countersign

lawA second signature added to a document to affirm the validity of the signature of the first person.

The response to a sign or signal.

“Only those who give the countersign may enter the Masonic Lodge.”

A secret word or phrase known only to a restricted group

“he forgot the password”

A second confirming signature endorsing a document already signed

To sign on the opposite side of (a document).

To add a second signature to a document, affirming the validity of the signature of another person.

Add one's signature to after another's to attest authenticity

“You must countersign on this line of the contract”