...
Delivery is performed by SIP over TLS protocol, so as you can see the whole end-to-end path is encrypted and as the message is stored after a local encryption on the server, then there's (almost) no way of deciphering it.
Send Secure Messages
It's possible to write Secure Messages to other parties both by answering them or by writing for your own initiative. In the first case you would probably use the "History" view, in the latter you would use the "Contacts" list instead.
...
- You cannot attach anything to a Secure Message: Secure Messages are actually text only. Binary Messages that could use attachments might come in next releases.
- Secure Message lenght is fixed to a maximum of 160 characters. Nowadays there's no way to create longer messages. If you need to write more you have to split your text in more messages by yourself.
- Each Secure Message has a time validity, after which it expires. Expiration means that the Secure Message content is no longer readable and it's gone for good. Instead of message content you'd rather read "Message received" or "Message sent".
...