counter-sign

All persons or objects that answer to the same counter-sign or that bear the same imprint are thereby stamped as somehow related.

But during the recent conversation he had forgotten for the moment the "counter sign," and continued calling back.

Youve been away so long that you must have forgotten the counter-sign.

Short as was the distance, they were challenged twice, and the sign and counter-sign had to be exchanged.

In the second fable the fox leaves her little ones at home, bidding them admit no one without a counter-sign.