crypto/tls: add PSK support

[gopherbot]

by tiebingzhang:

RFC 4279 (http://tools.ietf.org/html/rfc4279#page-10) added PSK to TLS.
OpenSSL and GnuTLS already have support for it.

The RFC defines three additional key exchange algorithms:
PSK
RSA-PSK
DHE-PSK

It would be nice to add at least PSK and DHE-PSK to GO's crypto/tls package. The work
seems to be reasonable size.

According to Wikipedia
(http://en.wikipedia.org/wiki/Comparison_of_TLS_implementations#Key_Exchange_Algorithms_.28Alternative_key-exchanges.29),
RSA-PSK has not been implemented by any of the listed implementations, so it is maybe
okay to push that one off for later.

[rsc]

Comment 1:

For now at least, very low priority.

Labels changed: added priority-someday, removed priority-triage.

Status changed to Accepted.

[rsc]

Comment 2:

Labels changed: added go1.3maybe.

[rsc]

Comment 3:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 4:

Labels changed: added repo-main.

[tchap]

Might be a good idea to revisit this. I am interested in IoT and some devices simply cannot do regular TLS. Having a solid implementation of TLS-PSK would enable people to use Go to implement a server communicating with a swarm of IoT devices. I am not so much into crypto to be able to implement this myself. Basically just saying that IoT is on hype today and the last comment in this thread is more than 3 years old. For someone knowing crypto and crypto/tls it seems that this would not be too much work...