Support for m.login.sso.

This is forked from @anandv96's #1374. Closes #1297.

clientapi/auth/auth.go

@@ -43,6 +43,7 @@ type AccountDatabase interface {
 	// Look up the account matching the given localpart.
 	GetAccountByLocalpart(ctx context.Context, localpart string) (*api.Account, error)
 	GetAccountByPassword(ctx context.Context, localpart, password string) (*api.Account, error)
+	GetLocalpartForThreePID(ctx context.Context, address, medium string) (string, error)
 }
 
 // VerifyUserFromRequest authenticates the HTTP request,

clientapi/auth/authtypes/logintypes.go

@@ -10,5 +10,6 @@ const (
 	LoginTypeSharedSecret       = "org.matrix.login.shared_secret"
 	LoginTypeRecaptcha          = "m.login.recaptcha"
 	LoginTypeApplicationService = "m.login.application_service"
+	LoginTypeSSO                = "m.login.sso"
 	LoginTypeToken              = "m.login.token"
 )

clientapi/auth/sso/github.go

@@ -0,0 +1,37 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sso
+
+import (
+	"github.com/matrix-org/dendrite/setup/config"
+)
+
+// GitHubIdentityProvider is a GitHub-flavored identity provider.
+var GitHubIdentityProvider IdentityProvider = githubIdentityProvider{
+	baseOIDCIdentityProvider: &baseOIDCIdentityProvider{
+		AuthURL:                     mustParseURLTemplate("https://github.com/login/oauth/authorize?scope=user:email"),
+		AccessTokenURL:              mustParseURLTemplate("https://github.com/login/oauth/access_token"),
+		UserInfoURL:                 mustParseURLTemplate("https://api.github.com/user"),
+		UserInfoAccept:              "application/vnd.github.v3+json",
+		UserInfoEmailPath:           "email",
+		UserInfoSuggestedUserIDPath: "login",
+	},
+}
+
+type githubIdentityProvider struct {
+	*baseOIDCIdentityProvider
+}
+
+func (githubIdentityProvider) DefaultBrand() string { return config.SSOBrandGitHub }

clientapi/auth/sso/oidc_base.go

@@ -0,0 +1,262 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sso
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"mime"
+	"net/http"
+	"net/url"
+	"strings"
+	"text/template"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/clientapi/userutil"
+	"github.com/tidwall/gjson"
+)
+
+type baseOIDCIdentityProvider struct {
+	AuthURL                     *urlTemplate
+	AccessTokenURL              *urlTemplate
+	UserInfoURL                 *urlTemplate
+	UserInfoAccept              string
+	UserInfoEmailPath           string
+	UserInfoSuggestedUserIDPath string
+}
+
+func (p *baseOIDCIdentityProvider) AuthorizationURL(ctx context.Context, req *IdentityProviderRequest) (string, error) {
+	u, err := p.AuthURL.Execute(map[string]interface{}{
+		"Config":      req.System,
+		"State":       req.DendriteNonce,
+		"RedirectURI": req.CallbackURL,
+	}, url.Values{
+		"client_id":     []string{req.System.OIDC.ClientID},
+		"response_type": []string{"code"},
+		"redirect_uri":  []string{req.CallbackURL},
+		"state":         []string{req.DendriteNonce},
+	})
+	if err != nil {
+		return "", err
+	}
+	return u.String(), nil
+}
+
+func (p *baseOIDCIdentityProvider) ProcessCallback(ctx context.Context, req *IdentityProviderRequest, values url.Values) (*CallbackResult, error) {
+	state := values.Get("state")
+	if state == "" {
+		return nil, jsonerror.MissingArgument("state parameter missing")
+	}
+	if state != req.DendriteNonce {
+		return nil, jsonerror.InvalidArgumentValue("state parameter not matching nonce")
+	}
+
+	if error := values.Get("error"); error != "" {
+		if euri := values.Get("error_uri"); euri != "" {
+			return &CallbackResult{RedirectURL: euri}, nil
+		}
+
+		desc := values.Get("error_description")
+		if desc == "" {
+			desc = error
+		}
+		switch error {
+		case "unauthorized_client", "access_denied":
+			return nil, jsonerror.Forbidden("SSO said no: " + desc)
+		default:
+			return nil, fmt.Errorf("SSO failed: %v", error)
+		}
+	}
+
+	code := values.Get("code")
+	if code == "" {
+		return nil, jsonerror.MissingArgument("code parameter missing")
+	}
+
+	oidcAccessToken, err := p.getOIDCAccessToken(ctx, req, code)
+	if err != nil {
+		return nil, err
+	}
+
+	id, userID, err := p.getUserInfo(ctx, req, oidcAccessToken)
+	if err != nil {
+		return nil, err
+	}
+
+	return &CallbackResult{Identifier: id, SuggestedUserID: userID}, nil
+}
+
+func (p *baseOIDCIdentityProvider) getOIDCAccessToken(ctx context.Context, req *IdentityProviderRequest, code string) (string, error) {
+	u, err := p.AccessTokenURL.Execute(nil, nil)
+	if err != nil {
+		return "", err
+	}
+
+	body := url.Values{
+		"grant_type":   []string{"authorization_code"},
+		"code":         []string{code},
+		"redirect_uri": []string{req.CallbackURL},
+		"client_id":    []string{req.System.OIDC.ClientID},
+	}
+
+	hreq, err := http.NewRequestWithContext(ctx, http.MethodPost, u.String(), strings.NewReader(body.Encode()))
+	if err != nil {
+		return "", err
+	}
+	hreq.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	hreq.Header.Set("Accept", "application/x-www-form-urlencoded")
+
+	hresp, err := http.DefaultClient.Do(hreq)
+	if err != nil {
+		return "", err
+	}
+	defer hresp.Body.Close()
+
+	ctype, _, err := mime.ParseMediaType(hresp.Header.Get("Content-Type"))
+	if err != nil {
+		return "", err
+	}
+	if ctype != "application/json" {
+		return "", fmt.Errorf("expected URL encoded response, got content type %q", ctype)
+	}
+
+	var resp struct {
+		TokenType   string `json:"token_type"`
+		AccessToken string `json:"access_token"`
+
+		Error            string `json:"error"`
+		ErrorDescription string `json:"error_description"`
+		ErrorURI         string `json:"error_uri"`
+	}
+	if err := json.NewDecoder(hresp.Body).Decode(&resp); err != nil {
+		return "", err
+	}
+
+	if resp.Error != "" {
+		desc := resp.ErrorDescription
+		if desc == "" {
+			desc = resp.Error
+		}
+		return "", fmt.Errorf("failed to retrieve OIDC access token: %s", desc)
+	}
+
+	if strings.ToLower(resp.TokenType) != "bearer" {
+		return "", fmt.Errorf("expected bearer token, got type %q", resp.TokenType)
+	}
+
+	return resp.AccessToken, nil
+}
+
+func (p *baseOIDCIdentityProvider) getUserInfo(ctx context.Context, req *IdentityProviderRequest, oidcAccessToken string) (*userutil.ThirdPartyIdentifier, string, error) {
+	u, err := p.UserInfoURL.Execute(map[string]interface{}{
+		"Config": req.System,
+	}, nil)
+	if err != nil {
+		return nil, "", err
+	}
+
+	hreq, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, "", err
+	}
+	hreq.Header.Set("Authorization", "token "+oidcAccessToken)
+	hreq.Header.Set("Accept", p.UserInfoAccept)
+
+	hresp, err := http.DefaultClient.Do(hreq)
+	if err != nil {
+		return nil, "", err
+	}
+	defer hresp.Body.Close()
+
+	ctype, _, err := mime.ParseMediaType(hresp.Header.Get("Content-Type"))
+	if err != nil {
+		return nil, "", err
+	}
+
+	var email string
+	var suggestedUserID string
+	switch ctype {
+	case "application/json":
+		body, err := ioutil.ReadAll(hresp.Body)
+		if err != nil {
+			return nil, "", err
+		}
+
+		emailRes := gjson.GetBytes(body, p.UserInfoEmailPath)
+		if !emailRes.Exists() {
+			return nil, "", fmt.Errorf("no email in user info response body")
+		}
+		email = emailRes.String()
+
+		// This is optional.
+		userIDRes := gjson.GetBytes(body, p.UserInfoSuggestedUserIDPath)
+		suggestedUserID = userIDRes.String()
+
+	default:
+		return nil, "", fmt.Errorf("got unknown content type %q for user info", ctype)
+	}
+
+	if email == "" {
+		return nil, "", fmt.Errorf("no email address in user info")
+	}
+
+	return &userutil.ThirdPartyIdentifier{Medium: "email", Address: email}, suggestedUserID, nil
+}
+
+type urlTemplate struct {
+	base *template.Template
+}
+
+func parseURLTemplate(s string) (*urlTemplate, error) {
+	t, err := template.New("").Parse(s)
+	if err != nil {
+		return nil, err
+	}
+	return &urlTemplate{base: t}, nil
+}
+
+func mustParseURLTemplate(s string) *urlTemplate {
+	t, err := parseURLTemplate(s)
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
+func (t *urlTemplate) Execute(params interface{}, defaultQuery url.Values) (*url.URL, error) {
+	var sb strings.Builder
+	err := t.base.Execute(&sb, params)
+	if err != nil {
+		return nil, err
+	}
+
+	u, err := url.Parse(sb.String())
+	if err != nil {
+		return nil, err
+	}
+
+	if defaultQuery != nil {
+		q := u.Query()
+		for k, vs := range defaultQuery {
+			if q.Get(k) == "" {
+				q[k] = vs
+			}
+		}
+		u.RawQuery = q.Encode()
+	}
+	return u, nil
+}

clientapi/auth/sso/sso.go

@@ -0,0 +1,57 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sso
+
+import (
+	"context"
+	"net/url"
+
+	"github.com/matrix-org/dendrite/clientapi/userutil"
+	"github.com/matrix-org/dendrite/setup/config"
+)
+
+type IdentityProvider interface {
+	DefaultBrand() string
+
+	AuthorizationURL(context.Context, *IdentityProviderRequest) (string, error)
+	ProcessCallback(context.Context, *IdentityProviderRequest, url.Values) (*CallbackResult, error)
+}
+
+type IdentityProviderRequest struct {
+	System        *config.IdentityProvider
+	CallbackURL   string
+	DendriteNonce string
+}
+
+type CallbackResult struct {
+	RedirectURL     string
+	Identifier      *userutil.ThirdPartyIdentifier
+	SuggestedUserID string
+}
+
+type IdentityProviderType string
+
+const (
+	TypeGitHub IdentityProviderType = config.SSOBrandGitHub
+)
+
+func GetIdentityProvider(t IdentityProviderType) IdentityProvider {
+	switch t {
+	case TypeGitHub:
+		return GitHubIdentityProvider
+	default:
+		return nil
+	}
+}

clientapi/routing/login.go

@@ -19,6 +19,8 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/auth"
+	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
+	"github.com/matrix-org/dendrite/clientapi/auth/sso"
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/clientapi/userutil"
 	"github.com/matrix-org/dendrite/setup/config"
@@ -35,20 +37,54 @@ type loginResponse struct {
 }
 
 type flows struct {
-	Flows []flow `json:"flows"`
+	Flows []stage `json:"flows"`
 }
 
-type flow struct {
+type stage struct {
-	Type string `json:"type"`
+	Type              string             `json:"type"`
+	IdentityProviders []identityProvider `json:"identity_providers,omitempty"`
 }
 
-func passwordLogin() flows {
+type identityProvider struct {
-	f := flows{}
+	ID    string `json:"id"`
-	s := flow{
+	Name  string `json:"name"`
-		Type: "m.login.password",
+	Brand string `json:"brand,omitempty"`
+	Icon  string `json:"icon,omitempty"`
+}
+
+func passwordLogin() []stage {
+	return []stage{
+		{Type: authtypes.LoginTypePassword},
+	}
+}
+
+func ssoLogin(cfg *config.ClientAPI) []stage {
+	var idps []identityProvider
+	for _, idp := range cfg.Login.SSO.Providers {
+		brand := idp.Brand
+		if brand == "" {
+			typ := idp.Type
+			if typ == "" {
+				typ = idp.ID
+			}
+			idpType := sso.GetIdentityProvider(sso.IdentityProviderType(typ))
+			if idpType != nil {
+				brand = idpType.DefaultBrand()
+			}
+		}
+		idps = append(idps, identityProvider{
+			ID:    idp.ID,
+			Name:  idp.Name,
+			Brand: brand,
+			Icon:  idp.Icon,
+		})
+	}
+	return []stage{
+		{
+			Type:              authtypes.LoginTypeSSO,
+			IdentityProviders: idps,
+		},
 	}
-	f.Flows = append(f.Flows, s)
-	return f
 }
 
 // Login implements GET and POST /login
@@ -57,10 +93,13 @@ func Login(
 	cfg *config.ClientAPI,
 ) util.JSONResponse {
 	if req.Method == http.MethodGet {
-		// TODO: support other forms of login other than password, depending on config options
+		allFlows := passwordLogin()
+		if cfg.Login.SSO.Enabled {
+			allFlows = append(allFlows, ssoLogin(cfg)...)
+		}
 		return util.JSONResponse{
 			Code: http.StatusOK,
-			JSON: passwordLogin(),
+			JSON: flows{Flows: allFlows},
 		}
 	} else if req.Method == http.MethodPost {
 		login, cleanup, authErr := auth.LoginFromJSONReader(req.Context(), req.Body, userAPI, userAPI, cfg)
@@ -72,6 +111,7 @@ func Login(
 		cleanup(req.Context(), &authErr2)
 		return authErr2
 	}
+
 	return util.JSONResponse{
 		Code: http.StatusMethodNotAllowed,
 		JSON: jsonerror.NotFound("Bad method"),

clientapi/routing/routing.go

@@ -563,6 +563,25 @@ func Setup(
 		}),
 	).Methods(http.MethodGet, http.MethodPost, http.MethodOptions)
 
+	v3mux.Handle("/login/sso/callback",
+		httputil.MakeExternalAPI("login", func(req *http.Request) util.JSONResponse {
+			return SSOCallback(req, userAPI, cfg)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
+	v3mux.Handle("/login/sso/redirect",
+		httputil.MakeExternalAPI("login", func(req *http.Request) util.JSONResponse {
+			return SSORedirect(req, "", cfg)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
+	v3mux.Handle("/login/sso/redirect/{idpID}",
+		httputil.MakeExternalAPI("login", func(req *http.Request) util.JSONResponse {
+			vars := mux.Vars(req)
+			return SSORedirect(req, vars["idpID"], cfg)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
 	v3mux.Handle("/auth/{authType}/fallback/web",
 		httputil.MakeHTMLAPI("auth_fallback", func(w http.ResponseWriter, req *http.Request) *util.JSONResponse {
 			vars := mux.Vars(req)

clientapi/routing/sso.go

@@ -0,0 +1,283 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package routing
+
+import (
+	"context"
+	"encoding/base64"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+
+	"github.com/matrix-org/dendrite/clientapi/auth/sso"
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/clientapi/userutil"
+	"github.com/matrix-org/dendrite/setup/config"
+	uapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+// SSORedirect implements /login/sso/redirect
+// https://spec.matrix.org/v1.2/client-server-api/#redirecting-to-the-authentication-server
+func SSORedirect(
+	req *http.Request,
+	idpID string,
+	cfg *config.ClientAPI,
+) util.JSONResponse {
+	if !cfg.Login.SSO.Enabled {
+		return util.JSONResponse{
+			Code: http.StatusNotImplemented,
+			JSON: jsonerror.NotFound("authentication method disabled"),
+		}
+	}
+
+	redirectURL := req.URL.Query().Get("redirectUrl")
+	if redirectURL == "" {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("redirectUrl parameter missing"),
+		}
+	}
+	_, err := url.Parse(redirectURL)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.InvalidArgumentValue("Invalid redirectURL: " + err.Error()),
+		}
+	}
+
+	if idpID == "" {
+		// Check configuration if the client didn't provide an ID.
+		idpID = cfg.Login.SSO.DefaultProviderID
+	}
+	if idpID == "" && len(cfg.Login.SSO.Providers) > 0 {
+		// Fall back to the first provider. If there are no providers, getProvider("") will fail.
+		idpID = cfg.Login.SSO.Providers[0].ID
+	}
+	idpCfg, idpType := getProvider(cfg, idpID)
+	if idpType == nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.InvalidArgumentValue("unknown identity provider"),
+		}
+	}
+
+	idpReq := &sso.IdentityProviderRequest{
+		System:        idpCfg,
+		CallbackURL:   req.URL.ResolveReference(&url.URL{Path: "../callback", RawQuery: url.Values{"provider": []string{idpID}}.Encode()}).String(),
+		DendriteNonce: formatNonce(redirectURL),
+	}
+	u, err := idpType.AuthorizationURL(req.Context(), idpReq)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: err,
+		}
+	}
+
+	resp := util.RedirectResponse(u)
+	resp.Headers["Set-Cookie"] = (&http.Cookie{
+		Name:     "oidc_nonce",
+		Value:    idpReq.DendriteNonce,
+		Expires:  time.Now().Add(10 * time.Minute),
+		Secure:   true,
+		SameSite: http.SameSiteStrictMode,
+	}).String()
+	return resp
+}
+
+// SSOCallback implements /login/sso/callback.
+// https://spec.matrix.org/v1.2/client-server-api/#handling-the-callback-from-the-authentication-server
+func SSOCallback(
+	req *http.Request,
+	userAPI userAPIForSSO,
+	cfg *config.ClientAPI,
+) util.JSONResponse {
+	ctx := req.Context()
+
+	query := req.URL.Query()
+	idpID := query.Get("provider")
+	if idpID == "" {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("provider parameter missing"),
+		}
+	}
+	idpCfg, idpType := getProvider(cfg, idpID)
+	if idpType == nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.InvalidArgumentValue("unknown identity provider"),
+		}
+	}
+
+	nonce, err := req.Cookie("oidc_nonce")
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("no nonce cookie: " + err.Error()),
+		}
+	}
+	finalRedirectURL, err := parseNonce(nonce.Value)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: err,
+		}
+	}
+
+	idpReq := &sso.IdentityProviderRequest{
+		System: idpCfg,
+		CallbackURL: (&url.URL{
+			Scheme: req.URL.Scheme,
+			Host:   req.URL.Host,
+			Path:   req.URL.Path,
+			RawQuery: url.Values{
+				"provider": []string{idpID},
+			}.Encode(),
+		}).String(),
+		DendriteNonce: nonce.Value,
+	}
+	result, err := idpType.ProcessCallback(ctx, idpReq, query)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: err,
+		}
+	}
+
+	if result.Identifier == nil {
+		// Not authenticated yet.
+		return util.RedirectResponse(result.RedirectURL)
+	}
+
+	id, err := verifyThirdPartyUserIdentifier(ctx, userAPI, result.Identifier, cfg.Matrix.ServerName)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).WithField("identifier", result.Identifier.String()).Error("failed to find user")
+		return util.JSONResponse{
+			Code: http.StatusUnauthorized,
+			JSON: jsonerror.Forbidden("ID not associated with a local account"),
+		}
+	}
+	if id == nil {
+		// The user doesn't exist.
+		// TODO: let the user select a localpart and register an account.
+		util.GetLogger(ctx).WithError(err).WithField("identifier", result.Identifier.String()).Error("failed to find user")
+		return util.JSONResponse{
+			Code: http.StatusNotImplemented,
+			JSON: jsonerror.Forbidden("SSO registration not implemented"),
+		}
+	}
+
+	token, err := createLoginToken(ctx, userAPI, id)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Errorf("PerformLoginTokenCreation failed")
+		return jsonerror.InternalServerError()
+	}
+
+	rquery := finalRedirectURL.Query()
+	rquery.Set("loginToken", token.Token)
+	resp := util.RedirectResponse(finalRedirectURL.ResolveReference(&url.URL{RawQuery: rquery.Encode()}).String())
+	resp.Headers["Set-Cookie"] = (&http.Cookie{
+		Name:   "oidc_nonce",
+		Value:  "",
+		MaxAge: -1,
+		Secure: true,
+	}).String()
+	return resp
+}
+
+type userAPIForSSO interface {
+	uapi.LoginTokenInternalAPI
+
+	QueryLocalpartForThreePID(ctx context.Context, req *uapi.QueryLocalpartForThreePIDRequest, res *uapi.QueryLocalpartForThreePIDResponse) error
+}
+
+// getProvider looks up the given provider in the
+// configuration. Returns nil if it wasn't found or was of unknown
+// type.
+func getProvider(cfg *config.ClientAPI, id string) (*config.IdentityProvider, sso.IdentityProvider) {
+	for _, idp := range cfg.Login.SSO.Providers {
+		if idp.ID == id {
+			switch sso.IdentityProviderType(id) {
+			case sso.TypeGitHub:
+				return &idp, sso.GitHubIdentityProvider
+			default:
+				return nil, nil
+			}
+		}
+	}
+	return nil, nil
+}
+
+// formatNonce creates a random nonce that also contains the URL.
+func formatNonce(redirectURL string) string {
+	return util.RandomString(16) + "." + base64.RawURLEncoding.EncodeToString([]byte(redirectURL))
+}
+
+// parseNonce extracts the embedded URL from the nonce. The nonce
+// should have been validated to be the original before calling this
+// function. The URL is not integrity protected.
+func parseNonce(s string) (redirectURL *url.URL, _ error) {
+	if s == "" {
+		return nil, jsonerror.MissingArgument("empty OIDC nonce cookie")
+	}
+
+	ss := strings.Split(s, ".")
+	if len(ss) < 2 {
+		return nil, jsonerror.InvalidArgumentValue("malformed OIDC nonce cookie")
+	}
+
+	urlbs, err := base64.RawURLEncoding.DecodeString(ss[1])
+	if err != nil {
+		return nil, jsonerror.InvalidArgumentValue("invalid redirect URL in OIDC nonce cookie")
+	}
+	u, err := url.Parse(string(urlbs))
+	if err != nil {
+		return nil, jsonerror.InvalidArgumentValue("invalid redirect URL in OIDC nonce cookie: " + err.Error())
+	}
+
+	return u, nil
+}
+
+// verifyThirdPartyUserIdentifier resolves a ThirdPartyIdentifier to a
+// UserIdentifier using the User API. Returns nil if there is no
+// associated user.
+func verifyThirdPartyUserIdentifier(ctx context.Context, userAPI userAPIForSSO, id *userutil.ThirdPartyIdentifier, serverName gomatrixserverlib.ServerName) (*userutil.UserIdentifier, error) {
+	req := &uapi.QueryLocalpartForThreePIDRequest{
+		ThreePID: id.Address,
+		Medium:   string(id.Medium),
+	}
+	var res uapi.QueryLocalpartForThreePIDResponse
+	if err := userAPI.QueryLocalpartForThreePID(ctx, req, &res); err != nil {
+		return nil, err
+	}
+	if res.Localpart == "" {
+		return nil, nil
+	}
+
+	return &userutil.UserIdentifier{UserID: userutil.MakeUserID(res.Localpart, serverName)}, nil
+}
+
+func createLoginToken(ctx context.Context, userAPI userAPIForSSO, id *userutil.UserIdentifier) (*uapi.LoginTokenMetadata, error) {
+	req := uapi.PerformLoginTokenCreationRequest{Data: uapi.LoginTokenData{UserID: id.UserID}}
+	var resp uapi.PerformLoginTokenCreationResponse
+	if err := userAPI.PerformLoginTokenCreation(ctx, &req, &resp); err != nil {
+		return nil, err
+	}
+	return &resp.Metadata, nil
+}

clientapi/userutil/identifier.go

@@ -0,0 +1,153 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package userutil
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+)
+
+// An Identifier identifies a user. There are many kinds, and this is
+// the common interface for them.
+//
+// If you need to handle an identifier as JSON, use the AnyIdentifier wrapper.
+// Passing around identifiers in code, the raw Identifier is enough.
+//
+// See https://matrix.org/docs/spec/client_server/r0.6.1#identifier-types
+type Identifier interface {
+	// IdentifierType returns the identifier type, like "m.id.user".
+	IdentifierType() IdentifierType
+
+	// String returns a debug-output string representation. The format
+	// is not specified.
+	String() string
+}
+
+// A UserIdentifier contains an MXID. It may be only the local part.
+type UserIdentifier struct {
+	UserID string `json:"user"`
+}
+
+func (i *UserIdentifier) IdentifierType() IdentifierType { return IdentifierUser }
+func (i *UserIdentifier) String() string                 { return i.UserID }
+
+// A ThirdPartyIdentifier references an identifier in another system.
+type ThirdPartyIdentifier struct {
+	// Medium is normally MediumEmail.
+	Medium Medium `json:"medium"`
+
+	// Address is the medium-specific identifier.
+	Address string `json:"address"`
+}
+
+func (i *ThirdPartyIdentifier) IdentifierType() IdentifierType { return IdentifierThirdParty }
+func (i *ThirdPartyIdentifier) String() string                 { return string(i.Medium) + ":" + i.Address }
+
+// A PhoneIdentifier references a phone number.
+type PhoneIdentifier struct {
+	// Country is a ISO-3166-1 alpha-2 country code.
+	Country string `json:"country"`
+
+	// PhoneNumber is a country-specific phone number, as it would be dialled from.
+	PhoneNumber string `json:"phone"`
+}
+
+func (i *PhoneIdentifier) IdentifierType() IdentifierType { return IdentifierPhone }
+func (i *PhoneIdentifier) String() string                 { return i.Country + ":" + i.PhoneNumber }
+
+// UnknownIdentifier is the catch-all for identifiers this code doesn't know about.
+// It simply stores raw JSON.
+type UnknownIdentifier struct {
+	json.RawMessage
+	Type IdentifierType
+}
+
+func (i *UnknownIdentifier) IdentifierType() IdentifierType { return i.Type }
+func (i *UnknownIdentifier) String() string                 { return "unknown/" + string(i.Type) }
+
+// AnyIdentifier is a wrapper that allows marshalling and unmarshalling the various
+// types of identifiers to/from JSON. Always use this in data types that will be
+// used in JSON manipulation.
+type AnyIdentifier struct {
+	Identifier
+}
+
+func (i AnyIdentifier) MarshalJSON() ([]byte, error) {
+	v := struct {
+		*UserIdentifier
+		*ThirdPartyIdentifier
+		*PhoneIdentifier
+		Type IdentifierType `json:"type"`
+	}{
+		Type: i.Identifier.IdentifierType(),
+	}
+	switch iid := i.Identifier.(type) {
+	case *UserIdentifier:
+		v.UserIdentifier = iid
+	case *ThirdPartyIdentifier:
+		v.ThirdPartyIdentifier = iid
+	case *PhoneIdentifier:
+		v.PhoneIdentifier = iid
+	case *UnknownIdentifier:
+		return iid.RawMessage, nil
+	}
+	return json.Marshal(v)
+}
+
+func (i *AnyIdentifier) UnmarshalJSON(bs []byte) error {
+	var hdr struct {
+		Type IdentifierType `json:"type"`
+	}
+	if err := json.Unmarshal(bs, &hdr); err != nil {
+		return err
+	}
+	switch hdr.Type {
+	case IdentifierUser:
+		var ui UserIdentifier
+		i.Identifier = &ui
+		return json.Unmarshal(bs, &ui)
+	case IdentifierThirdParty:
+		var tpi ThirdPartyIdentifier
+		i.Identifier = &tpi
+		return json.Unmarshal(bs, &tpi)
+	case IdentifierPhone:
+		var pi PhoneIdentifier
+		i.Identifier = &pi
+		return json.Unmarshal(bs, &pi)
+	case "":
+		return errors.New("missing identifier type")
+	default:
+		i.Identifier = &UnknownIdentifier{RawMessage: json.RawMessage(bytes.TrimSpace(bs)), Type: hdr.Type}
+		return nil
+	}
+}
+
+// IdentifierType describes the type of identifier.
+type IdentifierType string
+
+const (
+	IdentifierUser       IdentifierType = "m.id.user"
+	IdentifierThirdParty IdentifierType = "m.id.thirdparty"
+	IdentifierPhone      IdentifierType = "m.id.phone"
+)
+
+// Medium describes the interpretation of a third-party identifier.
+type Medium string
+
+const (
+	// MediumEmail signifies that the address is an email address.
+	MediumEmail Medium = "email"
+)

clientapi/userutil/identifier_test.go

@@ -0,0 +1,75 @@
+// Copyright 2022 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package userutil
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestAnyIdentifierJSON(t *testing.T) {
+	tsts := []struct {
+		Name string
+		JSON string
+		Want Identifier
+	}{
+		{Name: "empty", JSON: `{}`},
+		{Name: "user", JSON: `{"type":"m.id.user","user":"auser"}`, Want: &UserIdentifier{UserID: "auser"}},
+		{Name: "thirdparty", JSON: `{"type":"m.id.thirdparty","medium":"email","address":"auser@example.com"}`, Want: &ThirdPartyIdentifier{Medium: "email", Address: "auser@example.com"}},
+		{Name: "phone", JSON: `{"type":"m.id.phone","country":"GB","phone":"123456789"}`, Want: &PhoneIdentifier{Country: "GB", PhoneNumber: "123456789"}},
+		// This test is a little fragile since it compares the output of json.Marshal.
+		{Name: "unknown", JSON: `{"type":"other"}`, Want: &UnknownIdentifier{Type: "other", RawMessage: json.RawMessage(`{"type":"other"}`)}},
+	}
+	for _, tst := range tsts {
+		t.Run("Unmarshal/"+tst.Name, func(t *testing.T) {
+			var got AnyIdentifier
+			if err := json.Unmarshal([]byte(tst.JSON), &got); err != nil {
+				if tst.Want == nil {
+					return
+				}
+				t.Fatalf("Unmarshal failed: %v", err)
+			}
+
+			if !reflect.DeepEqual(got.Identifier, tst.Want) {
+				t.Errorf("got %+v, want %+v", got.Identifier, tst.Want)
+			}
+		})
+
+		if tst.Want == nil {
+			continue
+		}
+		t.Run("Marshal/"+tst.Name, func(t *testing.T) {
+			id := AnyIdentifier{Identifier: tst.Want}
+			bs, err := json.Marshal(id)
+			if err != nil {
+				t.Fatalf("Marshal failed: %v", err)
+			}
+			t.Logf("Marshalled JSON: %q", string(bs))
+
+			var got AnyIdentifier
+			if err := json.Unmarshal(bs, &got); err != nil {
+				if tst.Want == nil {
+					return
+				}
+				t.Fatalf("Unmarshal failed: %v", err)
+			}
+
+			if !reflect.DeepEqual(got.Identifier, tst.Want) {
+				t.Errorf("got %+v, want %+v", got.Identifier, tst.Want)
+			}
+		})
+	}
+}

setup/config/config_clientapi.go

@@ -42,6 +42,8 @@ type ClientAPI struct {
 	// was successful
 	RecaptchaSiteVerifyAPI string `yaml:"recaptcha_siteverify_api"`
 
+	Login Login `yaml:"login"`
+
 	// TURN options
 	TURN TURN `yaml:"turn"`
 
@@ -64,9 +66,11 @@ func (c *ClientAPI) Defaults(generate bool) {
 	c.RegistrationDisabled = true
 	c.OpenRegistrationWithoutVerificationEnabled = false
 	c.RateLimiting.Defaults()
+	c.Login.SSO.Enabled = false
 }
 
 func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
+	c.Login.Verify(configErrs)
 	c.TURN.Verify(configErrs)
 	c.RateLimiting.Verify(configErrs)
 	if c.RecaptchaEnabled {
@@ -95,6 +99,125 @@ func (c *ClientAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
 	checkURL(configErrs, "client_api.external_api.listen", string(c.ExternalAPI.Listen))
 }
 
+type Login struct {
+	SSO SSO `yaml:"sso"`
+}
+
+func (l *Login) Verify(configErrs *ConfigErrors) {
+	l.SSO.Verify(configErrs)
+}
+
+type SSO struct {
+	// Enabled determines whether SSO should be allowed.
+	Enabled bool `yaml:"enabled"`
+
+	// Providers list the identity providers this server is capable of confirming an
+	// identity with.
+	Providers []IdentityProvider `yaml:"providers"`
+
+	// DefaultProviderID is the provider to use when the client doesn't indicate one.
+	// This is legacy support. If empty, the first provider listed is used.
+	DefaultProviderID string `yaml:"default_provider"`
+}
+
+func (sso *SSO) Verify(configErrs *ConfigErrors) {
+	var foundDefaultProvider bool
+	seenPIDs := make(map[string]bool, len(sso.Providers))
+	for _, p := range sso.Providers {
+		p.Verify(configErrs)
+		if p.ID == sso.DefaultProviderID {
+			foundDefaultProvider = true
+		}
+		if seenPIDs[p.ID] {
+			configErrs.Add(fmt.Sprintf("duplicate identity provider for config key %q: %s", "client_api.sso.providers", p.ID))
+		}
+		seenPIDs[p.ID] = true
+	}
+	if sso.DefaultProviderID != "" && !foundDefaultProvider {
+		configErrs.Add(fmt.Sprintf("identity provider ID not found for config key %q: %s", "client_api.sso.default_provider", sso.DefaultProviderID))
+	}
+
+	if sso.Enabled {
+		if len(sso.Providers) == 0 {
+			configErrs.Add(fmt.Sprintf("empty list for config key %q", "client_api.sso.providers"))
+		}
+	}
+}
+
+// See https://github.com/matrix-org/matrix-doc/blob/old_master/informal/idp-brands.md.
+type IdentityProvider struct {
+	// ID is the unique identifier of this IdP. We use the brand identifiers as provider
+	// identifiers for simplicity.
+	ID string `yaml:"id"`
+
+	// Name is a human-friendly name of the provider.
+	Name string `yaml:"name"`
+
+	// Brand is a hint on how to display the IdP to the user. If this is empty, a default
+	// based on the type is used.
+	Brand string `yaml:"brand"`
+
+	// Icon is an MXC URI describing how to display the IdP to the user. Prefer using `brand`.
+	Icon string `yaml:"icon"`
+
+	// Type describes how this provider is implemented. It must match "github". If this is
+	// empty, the ID is used, which means there is a weak expectation that ID is also a
+	// valid type, unless you have a complicated setup.
+	Type string `yaml:"type"`
+
+	// OIDC contains settings for providers based on OpenID Connect (OAuth 2).
+	OIDC struct {
+		ClientID     string `yaml:"client_id"`
+		ClientSecret string `yaml:"client_secret"`
+	} `yaml:"oidc"`
+}
+
+func (idp *IdentityProvider) Verify(configErrs *ConfigErrors) {
+	checkNotEmpty(configErrs, "client_api.sso.providers.id", idp.ID)
+	if !checkIdentityProviderBrand(idp.ID) {
+		configErrs.Add(fmt.Sprintf("unrecognized ID config key %q: %s", "client_api.sso.providers", idp.ID))
+	}
+	checkNotEmpty(configErrs, "client_api.sso.providers.name", idp.Name)
+	if idp.Brand != "" && !checkIdentityProviderBrand(idp.Brand) {
+		configErrs.Add(fmt.Sprintf("unrecognized brand in identity provider %q for config key %q: %s", idp.ID, "client_api.sso.providers", idp.Brand))
+	}
+	if idp.Icon != "" {
+		checkURL(configErrs, "client_api.sso.providers.icon", idp.Icon)
+	}
+	typ := idp.Type
+	if idp.Type == "" {
+		typ = idp.ID
+	}
+
+	switch typ {
+	case "github":
+		checkNotEmpty(configErrs, "client_api.sso.providers.oidc.client_id", idp.OIDC.ClientID)
+		checkNotEmpty(configErrs, "client_api.sso.providers.oidc.client_secret", idp.OIDC.ClientSecret)
+
+	default:
+		configErrs.Add(fmt.Sprintf("unrecognized type in identity provider %q for config key %q: %s", idp.ID, "client_api.sso.providers", typ))
+	}
+}
+
+// See https://github.com/matrix-org/matrix-doc/blob/old_master/informal/idp-brands.md.
+func checkIdentityProviderBrand(s string) bool {
+	switch s {
+	case SSOBrandApple, SSOBrandFacebook, SSOBrandGitHub, SSOBrandGitLab, SSOBrandGoogle, SSOBrandTwitter:
+		return true
+	default:
+		return false
+	}
+}
+
+const (
+	SSOBrandApple    = "apple"
+	SSOBrandFacebook = "facebook"
+	SSOBrandGitHub   = "github"
+	SSOBrandGitLab   = "gitlab"
+	SSOBrandGoogle   = "google"
+	SSOBrandTwitter  = "twitter"
+)
+
 type TURN struct {
 	// TODO Guest Support
 	// Whether or not guests can request TURN credentials