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// Copyright 2021 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 authhandler implements a TokenSource to support
// "three-legged OAuth 2.0" via a custom AuthorizationHandler.
package authhandler
import (
const (
// Parameter keys for AuthCodeURL method to support PKCE.
codeChallengeKey = "code_challenge"
codeChallengeMethodKey = "code_challenge_method"
// Parameter key for Exchange method to support PKCE.
codeVerifierKey = "code_verifier"
// PKCEParams holds parameters to support PKCE.
type PKCEParams struct {
Challenge string // The unpadded, base64-url-encoded string of the encrypted code verifier.
ChallengeMethod string // The encryption method (ex. S256).
Verifier string // The original, non-encrypted secret.
// AuthorizationHandler is a 3-legged-OAuth helper that prompts
// the user for OAuth consent at the specified auth code URL
// and returns an auth code and state upon approval.
type AuthorizationHandler func(authCodeURL string) (code string, state string, err error)
// TokenSourceWithPKCE is an enhanced version of TokenSource with PKCE support.
// The pkce parameter supports PKCE flow, which uses code challenge and code verifier
// to prevent CSRF attacks. A unique code challenge and code verifier should be generated
// by the caller at runtime. See for more info.
func TokenSourceWithPKCE(ctx context.Context, config *oauth2.Config, state string, authHandler AuthorizationHandler, pkce *PKCEParams) oauth2.TokenSource {
return oauth2.ReuseTokenSource(nil, authHandlerSource{config: config, ctx: ctx, authHandler: authHandler, state: state, pkce: pkce})
// TokenSource returns an oauth2.TokenSource that fetches access tokens
// using 3-legged-OAuth flow.
// The provided context.Context is used for oauth2 Exchange operation.
// The provided oauth2.Config should be a full configuration containing AuthURL,
// TokenURL, and Scope.
// An environment-specific AuthorizationHandler is used to obtain user consent.
// Per the OAuth protocol, a unique "state" string should be specified here.
// This token source will verify that the "state" is identical in the request
// and response before exchanging the auth code for OAuth token to prevent CSRF
// attacks.
func TokenSource(ctx context.Context, config *oauth2.Config, state string, authHandler AuthorizationHandler) oauth2.TokenSource {
return TokenSourceWithPKCE(ctx, config, state, authHandler, nil)
type authHandlerSource struct {
ctx context.Context
config *oauth2.Config
authHandler AuthorizationHandler
state string
pkce *PKCEParams
func (source authHandlerSource) Token() (*oauth2.Token, error) {
// Step 1: Obtain auth code.
var authCodeUrlOptions []oauth2.AuthCodeOption
if source.pkce != nil && source.pkce.Challenge != "" && source.pkce.ChallengeMethod != "" {
authCodeUrlOptions = []oauth2.AuthCodeOption{oauth2.SetAuthURLParam(codeChallengeKey, source.pkce.Challenge),
oauth2.SetAuthURLParam(codeChallengeMethodKey, source.pkce.ChallengeMethod)}
url := source.config.AuthCodeURL(source.state, authCodeUrlOptions...)
code, state, err := source.authHandler(url)
if err != nil {
return nil, err
if state != source.state {
return nil, errors.New("state mismatch in 3-legged-OAuth flow")
// Step 2: Exchange auth code for access token.
var exchangeOptions []oauth2.AuthCodeOption
if source.pkce != nil && source.pkce.Verifier != "" {
exchangeOptions = []oauth2.AuthCodeOption{oauth2.SetAuthURLParam(codeVerifierKey, source.pkce.Verifier)}
return source.config.Exchange(source.ctx, code, exchangeOptions...)