Add support private_key_jwt authentication type in client credentials flow

It is implementation of rfc7523 JWT Profile for client authentication.
See https://tools.ietf.org/html/rfc7523
See https://openid.net/specs/openid-connect-core-1_0.html

Fixes golang#433

Author: Lilit Smotrova

clientcredentials/clientcredentials.go

@@ -11,6 +11,7 @@
 // server.
 //
 // See https://tools.ietf.org/html/rfc6749#section-4.4
+// See https://tools.ietf.org/html/rfc7523
 package clientcredentials // import "golang.org/x/oauth2/clientcredentials"
 
 import (
@@ -19,6 +20,7 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/internal"
@@ -46,7 +48,25 @@ type Config struct {
 	// AuthStyle optionally specifies how the endpoint wants the
 	// client ID & client secret sent. The zero value means to
 	// auto-detect.
+	// See https://openid.net/specs/openid-connect-core-1_0.html.
 	AuthStyle oauth2.AuthStyle
+
+	// JWTExpires optionally specifies how long the jwt token is valid for.
+	JWTExpires time.Duration
+
+	// PrivateKey contains the contents of an RSA private key or the
+	// contents of a PEM file that contains a private key. The provided
+	// private key is used to sign JWT payloads.
+	// PEM containers with a passphrase are not supported.
+	// Use the following command to convert a PKCS 12 file into a PEM.
+	//
+	//    $ openssl pkcs12 -in key.p12 -out key.pem -nodes
+	//
+	PrivateKey []byte
+
+	// KeyID contains an optional hint indicating which key is being
+	// used.
+	KeyID string
 }
 
 // Token uses client credentials to retrieve a token.
@@ -91,6 +111,14 @@ func (c *tokenSource) Token() (*oauth2.Token, error) {
 	v := url.Values{
 		"grant_type": {"client_credentials"},
 	}
+	if c.conf.AuthStyle == oauth2.AuthStylePrivateKeyJWT {
+		var err error
+		v, err = c.jwtAssertionValues()
+		if err != nil {
+			return nil, err
+		}
+
+	}
 	if len(c.conf.Scopes) > 0 {
 		v.Set("scope", strings.Join(c.conf.Scopes, " "))
 	}

clientcredentials/clientcredentials_test.go

@@ -6,14 +6,20 @@ package clientcredentials
 
 import (
 	"context"
+	"encoding/base64"
+	"encoding/json"
 	"io"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
+	"time"
 
+	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/internal"
+	"golang.org/x/oauth2/jws"
 )
 
 func newConf(serverURL string) *Config {
@@ -113,6 +119,137 @@ func TestTokenRequest(t *testing.T) {
 	}
 }
 
+var dummyPrivateKey = []byte(`-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----`)
+
+func TestTokenJWTRequest(t *testing.T) {
+	var assertion string
+	audience := "audience1"
+	scopes := "scope1 scope2"
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.String() != "/token" {
+			t.Errorf("authenticate client request URL = %q; want %q", r.URL, "/token")
+		}
+		if got, want := r.Header.Get("Content-Type"), "application/x-www-form-urlencoded"; got != want {
+			t.Errorf("Content-Type header = %q; want %q", got, want)
+		}
+		err := r.ParseForm()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got, want := r.Form.Get("scope"), scopes; got != want {
+			t.Errorf("scope = %q; want %q", got, want)
+		}
+		if got, want := r.Form.Get("audience"), audience; got != want {
+			t.Errorf("audience = %q; want %q", got, want)
+		}
+		if got, want := r.Form.Get("grant_type"), "client_credentials"; got != want {
+			t.Errorf("grant_type = %q; want %q", got, want)
+		}
+		expectedAssertionType := "urn:ietf:params:oauth:client-assertion-type:jwt-bearer"
+		if got, want := r.Form.Get("client_assertion_type"), expectedAssertionType; got != want {
+			t.Errorf("client_assertion_type = %q; want %q", got, want)
+		}
+
+		assertion = r.Form.Get("client_assertion")
+
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{
+			"access_token": "90d64460d14870c08c81352a05dedd3465940a7c",
+			"scope": "user",
+			"token_type": "bearer",
+			"expires_in": 3600
+		}`))
+	}))
+	defer ts.Close()
+
+	for _, conf := range []*Config{
+		{
+			ClientID:       "CLIENT_ID",
+			Scopes:         strings.Split(scopes, " "),
+			TokenURL:       ts.URL + "/token",
+			EndpointParams: url.Values{"audience": {audience}},
+			AuthStyle:      oauth2.AuthStylePrivateKeyJWT,
+			PrivateKey:     dummyPrivateKey,
+			KeyID:          "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+		},
+		{
+			ClientID:       "CLIENT_ID_set_jwt_expiration_time",
+			Scopes:         strings.Split(scopes, " "),
+			TokenURL:       ts.URL + "/token",
+			EndpointParams: url.Values{"audience": {audience}},
+			AuthStyle:      oauth2.AuthStylePrivateKeyJWT,
+			PrivateKey:     dummyPrivateKey,
+			KeyID:          "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+			JWTExpires:     time.Minute,
+		},
+	} {
+		t.Run(conf.ClientID, func(t *testing.T) {
+			_, err := conf.TokenSource(context.Background()).Token()
+			if err != nil {
+				t.Fatalf("Failed to fetch token: %v", err)
+			}
+			parts := strings.Split(assertion, ".")
+			if len(parts) != 3 {
+				t.Fatalf("assertion = %q; want 3 parts", assertion)
+			}
+			gotJson, err := base64.RawURLEncoding.DecodeString(parts[1])
+			if err != nil {
+				t.Fatalf("invalid token payload; err = %v", err)
+			}
+			claimSet := jws.ClaimSet{}
+			if err := json.Unmarshal(gotJson, &claimSet); err != nil {
+				t.Errorf("failed to unmarshal json token payload = %q; err = %v", gotJson, err)
+			}
+			if got, want := claimSet.Iss, conf.ClientID; got != want {
+				t.Errorf("payload iss = %q; want %q", got, want)
+			}
+			if claimSet.Jti == "" {
+				t.Errorf("payload jti is empty")
+			}
+			expectedDuration := time.Hour
+			if conf.JWTExpires > 0 {
+				expectedDuration = conf.JWTExpires
+			}
+			if got, want := claimSet.Exp, time.Now().Add(expectedDuration).Unix(); got != want {
+				t.Errorf("payload exp = %q; want %q", got, want)
+			}
+			if got, want := claimSet.Aud, conf.TokenURL; got != want {
+				t.Errorf("payload aud = %q; want %q", got, want)
+			}
+			if got, want := claimSet.Sub, conf.ClientID; got != want {
+				t.Errorf("payload sub = %q; want %q", got, want)
+			}
+		})
+	}
+}
+
 func TestTokenRefreshRequest(t *testing.T) {
 	internal.ResetAuthCache()
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

clientcredentials/jwt.go

@@ -0,0 +1,69 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package clientcredentials
+
+import (
+	"math/rand"
+	"net/url"
+	"time"
+
+	"golang.org/x/oauth2/internal"
+	"golang.org/x/oauth2/jws"
+)
+
+const (
+	clientAssertionType = "urn:ietf:params:oauth:client-assertion-type:jwt-bearer"
+)
+
+var (
+	defaultHeader = &jws.Header{Algorithm: "RS256", Typ: "JWT"}
+)
+
+func init() {
+	rand.Seed(time.Now().UnixNano())
+}
+
+var letters = []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
+
+func randJWTID(n int) string {
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = letters[rand.Intn(len(letters))]
+	}
+	return string(b)
+}
+
+func (c *tokenSource) jwtAssertionValues() (url.Values, error) {
+	v := url.Values{
+		"grant_type": {"client_credentials"},
+	}
+	pk, err := internal.ParseKey(c.conf.PrivateKey)
+	if err != nil {
+		return nil, err
+	}
+	claimSet := &jws.ClaimSet{
+		Iss: c.conf.ClientID,
+		Sub: c.conf.ClientID,
+		Aud: c.conf.TokenURL,
+	}
+
+	claimSet.Jti = randJWTID(36)
+	if t := c.conf.JWTExpires; t > 0 {
+		claimSet.Exp = time.Now().Add(t).Unix()
+	} else {
+		claimSet.Exp = time.Now().Add(time.Hour).Unix()
+	}
+
+	h := *defaultHeader
+	h.KeyID = c.conf.KeyID
+	payload, err := jws.Encode(&h, claimSet, pk)
+	if err != nil {
+		return nil, err
+	}
+	v.Set("client_assertion", payload)
+	v.Set("client_assertion_type", clientAssertionType)
+
+	return v, nil
+}

jws/jws.go

@@ -49,6 +49,10 @@ type ClaimSet struct {
 	// See http://tools.ietf.org/html/draft-jones-json-web-token-10#section-4.3
 	// This array is marshalled using custom code (see (c *ClaimSet) encode()).
 	PrivateClaims map[string]interface{} `json:"-"`
+
+	// See https://tools.ietf.org/html/rfc7523#section-3.
+	// Unique identifier for the jwt token.
+	Jti string `json:"jti"`
 }
 
 func (c *ClaimSet) encode() (string, error) {

oauth2.go

@@ -97,6 +97,11 @@ const (
 	// using HTTP Basic Authorization. This is an optional style
 	// described in the OAuth2 RFC 6749 section 2.3.1.
 	AuthStyleInHeader AuthStyle = 2
+
+	// AuthStylePrivateKeyJWT send jwt token signed by private key.
+	// See https://openid.net/specs/openid-connect-core-1_0.html.
+	// See https://tools.ietf.org/html/rfc7523.
+	AuthStylePrivateKeyJWT AuthStyle = 3
 )
 
 var (