Implement RSASSA-PSS (PS) signing and verifying

[csprl]

This would be useful for https://github.com/brianloveswords/node-jws/issues/47

[omsmith]

Hi @csprl. Thanks for the contribution.

Would it be possible for you to add some tests to the test suite for this?

[csprl]

Hey, I added tests (based on the existing ones for "regular" RSA). It does however seem like support for PSS padding wasn't added until Node.js v6. Function arguments for Verify.verify() were also changed in v7, but back again to the v6 behaviour in v8 which is why the v7 build is failing. Considering these Node.js versions aren't officially supported anymore it might make sense to drop support for them.

[omsmith]

Perfect thanks. I'll update the support matrix on travis later today. I'm still going to keep the old old versions, as I don't want to do a major bump at the moment. I think what we can do is

• add this as semver-minor
• check the node version in tests (I think we're doing this elsewhere already)
• add a note to the readme that PS* is Node 6+

[csprl]

Sounds good. I added checks for Node version and pinpointed 6.12.0 to be the exact release when PSS support was added. I also added a notice in README.md.

[omsmith]

That's awesome, thanks for being so responsive to feedback and sorry that I took a while to hop on this!

I'll aim to get this out tonight, but I realized that https://github.com/brianloveswords/node-jwa/pull/26 may have a couple issues with https://github.com/crypto-browserify/browserify-sign that I'll want to address first (perhaps by just reverting my change for now in order to get yours out).

[csprl]

No worries at all! You absolutely don't have to rush this change, but getting PS support in node-jws in the near future would be great.

[nikolaymatrosov]

It would be great if this PR was merged, so all dependent packages obtain the ability to use PS* algorithms.