feat(op): implemented support for client_credentials grant (#172)

* implemented support for client_credentials grant

* first draft

* Update pkg/op/token_client_credentials.go

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

* updated placeholder interface name

* updated import paths

* ran mockgen

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

pkg/op/token_client_credentials.go

@@ -0,0 +1,115 @@
+package op
+
+import (
+	"context"
+	"net/http"
+	"net/url"
+
+	httphelper "github.com/zitadel/oidc/pkg/http"
+	"github.com/zitadel/oidc/pkg/oidc"
+)
+
+//ClientCredentialsExchange handles the OAuth 2.0 client_credentials grant, including
+//parsing, validating, authorizing the client and finally returning a token
+func ClientCredentialsExchange(w http.ResponseWriter, r *http.Request, exchanger Exchanger) {
+	request, err := ParseClientCredentialsRequest(r, exchanger.Decoder())
+	if err != nil {
+		RequestError(w, r, err)
+	}
+
+	validatedRequest, client, err := ValidateClientCredentialsRequest(r.Context(), request, exchanger)
+	if err != nil {
+		RequestError(w, r, err)
+		return
+	}
+
+	resp, err := CreateClientCredentialsTokenResponse(r.Context(), validatedRequest, exchanger, client)
+	if err != nil {
+		RequestError(w, r, err)
+		return
+	}
+
+	httphelper.MarshalJSON(w, resp)
+}
+
+//ParseClientCredentialsRequest parsed the http request into a oidc.ClientCredentialsRequest
+func ParseClientCredentialsRequest(r *http.Request, decoder httphelper.Decoder) (*oidc.ClientCredentialsRequest, error) {
+	err := r.ParseForm()
+	if err != nil {
+		return nil, oidc.ErrInvalidRequest().WithDescription("error parsing form").WithParent(err)
+	}
+
+	request := new(oidc.ClientCredentialsRequest)
+	err = decoder.Decode(request, r.Form)
+	if err != nil {
+		return nil, oidc.ErrInvalidRequest().WithDescription("error decoding form").WithParent(err)
+	}
+
+	if clientID, clientSecret, ok := r.BasicAuth(); ok {
+		clientID, err = url.QueryUnescape(clientID)
+		if err != nil {
+			return nil, oidc.ErrInvalidClient().WithDescription("invalid basic auth header").WithParent(err)
+		}
+
+		clientSecret, err = url.QueryUnescape(clientSecret)
+		if err != nil {
+			return nil, oidc.ErrInvalidClient().WithDescription("invalid basic auth header").WithParent(err)
+		}
+
+		request.ClientID = clientID
+		request.ClientSecret = clientSecret
+	}
+
+	return request, nil
+}
+
+//ValidateClientCredentialsRequest validates the refresh_token request parameters including authorization check of the client
+//and returns the data representing the original auth request corresponding to the refresh_token
+func ValidateClientCredentialsRequest(ctx context.Context, request *oidc.ClientCredentialsRequest, exchanger Exchanger) (TokenRequest, Client, error) {
+	storage, ok := exchanger.Storage().(ClientCredentialsStorage)
+	if !ok {
+		return nil, nil, oidc.ErrUnsupportedGrantType().WithDescription("client_credentials grant not supported")
+	}
+
+	client, err := AuthorizeClientCredentialsClient(ctx, request, exchanger)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	tokenRequest, err := storage.ClientCredentialsTokenRequest(ctx, request.ClientID, request.Scope)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return tokenRequest, client, nil
+}
+
+func AuthorizeClientCredentialsClient(ctx context.Context, request *oidc.ClientCredentialsRequest, exchanger Exchanger) (Client, error) {
+	if err := AuthorizeClientIDSecret(ctx, request.ClientID, request.ClientSecret, exchanger.Storage()); err != nil {
+		return nil, err
+	}
+
+	client, err := exchanger.Storage().GetClientByClientID(ctx, request.ClientID)
+	if err != nil {
+		return nil, oidc.ErrInvalidClient().WithParent(err)
+	}
+
+	if !ValidateGrantType(client, oidc.GrantTypeClientCredentials) {
+		return nil, oidc.ErrUnauthorizedClient()
+	}
+
+	return client, nil
+}
+
+func CreateClientCredentialsTokenResponse(ctx context.Context, tokenRequest TokenRequest, creator TokenCreator, client Client) (*oidc.AccessTokenResponse, error) {
+	accessToken, _, validity, err := CreateAccessToken(ctx, tokenRequest, AccessTokenTypeJWT, creator, client, "")
+	if err != nil {
+		return nil, err
+	}
+
+	return &oidc.AccessTokenResponse{
+		AccessToken: accessToken,
+		TokenType:   oidc.BearerToken,
+		ExpiresIn:   uint64(validity.Seconds()),
+	}, nil
+}