Support DTLS encryption about coap-rs

[StEvUgnIn]

I started working on the implementation based on the latter example. I would like @Covertness to assign this issue to me before I present my PR.

[Covertness]

Thank you for your advice. I have the similar idea before. Unfortunately, I haven't enough time to implement this feature.
Of cource , any PR about the feature are welcome.

[Covertness]

DTLS written in Rust seems not available. I found two issues about this:
rustls/rustls#40
sfackler/rust-openssl#528
Waiting for the implement.

[StEvUgnIn]

I thought Core CoAP Standard included DTLS 1.2 .

[StEvUgnIn]

I found this code based on tokio-openssl which includes DTLS encryption:
https://github.com/simmons/tokio-dtls-example/blob/master/src/main.rs

Edit:
There are multiple implementations of rust-openssl for using sslMethod::dtls()
I suggest that we take inspiration from this clean example https://github.com/JRF63/DTLS-Example/blob/master/src/main.rs

[StEvUgnIn]

I am working on now after a family member of mine had health issue. Have you tried this project? https://github.com/google/rust-async-coap

[Covertness]

sorry for hear that. I noticed it before. It seems only implement some basic functions and haven't contribute for a long time

[fdeantoni]

Hi guys! Has anybody been able to make progress on this? I looked at this a couple of years ago implementing DTLS with rust-async-coap (see async-coap-dtls) but my very crude attempt is quite clunky. DTLS 1.3 should make things a lot simpler but this is still a draft if I'm not mistaken...

[Covertness]

I think it's not easy to implement. I would appreciate if you could finish it.

[StEvUgnIn]

@fdeantoni
Feel free to continue if you are interested.

[Covertness]

#88