PKCE implementation (#1784)

* Basic implementation of PKCE

Signed-off-by: Tadeusz Magura-Witkowski <tadeuszmw@gmail.com>

* @mfmarche on 24 Feb: when code_verifier is set, don't check client_secret

In PKCE flow, no client_secret is used, so the check for a valid client_secret
would always fail.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* @deric on 16 Jun: return invalid_grant when wrong code_verifier

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Enforce PKCE flow on /token when PKCE flow was started on /auth
Also dissallow PKCE on /token, when PKCE flow was not started on /auth

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* fixed error messages when mixed PKCE/no PKCE flow.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* server_test.go: Added PKCE error cases on /token endpoint

* Added test for invalid_grant, when wrong code_verifier is sent
* Added test for mixed PKCE / no PKCE auth flows.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* cleanup: extracted method checkErrorResponse and type TestDefinition

* fixed connector being overwritten

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* /token endpoint: skip client_secret verification only for grand type authorization_code with PKCE extension

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Allow "Authorization" header in CORS handlers

* Adds "Authorization" to the default CORS headers{"Accept", "Accept-Language", "Content-Language", "Origin"}

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Add "code_challenge_methods_supported" to discovery endpoint

discovery endpoint /dex/.well-known/openid-configuration
now has the following entry:

"code_challenge_methods_supported": [
  "S256",
  "plain"
]

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Updated tests (mixed-up comments), added a PKCE test

* @asoorm added test that checks if downgrade to "plain" on /token endpoint

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* remove redefinition of providedCodeVerifier, fixed spelling (#6)

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>
Signed-off-by: Bernd Eckstein <HEllRZA@users.noreply.github.com>

* Rename struct CodeChallenge to PKCE

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* PKCE: Check clientSecret when available

In authorization_code flow with PKCE, allow empty client_secret on /auth and /token endpoints. But check the client_secret when it is given.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Enable PKCE with public: true

dex configuration public on staticClients now enables the following behavior in PKCE:
- Public: false, PKCE will always check client_secret. This means PKCE in it's natural form is disabled.
- Public: true, PKCE is enabled. It will only check client_secret if the client has sent one. But it allows the code flow if the client didn't sent one.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Redirect error on unsupported code_challenge_method

- Check for unsupported code_challenge_method after redirect uri is validated, and use newErr() to return the error.
- Add PKCE tests to oauth2_test.go

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Reverted go.mod and go.sum to the state of master

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Don't omit client secret check for PKCE

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Allow public clients (e.g. with PKCE) to have redirect URIs configured

Signed-off-by: Martin Heide <martin.heide@faro.com>

* Remove "Authorization" as Accepted Headers on CORS, small fixes

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Revert "Allow public clients (e.g. with PKCE) to have redirect URIs configured"

This reverts commit b6e297b78537dc44cd3e1374f0b4d34bf89404ac.

Signed-off-by: Martin Heide <martin.heide@faro.com>

* PKCE on client_secret client error message

* When connecting to the token endpoint with PKCE without client_secret, but the client is configured with a client_secret, generate a special error message.

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* Output info message when PKCE without client_secret used on confidential client

* removes the special error message

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

* General missing/invalid client_secret message on token endpoint

Signed-off-by: Bernd Eckstein <Bernd.Eckstein@faro.com>

Co-authored-by: Tadeusz Magura-Witkowski <tadeuszmw@gmail.com>
Co-authored-by: Martin Heide <martin.heide@faro.com>
Co-authored-by: M. Heide <66078329+heidemn-faro@users.noreply.github.com>