Issue #1831 - Add an option to enable TLS 1.3 "compatibility" mode.

Critical note: this potentially reduces the strength of TLS 1.3 and
should only be enabled if absolutely necessary to access a site.
A browser restart is required for the pref change to take effect as it
is set on NSS initialization.

Resolves #1831

netwerk/base/security-prefs.js

@@ -112,6 +112,12 @@ pref("security.webauth.u2f_enable_usbtoken", false);
 // OCSP must-staple
 pref("security.ssl.enable_ocsp_must_staple", true);
 
+// Enable TLS 1.3 compatmode version for bad middleware boxes?
+// This is a holdover from the later draft specs and SHOULD NOT be enabled by
+// default. ONLY use this when you explicitly need it. You have been warned!
+// Restart required.
+pref("security.ssl.enable_tls13_compat_mode", false);
+
 // If a request is mixed-content, send an HSTS priming request to attempt to
 // see if it is available over HTTPS.
 pref("security.mixed_content.send_hsts_priming", true);

security/manager/ssl/nsNSSComponent.cpp

@@ -1443,6 +1443,7 @@ static const bool FALSE_START_ENABLED_DEFAULT = true;
 static const bool NPN_ENABLED_DEFAULT = true;
 static const bool ALPN_ENABLED_DEFAULT = false;
 static const bool ENABLED_0RTT_DATA_DEFAULT = false;
+static const bool TLS13_COMPAT_MODE_DEFAULT = false;
 
 static void
 ConfigureTLSSessionIdentifiers()
@@ -1876,6 +1877,11 @@ nsNSSComponent::InitializeNSS()
                        Preferences::GetBool("security.tls.enable_0rtt_data",
                                             ENABLED_0RTT_DATA_DEFAULT));
 
+  // Set TLS 1.3 compatibility mode for bad middleware boxes?
+  SSL_OptionSetDefault(SSL_ENABLE_TLS13_COMPAT_MODE,
+                       Preferences::GetBool("security.ssl.enable_tls13_compat_mode",
+                                            TLS13_COMPAT_MODE_DEFAULT));  
+  
   if (NS_FAILED(InitializeCipherSuite())) {
 
     MOZ_LOG(gPIPNSSLog, LogLevel::Error, ("Unable to initialize cipher suite settings\n"));