Hey there! I’ve run into this exact snag before when building a chat app with Smack and OMEMO. The core issue here is that OMEMO works fundamentally differently for group chats (MUCs) vs. one-on-one conversations—since groups have multiple participants (each with potentially multiple devices), you can’t reuse the same fingerprint logic you use for single chats. Let’s break down how to fix this step by step:

Why It’s Failing for MUCs #

In one-on-one chats, you’re dealing with a single remote JID and their associated devices. For MUCs, every participant in the group has their own OMEMO device(s), and fingerprints are tied to individual devices—not just the user’s JID. If you’re trying to generate a single "group fingerprint," that’s not how OMEMO operates—you need to fetch fingerprints for each participant’s devices separately.

Step-by-Step Solution #

1. Fetch Active MUC Participants #

First, make sure you’re correctly pulling the list of current occupants in the MUC using Smack’s MultiUserChat class:

// Get your MUC instance
MultiUserChat muc = MultiUserChatManager.getInstanceFor(connection).getMultiUserChat(mucJid);
// Fetch all current group participants
List<Occupant> occupants = muc.getOccupants();

2. Discover OMEMO Devices for Each Participant #

OMEMO requires you to discover what devices each participant is using before you can retrieve their fingerprints. Use Smack’s OmemoManager to trigger device discovery (you can do this upfront or lazily as needed):

OmemoManager omemoManager = OmemoManager.getInstanceFor(connection, yourOwnJid);

for (Occupant occupant : occupants) {
    BareJid participantBareJid = occupant.getJid().asBareJid();
    // Skip your own account to avoid unnecessary work
    if (participantBareJid.equals(yourOwnJid.asBareJid())) continue;
    
    // Trigger device discovery for the participant
    omemoManager.discoverDevices(participantBareJid);
}

3. Generate Fingerprints for Each Device #

Once you have the device list for a participant, fetch the fingerprint for each individual device (each device has its own unique fingerprint):

for (Occupant occupant : occupants) {
    BareJid participantBareJid = occupant.getJid().asBareJid();
    if (participantBareJid.equals(yourOwnJid.asBareJid())) continue;
    
    // Get all OMEMO devices registered to this participant
    Set<Integer> participantDevices = omemoManager.getDevices(participantBareJid);
    
    for (Integer deviceId : participantDevices) {
        // Generate the human-readable fingerprint for this device
        String fingerprint = omemoManager.getFingerprint(participantBareJid, deviceId);
        
        // Use the fingerprint (e.g., display it in your UI with the participant's name and device ID)
        Log.d("OMEMO MUC", String.format("%s (Device %d): %s", participantBareJid, deviceId, fingerprint));
    }
}

4. Handle Edge Cases #

• Participants without OMEMO: If getDevices() returns an empty set, that participant isn’t using OMEMO—you can skip fingerprint generation for them.
• Async Updates: Device discovery is asynchronous, so listen for OmemoDeviceListener events to refresh fingerprints when a participant’s device list changes:

  omemoManager.addOmemoDeviceListener(new OmemoDeviceListener() {
      @Override
      public void onOmemoDevicesUpdated(OmemoManager manager, BareJid jid, Set<Integer> devices) {
          // Refresh fingerprints for this participant when their devices change
      }
  });
  

Key Takeaway #

OMEMO MUC fingerprints aren’t a single group-wide value—they’re per-participant, per-device. By adjusting your logic to iterate through each group member and their individual devices, you’ll be able to generate and display fingerprints just like you do for one-on-one chats.

内容的提问来源于stack exchange，提问作者Jason Jack