docs: function (prf, plaintext, iv, adata, tlen) {
      var out, data = plaintext.slice(0), w=sjcl.bitArray;
      tlen = tlen || 128;
      adata = adata || [];
  
      // encrypt and tag
      out = sjcl.mode.gcm._ctrMode(true, prf, data, adata, iv, tlen);
  
      return w.concat(out.data, out.tag);
    }