authenticate vs sign off on

authenticate

verb
  • To render authentic; to give authority to, by the proof, attestation, or formalities required by law, or sufficient to entitle to credit. 

  • To prove authentic; to determine as real and true. 

adj
  • Synonym of authentic. 

sign off on

How often have the words authenticate and sign off on occurred in a corpus of books? (source: Google Ngram Viewer )