countersign

Noun

• a second confirming signature endorsing a document already signed

• a secret word or phrase known only to a restricted group
  • he forgot the password
    

Verb

• add one's signature to after another's to attest authenticity
  • You must countersign on this line of the contract
    

Related words

• Synonyms: 
  countersignature , password , watchword , word , parole