It's SSL, not SLS.
If I'm not wrong, I don't think it's possible. Either the person
sign on to the server or you sign off the server to message him.
The way PKI works requires a public and private key, of which is
three-way. It's not a one-way communication thing.
I won't go into the technical aspects, but since you ask, you
should know why it's 3-way. Basically, it goes this way:
1. Client ask for an SSL-secured website
2. Website sends back the public key
3. Client checks the public key, if OK, continue
Secure communications data packets are encrypted, and this applies
to files as well. As per usual, no one-way communication. Either
both are communicating via secured communications or no.
Yes, the data is still encrypted. However, once it leaves the
server, there's no guarantee that it will still be encrypted.
That means, even though you are connected securely to a server,
your files are not. Once it reaches the other person, there's also
no guarantee that he keeps the files encrypted as well.