noun (n)

• (law) a person who attests to the genuineness of a document or signature by adding their own signature(noun.person)

  Synonym:
  attestator, attestor, witness
  

  source: wordnet30
• someone who affirms or vouches for the correctness or truth or genuineness of something(noun.person)

  Synonym:
  attester
  

  source: wordnet30