code: func (v SecretsVerifier) Ensure() error {
  	computed := v.hmac.Sum(nil)
  	// use hmac.Equal prevent leaking timing information.
  	if hmac.Equal(computed, v.signature) {
  		return nil
  	}
  
  	return fmt.Errorf("Expected signing signature: %s, but computed: %s", hex.EncodeToString(v.signature), hex.EncodeToString(computed))
  }
docs: // Ensure compares the signature sent from Slack with the actual computed hash to judge validity