Add PKCE verification support

[skycocker]

This is pretty much a 1:1 copy of https://github.com/omniauth/omniauth_openid_connect/pull/80/commits with addressed PR suggestions and removed unused code.

You may want to review it with ?w=1 appended to the URL - it'll make reading much easier.

Also, there's one small improvement to the original PR: https://github.com/omniauth/omniauth_openid_connect/pull/128/files#diff-ee829bbbbd9d35ee93126d96347fffbfa8756868001c1d4ad6aff9c4650e8151R251

This allows providing a parameter in case one does not use sessions.

[stanhu]

Looks good to me. Thanks!

[stanhu]

@omniauth/omniauth-oidc Can anyone else review these changes?

[danjay]

lgtm too. I'll close my PR now the improvements have been implemented in this one.

[skycocker]

@BobbyMcWho @pboling ping ping

this should introduce no changes to people that already have been using #80, except they would now be able to use a version from the master upstream

btw - what do you think about reducing the number of required reviews before workflows are enabled?

[BobbyMcWho]

I unfortunately don't know much about this strategy, I'll have to find some time to get up to speed on what this proposes

[skycocker]

I unfortunately don't know much about this strategy, I'll have to find some time to get up to speed on what this proposes

here's a good, simple explanation: https://doorkeeper.gitbook.io/guides/ruby-on-rails/pkce-flow

here's less simplified, but still a good one: https://developer.okta.com/docs/guides/implement-grant-type/authcodepkce/main/#about-the-authorization-code-grant-with-pkce

Not supporting PKCE these days excludes you from a lot of OIDC providers.

[pboling]

LGTM

[BobbyMcWho]

I'm going to merge this, but I don't manage releases for this gem, @stanhu looks like the last one who pushed this to rubygems, so I'll defer to them to release

[skycocker]

Thank youuu! Before the release, we may want to merge #130 and #127, so the complete support of the PKCE flow with optional checks is added as a feature.

[stanhu]

v0.5.0 has been released.