("Unexpected server certificate");
+    }
   }
 
   if (gnutls_store_pubkey(dbPath.buf, NULL, client->getServerName(),