countersign

nouns

a secret word or phrase known only to a restricted group
"he forgot the password"
countersign, parole, password, watchword, word
a second confirming signature endorsing a document already signed
countersign, countersignature

verbs

add one's signature to after another's to attest authenticity
"You must countersign on this line of the contract"
countersign
is derived from
countersignature
is a type of (narrower)
be engaged by a written agreement
"He signed to play the casino on Dec. 18"; "The soprano signed to sing the new opera"
sign