code: public HashMap<Integer, byte[]> getPreKeys() {
          if (preKeys == null) {
              preKeys = new HashMap<>();
              for (int id : preKeysB64.keySet()) {
                  preKeys.put(id, Base64.decode(preKeysB64.get(id)));
              }
          }
          return this.preKeys;
      }
docs: Return the HashMap of preKeys in the bundle.
  The map uses the preKeys ids as key and the preKeys as value.
  
  @return preKeys