code: func validateSignatureHeader(sHdr *common.SignatureHeader) error {
  	// check for nil argument
  	if sHdr == nil {
  		return errors.New("nil SignatureHeader provided")
  	}
  
  	// ensure that there is a nonce
  	if sHdr.Nonce == nil || len(sHdr.Nonce) == 0 {
  		return errors.New("invalid nonce specified in the header")
  	}
  
  	// ensure that there is a creator
  	if sHdr.Creator == nil || len(sHdr.Creator) == 0 {
  		return errors.New("invalid creator specified in the header")
  	}
  
  	return nil
  }
docs: // checks for a valid SignatureHeader