google: add utilities supporting upcoming oauth2 functionality

These are used to support some extended utilities to help with STS requests.

Change-Id: Iafc145b06ca42374cfc2ac6572762a50bcf560f2
GitHub-Last-Rev: 3085fe570382318e6690304640751bf312e1a0b8
GitHub-Pull-Request: golang/oauth2#439
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/259777
Trust: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

google/internal/externalaccount/clientauth.go

@@ -0,0 +1,41 @@
+// 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 externalaccount
+
+import (
+	"encoding/base64"
+	"golang.org/x/oauth2"
+	"net/http"
+	"net/url"
+)
+
+// ClientAuthentication represents an OAuth client ID and secret and the mechanism for passing these credentials as stated in rfc6749#2.3.1.
+type ClientAuthentication struct {
+	// AuthStyle can be either basic or request-body
+	AuthStyle    oauth2.AuthStyle
+	ClientID     string
+	ClientSecret string
+}
+
+func (c *ClientAuthentication) InjectAuthentication(values url.Values, headers http.Header) {
+	if c.ClientID == "" || c.ClientSecret == "" || values == nil || headers == nil {
+		return
+	}
+
+	switch c.AuthStyle {
+	case oauth2.AuthStyleInHeader: // AuthStyleInHeader corresponds to basic authentication as defined in rfc7617#2
+		plainHeader := c.ClientID + ":" + c.ClientSecret
+		headers.Add("Authorization", "Basic "+base64.StdEncoding.EncodeToString([]byte(plainHeader)))
+	case oauth2.AuthStyleInParams: // AuthStyleInParams corresponds to request-body authentication with ClientID and ClientSecret in the message body.
+		values.Set("client_id", c.ClientID)
+		values.Set("client_secret", c.ClientSecret)
+	case oauth2.AuthStyleAutoDetect:
+		values.Set("client_id", c.ClientID)
+		values.Set("client_secret", c.ClientSecret)
+	default:
+		values.Set("client_id", c.ClientID)
+		values.Set("client_secret", c.ClientSecret)
+	}
+}