[sasl] order mechanisms to prefer the most secure

The previous logic was:

- use the preferred SASL mechanism if available
- otherwise use the first supported mechanism offered by the server

However RFC 6120, section 6.3.3 states:

"The initiating entity MUST maintain its own preference order independent
of the preference order of the receiving entity."

The new logic is:

- order our supported mechanisms from most secure to least secure
- if the user sets QXmppConfiguration::saslMechanism, put it first
- use the best mechanism supported by the server

1 files changed, 2 insertions, 4 deletions

diff --git a/src/client/QXmppConfiguration.cpp b/src/client/QXmppConfiguration.cpp
index 817e8ee6..9b56b7c5 100644
--- a/src/client/QXmppConfiguration.cpp
+++ b/src/client/QXmppConfiguration.cpp
@@ -90,7 +90,6 @@ QXmppConfigurationPrivate::QXmppConfigurationPrivate()
     , ignoreSslErrors(false)
     , streamSecurityMode(QXmppConfiguration::TLSEnabled)
     , nonSASLAuthMechanism(QXmppConfiguration::NonSASLDigest)
-    , saslAuthMechanism("DIGEST-MD5")
 {
 }
 
@@ -501,8 +500,6 @@ void QXmppConfiguration::setNonSASLAuthMechanism(
 }
 
 /// Returns the preferred SASL authentication mechanism.
-///
-/// Default value: "DIGEST-MD5"
 
 QString QXmppConfiguration::saslAuthMechanism() const
 {
@@ -511,7 +508,8 @@ QString QXmppConfiguration::saslAuthMechanism() const
 
 /// Sets the preferred SASL authentication \a mechanism.
 ///
-/// Valid values: "PLAIN", "DIGEST-MD5", "ANONYMOUS", "X-FACEBOOK-PLATFORM"
+/// Valid values: "SCRAM-SHA-256", "SCRAM-SHA-1", "DIGEST-MD5", "PLAIN", "ANONYMOUS",
+//                "X-FACEBOOK-PLATFORM", "X-MESSENGER-OAUTH2", "X-OAUTH2"
 
 void QXmppConfiguration::setSaslAuthMechanism(const QString &mechanism)
 {