Implement PKCE

[Ross65536]

Implement PKCE for authorization code grant type, as per RFC-7637:

• add string fields code_challenge and code_challenge_method to oauth_access_grants table which will contain the PKCE information. Add instructions for upgrading.
• add config option use_pkce. If true when issuing grants the code_challenge and code_challenge_method query fields are mandatory and are saved to the grant model. If false these fields are ignored and the corresponding grant models are set to nil
• when emitting an access token, the grant model is checking for having the code_challenge_method field set. If set to a value not nil then the code_verifier query field is mandatory and it's used to check against the grant's code_challenge field. If set to nil this query parameter is ignored and the grant acts as though PKCE is disabled.

Partially inspired by #61

[dagumak]

Aww man! I was going to implement this, but you beat me to it.

[dagumak]

I think it would be a good idea to add a test with the examples in Appendix A and Appendix B.

[Ross65536]

Appendix B

I'll see If I have time later for this. But feel free to contribute with the tests if you need it right away :)

[dagumak]

Appendix B

I'll see If I have time later for this. But feel free to contribute with the tests if you need it right away :)

I'm still new to Elixir, but I may give it a try later :)

[lukyanov]

Waiting for this to be merged to upstream. A very good PR!

Just one note. use_pkce: true forces PKCE for all applications. There are use cases when you want to enable it for some applications only. For example, I want to force it for javascript apps, but don't for server-side apps. What if instead of using the global config parameter we do these things instead:

1. If code_challenge is present in the authorization request, we assume the client wants PKCE check and enable it.
2. We add a field to the oauth_applications table something like require_pkce, and if it's true, we fail the authorization if the parameters are not present.

What do you think?

[lukyanov]

@Ross65536 Made a PR to your repo :)
Ross65536#1