private Collection<String> getCutSet(Collection<String> networkMembers) {
		synchronized(keychain) {
			ArrayList<String> cutSet = new ArrayList<String>();
			// networkMembers might be bigger than keychain.keySet()
			// but lookups are faster in HashMaps than in Collections.
			for(String s: networkMembers) {
				if(keychain.containsKey(s)) {
					cutSet.add(s);
				}
			}
			return cutSet;
		}
	}