code: def snapshot_peek_sigb64( fd, off, bytelen ):
      
      fd.seek( off - bytelen, os.SEEK_SET )
      sigb64 = fd.read(bytelen)
      if len(sigb64) != bytelen:
          return None
  
      try:
          base64.b64decode(sigb64)
      except:
          return None
  
      return sigb64
docs: Read the last :bytelen bytes of
      fd and interpret it as a base64-encoded
      string