consider going from unique or exporter to endpoint a downgrade

src/main/java/eu/siacs/conversations/crypto/sasl/ChannelBinding.java

@@ -117,4 +117,14 @@ public enum ChannelBinding {
                 throw new AssertionError("Missing short name for " + channelBinding);
         }
     }
+
+    public static int priority(final ChannelBinding channelBinding) {
+        if (Arrays.asList(TLS_EXPORTER,TLS_UNIQUE).contains(channelBinding)) {
+            return 2;
+        } else if (channelBinding == ChannelBinding.TLS_SERVER_END_POINT) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
 }

src/main/java/eu/siacs/conversations/crypto/sasl/ChannelBindingMechanism.java

@@ -97,4 +97,13 @@ public interface ChannelBindingMechanism {
         messageDigest.update(encodedCertificate);
         return messageDigest.digest();
     }
+
+    static int getPriority(final SaslMechanism mechanism) {
+        if (mechanism instanceof ChannelBindingMechanism) {
+            final ChannelBindingMechanism channelBindingMechanism = (ChannelBindingMechanism) mechanism;
+            return ChannelBinding.priority(channelBindingMechanism.getChannelBinding());
+        } else {
+            return 0;
+        }
+    }
 }

src/main/java/eu/siacs/conversations/crypto/sasl/ScramSha1Plus.java

@@ -27,7 +27,7 @@ public class ScramSha1Plus extends ScramPlusMechanism {
 
     @Override
     public int getPriority() {
-        return 35; // higher than SCRAM-SHA512 (30)
+        return 35 + ChannelBinding.priority(this.channelBinding); // higher than SCRAM-SHA512 (30)
     }
 
     @Override

src/main/java/eu/siacs/conversations/crypto/sasl/ScramSha256Plus.java

@@ -27,7 +27,7 @@ public class ScramSha256Plus extends ScramPlusMechanism {
 
     @Override
     public int getPriority() {
-        return 40;
+        return 40 + ChannelBinding.priority(this.channelBinding);
     }
 
     @Override

src/main/java/eu/siacs/conversations/crypto/sasl/ScramSha512Plus.java

@@ -27,7 +27,7 @@ public class ScramSha512Plus extends ScramPlusMechanism {
 
     @Override
     public int getPriority() {
-        return 45;
+        return 45 + ChannelBinding.priority(this.channelBinding);
     }
 
     @Override