countersign

(noun) a secret word or phrase known only to a restricted group. Synonyms: password, watchword, word, parole. Example: "He forgot the password."

(noun) a second confirming signature endorsing a document already signed. Synonyms: countersignature.

(verb) add one's signature to after another's to attest authenticity. Example: "You must countersign on this line of the contract."