countersign
     n 1: a secret word or phrase known only to a restricted group;
          "he forgot the password" [syn: {password}, {watchword},
          {word}, {parole}]
     2: a second confirming signature endorsing a document already
        signed [syn: {countersignature}]
     v : add one's signature to after another's to attest
         authenticity; "You must countersign on this line of the
         contract"
Source: WordNet® 2.0