google: add support for JWT Access Tokens

This is a new form of authentication for Google services, where instead
of passing a signed claim to obtain a token from the OAuth endpoint, you
present the signed claim *as* the token to the API endpoint.

Fixes #139.
Fixes #140.

Change-Id: Ibf0f168a0ec111660ac08b86121c943fb96e146c
Reviewed-on: https://go-review.googlesource.com/10667
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Dave Day <djd@golang.org>

google/jwt.go

@@ -0,0 +1,67 @@
+// Copyright 2015 The oauth2 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 google
+
+import (
+	"crypto/rsa"
+	"fmt"
+	"time"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/internal"
+	"golang.org/x/oauth2/jws"
+)
+
+// JWTAccessTokenSourceFromJSON uses a Google Developers service account JSON
+// key file to read the credentials that authorize and authenticate the
+// requests, and returns a TokenSource that does not use any OAuth2 flow but
+// instead creates a JWT and sends that as the access token.
+// The audience is typically a URL that specifies the scope of the credentials.
+func JWTAccessTokenSourceFromJSON(jsonKey []byte, audience string) (oauth2.TokenSource, error) {
+	cfg, err := JWTConfigFromJSON(jsonKey)
+	if err != nil {
+		return nil, fmt.Errorf("google: could not parse JSON key: %v", err)
+	}
+	pk, err := internal.ParseKey(cfg.PrivateKey)
+	if err != nil {
+		return nil, fmt.Errorf("google: could not parse key: %v", err)
+	}
+	ts := &jwtAccessTokenSource{
+		email:    cfg.Email,
+		audience: audience,
+		pk:       pk,
+	}
+	tok, err := ts.Token()
+	if err != nil {
+		return nil, err
+	}
+	return oauth2.ReuseTokenSource(tok, ts), nil
+}
+
+type jwtAccessTokenSource struct {
+	email, audience string
+	pk              *rsa.PrivateKey
+}
+
+func (ts *jwtAccessTokenSource) Token() (*oauth2.Token, error) {
+	iat := time.Now()
+	exp := iat.Add(time.Hour)
+	cs := &jws.ClaimSet{
+		Iss: ts.email,
+		Sub: ts.email,
+		Aud: ts.audience,
+		Iat: iat.Unix(),
+		Exp: exp.Unix(),
+	}
+	hdr := &jws.Header{
+		Algorithm: "RS256",
+		Typ:       "JWT",
+	}
+	msg, err := jws.Encode(hdr, cs, ts.pk)
+	if err != nil {
+		return nil, fmt.Errorf("google: could not encode JWT: %v", err)
+	}
+	return &oauth2.Token{AccessToken: msg}, nil
+}

jws/jws.go

@@ -27,8 +27,8 @@ type ClaimSet struct {
 	Iss   string `json:"iss"`             // email address of the client_id of the application making the access token request
 	Scope string `json:"scope,omitempty"` // space-delimited list of the permissions the application requests
 	Aud   string `json:"aud"`             // descriptor of the intended target of the assertion (Optional).
-	Exp   int64  `json:"exp"`             // the expiration time of the assertion
-	Iat   int64  `json:"iat"`             // the time the assertion was issued.
+	Exp   int64  `json:"exp"`             // the expiration time of the assertion (seconds since Unix epoch)
+	Iat   int64  `json:"iat"`             // the time the assertion was issued (seconds since Unix epoch)
 	Typ   string `json:"typ,omitempty"`   // token type (Optional).
 
 	// Email for which the application is requesting delegated access (Optional).