use less entropy in SASL2 device id

src/main/java/eu/siacs/conversations/utils/AccountUtils.java

@@ -8,6 +8,7 @@ import android.widget.Toast;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.UUID;
 
 import eu.siacs.conversations.Config;
 import eu.siacs.conversations.R;
@@ -34,6 +35,23 @@ public class AccountUtils {
         return false;
     }
 
+    public static String publicDeviceId(final Account account) {
+        final UUID uuid;
+        try {
+            uuid = UUID.fromString(account.getUuid());
+        } catch (final IllegalArgumentException e) {
+            return account.getUuid();
+        }
+        final UUID publicDeviceId = getUuid(uuid.getLeastSignificantBits(), uuid.getLeastSignificantBits());
+        return publicDeviceId.toString();
+    }
+
+    protected static UUID getUuid(final long msb, final long lsb) {
+        final long msb0 = (msb & 0xffffffffffff0fffL) | 4; // set version
+        final long lsb0 = (lsb & 0x3fffffffffffffffL) | 0x8000000000000000L; // set variant
+        return new UUID(msb0, lsb0);
+    }
+
     public static List<String> getEnabledAccounts(final XmppConnectionService service) {
         ArrayList<String> accounts = new ArrayList<>();
         for (Account account : service.getAccounts()) {

src/main/java/eu/siacs/conversations/xmpp/XmppConnection.java

@@ -77,6 +77,7 @@ import eu.siacs.conversations.services.MemorizingTrustManager;
 import eu.siacs.conversations.services.MessageArchiveService;
 import eu.siacs.conversations.services.NotificationService;
 import eu.siacs.conversations.services.XmppConnectionService;
+import eu.siacs.conversations.utils.AccountUtils;
 import eu.siacs.conversations.utils.CryptoHelper;
 import eu.siacs.conversations.utils.Patterns;
 import eu.siacs.conversations.utils.PhoneHelper;
@@ -1534,7 +1535,7 @@ public class XmppConnection implements Runnable {
             authenticate.addChild("initial-response").setContent(firstMessage);
         }
         final Element userAgent = authenticate.addChild("user-agent");
-        userAgent.setAttribute("id", account.getUuid());
+        userAgent.setAttribute("id", AccountUtils.publicDeviceId(account));
         userAgent
                 .addChild("software")
                 .setContent(mXmppConnectionService.getString(R.string.app_name));