Minimal OpenID Connect implementation (#14139)

This is "minimal" in the sense that only the Authorization Code Flow
from OpenID Connect Core is implemented.  No discovery, no configuration
endpoint, and no user scope management.

OpenID Connect is an extension to the (already implemented) OAuth 2.0
protocol, and essentially an `id_token` JWT is added to the access token
endpoint response when using the Authorization Code Flow.  I also added
support for the "nonce" field since it is required to be used in the
id_token if the client decides to include it in its initial request.

In order to enable this extension an OAuth 2.0 scope containing
"openid" is needed. Other OAuth 2.0 requests should not be impacted by
this change.

This minimal implementation is enough to enable single sign-on (SSO)
for other sites, e.g. by using something like `mod_auth_openidc` to
only allow access to a CI server if a user has logged into Gitea.

Fixes: #1310

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

models/oauth2_application.go

@@ -9,6 +9,7 @@ import (
 	"encoding/base64"
 	"fmt"
 	"net/url"
+	"strings"
 	"time"
 
 	"code.gitea.io/gitea/modules/secret"
@@ -103,14 +104,15 @@ func (app *OAuth2Application) getGrantByUserID(e Engine, userID int64) (grant *O
 }
 
 // CreateGrant generates a grant for an user
-func (app *OAuth2Application) CreateGrant(userID int64) (*OAuth2Grant, error) {
-	return app.createGrant(x, userID)
+func (app *OAuth2Application) CreateGrant(userID int64, scope string) (*OAuth2Grant, error) {
+	return app.createGrant(x, userID, scope)
 }
 
-func (app *OAuth2Application) createGrant(e Engine, userID int64) (*OAuth2Grant, error) {
+func (app *OAuth2Application) createGrant(e Engine, userID int64, scope string) (*OAuth2Grant, error) {
 	grant := &OAuth2Grant{
 		ApplicationID: app.ID,
 		UserID:        userID,
+		Scope:         scope,
 	}
 	_, err := e.Insert(grant)
 	if err != nil {
@@ -380,6 +382,8 @@ type OAuth2Grant struct {
 	Application   *OAuth2Application `xorm:"-"`
 	ApplicationID int64              `xorm:"INDEX unique(user_application)"`
 	Counter       int64              `xorm:"NOT NULL DEFAULT 1"`
+	Scope         string             `xorm:"TEXT"`
+	Nonce         string             `xorm:"TEXT"`
 	CreatedUnix   timeutil.TimeStamp `xorm:"created"`
 	UpdatedUnix   timeutil.TimeStamp `xorm:"updated"`
 }
@@ -431,6 +435,30 @@ func (grant *OAuth2Grant) increaseCount(e Engine) error {
 	return nil
 }
 
+// ScopeContains returns true if the grant scope contains the specified scope
+func (grant *OAuth2Grant) ScopeContains(scope string) bool {
+	for _, currentScope := range strings.Split(grant.Scope, " ") {
+		if scope == currentScope {
+			return true
+		}
+	}
+	return false
+}
+
+// SetNonce updates the current nonce value of a grant
+func (grant *OAuth2Grant) SetNonce(nonce string) error {
+	return grant.setNonce(x, nonce)
+}
+
+func (grant *OAuth2Grant) setNonce(e Engine, nonce string) error {
+	grant.Nonce = nonce
+	_, err := e.ID(grant.ID).Cols("nonce").Update(grant)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
 // GetOAuth2GrantByID returns the grant with the given ID
 func GetOAuth2GrantByID(id int64) (*OAuth2Grant, error) {
 	return getOAuth2GrantByID(x, id)
@@ -533,3 +561,16 @@ func (token *OAuth2Token) SignToken() (string, error) {
 	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS512, token)
 	return jwtToken.SignedString(setting.OAuth2.JWTSecretBytes)
 }
+
+// OIDCToken represents an OpenID Connect id_token
+type OIDCToken struct {
+	jwt.StandardClaims
+	Nonce string `json:"nonce,omitempty"`
+}
+
+// SignToken signs an id_token with the (symmetric) client secret key
+func (token *OIDCToken) SignToken(clientSecret string) (string, error) {
+	token.IssuedAt = time.Now().Unix()
+	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, token)
+	return jwtToken.SignedString([]byte(clientSecret))
+}