code: func NewEnvelope(ttl uint32, topic TopicType, msg *sentMessage) *Envelope {
  	env := Envelope{
  		Expiry: uint32(time.Now().Add(time.Second * time.Duration(ttl)).Unix()),
  		TTL:    ttl,
  		Topic:  topic,
  		Data:   msg.Raw,
  		Nonce:  0,
  	}
  
  	return &env
  }
docs: // NewEnvelope wraps a Whisper message with expiration and destination data
  // included into an envelope for network forwarding.