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