cmd.go

package oauth

import (
	"bufio"
	"crypto/sha256"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"io/ioutil"
	"net/http"
	"net/http/httptest"
	"net/url"
	"os"
	"path"
	"strconv"
	"strings"
	"time"

	"github.com/pkg/errors"
	"github.com/smallstep/cli/command"
	"github.com/smallstep/cli/crypto/randutil"
	"github.com/smallstep/cli/errs"
	"github.com/smallstep/cli/exec"
	"github.com/smallstep/cli/jose"
	"github.com/urfave/cli"
)

// These are the OAuth2.0 client IDs from the Step CLI. This application is
// using the OAuth2.0 flow for installed applications described on
// https://developers.google.com/identity/protocols/OAuth2InstalledApp
//
// The Step CLI app and these client IDs do not have any APIs or services are
// enabled and it should be only used for OAuth 2.0 authorization.
//
// Due to the fact that the app cannot keep the client_secret confidential,
// incremental authorization with installed apps are not supported by Google.
//
// Google is also distributing the client ID and secret on the cloud SDK
// available here https://cloud.google.com/sdk/docs/quickstarts
const (
	defaultClientID          = "1087160488420-8qt7bavg3qesdhs6it824mhnfgcfe8il.apps.googleusercontent.com"
	defaultClientNotSoSecret = "udTrOT3gzrO7W9fDPgZQLfYJ"

	// The URN for getting verification token offline
	oobCallbackUrn = "urn:ietf:wg:oauth:2.0:oob"
	// The URN for token request grant type jwt-bearer
	jwtBearerUrn = "urn:ietf:params:oauth:grant-type:jwt-bearer"
)

type token struct {
	AccessToken  string `json:"access_token"`
	IDToken      string `json:"id_token"`
	RefreshToken string `json:"refresh_token"`
	ExpiresIn    int    `json:"expires_in"`
	TokenType    string `json:"token_type"`
	Err          string `json:"error,omitempty"`
	ErrDesc      string `json:"error_description,omitempty"`
}

func init() {
	cmd := cli.Command{
		Name:  "oauth",
		Usage: "authorization and single sign-on using OAuth & OIDC",
		UsageText: `
**step oauth** [**--provider**=<provider>] [**--client-id**=<client-id> **--client-secret**=<client-secret>]
  [**--scope**=<scope> ...] [**--bare** [**--oidc**]] [**--header** [**--oidc**]]

**step oauth** **--authorization-endpoint**=<authorization-endpoint> **--token-endpoint**=<token-endpoint>
  **--client-id**=<client-id> **--client-secret**=<client-secret> [**--scope**=<scope> ...] [**--bare** [**--oidc**]] [**--header** [**--oidc**]]

**step oauth** [**--account**=<account>] [**--authorization-endpoint**=<authorization-endpoint> **--token-endpoint**=<token-endpoint>]
  [**--scope**=<scope> ...] [**--bare** [**--oidc**]] [**--header** [**--oidc**]]

**step oauth** **--account**=<account> **--jwt** [**--scope**=<scope> ...] [**--header**] [**-bare**]
`,
		Flags: []cli.Flag{
			cli.StringFlag{
				Name:  "provider, idp",
				Usage: "OAuth provider for authentication",
				Value: "google",
			},
			cli.StringFlag{
				Name:  "email, e",
				Usage: "Email to authenticate",
			},
			cli.BoolFlag{
				Name:  "console, c",
				Usage: "Complete the flow while remaining only inside the terminal",
			},
			cli.StringFlag{
				Name:  "client-id",
				Usage: "OAuth Client ID",
			},
			cli.StringFlag{
				Name:  "client-secret",
				Usage: "OAuth Client Secret",
			},
			cli.StringFlag{
				Name:  "account",
				Usage: "JSON file containing account details",
			},
			cli.StringFlag{
				Name:  "authorization-endpoint",
				Usage: "OAuth Authorization Endpoint",
			},
			cli.StringFlag{
				Name:  "token-endpoint",
				Usage: "OAuth Token Endpoint",
			},
			cli.BoolFlag{
				Name:  "header",
				Usage: "Output HTTP Authorization Header (suitable for use with curl)",
			},
			cli.BoolFlag{
				Name:  "oidc",
				Usage: "Output OIDC Token instead of OAuth Access Token",
			},
			cli.BoolFlag{
				Name:  "bare",
				Usage: "Only output the token",
			},
			cli.StringSliceFlag{
				Name:  "scope",
				Usage: "OAuth scopes",
			},
			cli.BoolFlag{
				Name:  "jwt",
				Usage: "Generate a JWT Auth token instead of an OAuth Token (only works with service accounts)",
			},
			cli.BoolFlag{
				Name:   "implicit",
				Usage:  "Uses the implicit flow to authenticate the user. Requires **--insecure** and **--client-id** flags.",
				Hidden: true,
			},
			cli.BoolFlag{
				Name:   "insecure",
				Usage:  "Allows the use of insecure flows.",
				Hidden: true,
			},
		},
		Action: oauthCmd,
	}

	command.Register(cmd)
}

func oauthCmd(c *cli.Context) error {
	opts := &options{
		Provider: c.String("provider"),
		Email:    c.String("email"),
		Console:  c.Bool("console"),
		Implicit: c.Bool("implicit"),
	}
	if err := opts.Validate(); err != nil {
		return err
	}
	if (opts.Provider != "google" || c.IsSet("authorization-endpoint")) && !c.IsSet("client-id") {
		return errors.New("flag '--client-id' required with '--provider'")
	}

	var clientID, clientSecret string
	if opts.Implicit {
		if !c.Bool("insecure") {
			return errs.RequiredInsecureFlag(c, "implicit")
		}
		if !c.IsSet("client-id") {
			return errs.RequiredWithFlag(c, "implicit", "client-id")
		}
	} else {
		clientID = defaultClientID
		clientSecret = defaultClientNotSoSecret
	}
	if c.IsSet("client-id") {
		clientID = c.String("client-id")
		clientSecret = c.String("client-secret")
	}

	authzEp := ""
	tokenEp := ""
	if c.IsSet("authorization-endpoint") {
		if !c.IsSet("token-endpoint") {
			return errors.New("flag '--authorization-endpoint' requires flag '--token-endpoint'")
		}
		opts.Provider = ""
		authzEp = c.String("authorization-endpoint")
		tokenEp = c.String("token-endpoint")
	}

	do2lo := false
	issuer := ""
	// This code supports Google service accounts. Probably maybe also support JWKs?
	if c.IsSet("account") {
		opts.Provider = ""
		filename := c.String("account")
		b, err := ioutil.ReadFile(filename)
		if err != nil {
			return errors.Wrapf(err, "error reading account from %s", filename)
		}
		account := make(map[string]interface{})
		if err := json.Unmarshal(b, &account); err != nil {
			return errors.Wrapf(err, "error reading %s: unsupported format", filename)
		}

		if _, ok := account["installed"]; ok {
			details := account["installed"].(map[string]interface{})
			authzEp = details["auth_uri"].(string)
			tokenEp = details["token_uri"].(string)
			clientID = details["client_id"].(string)
			clientSecret = details["client_secret"].(string)
		} else if accountType, ok := account["type"]; ok && "service_account" == accountType {
			authzEp = account["auth_uri"].(string)
			tokenEp = account["token_uri"].(string)
			clientID = account["private_key_id"].(string)
			clientSecret = account["private_key"].(string)
			issuer = account["client_email"].(string)
			do2lo = true
		} else {
			return errors.Wrapf(err, "error reading %s: unsupported account type", filename)
		}
	}

	scope := "openid email"
	if c.IsSet("scope") {
		scope = strings.Join(c.StringSlice("scope"), " ")
	}

	o, err := newOauth(opts.Provider, clientID, clientSecret, authzEp, tokenEp, scope, opts)
	if err != nil {
		return err
	}

	var tok *token
	if do2lo {
		if c.Bool("jwt") {
			tok, err = o.DoJWTAuthorization(issuer, scope)
		} else {
			tok, err = o.DoTwoLeggedAuthorization(issuer)
		}
	} else if opts.Console {
		tok, err = o.DoManualAuthorization()
	} else {
		tok, err = o.DoLoopbackAuthorization()
	}

	if err != nil {
		return err
	}

	if c.Bool("header") {
		if c.Bool("oidc") {
			fmt.Println("Authorization: Bearer", tok.IDToken)
		} else {
			fmt.Println("Authorization: Bearer", tok.AccessToken)
		}
	} else {
		if c.Bool("bare") {
			if c.Bool("oidc") {
				fmt.Println(tok.IDToken)
			} else {
				fmt.Println(tok.AccessToken)
			}
		} else {
			b, err := json.MarshalIndent(tok, "", "  ")
			if err != nil {
				return errors.Wrapf(err, "error marshaling token data")
			}
			fmt.Println(string(b))
		}
	}

	return nil
}

type options struct {
	Provider string
	Email    string
	Console  bool
	Implicit bool
}

// Validate validates the options.
func (o *options) Validate() error {
	if o.Provider != "google" && !strings.HasPrefix(o.Provider, "https://") {
		return errors.New("Use a valid provider: google")
	}
	return nil
}

type oauth struct {
	provider         string
	clientID         string
	clientSecret     string
	scope            string
	loginHint        string
	redirectURI      string
	tokenEndpoint    string
	authzEndpoint    string
	userInfoEndpoint string // For testing
	state            string
	codeChallenge    string
	nonce            string
	implicit         bool
	errCh            chan error
	tokCh            chan *token
}

func newOauth(provider, clientID, clientSecret, authzEp, tokenEp, scope string, opts *options) (*oauth, error) {
	state, err := randutil.Alphanumeric(32)
	if err != nil {
		return nil, err
	}

	challenge, err := randutil.Alphanumeric(64)
	if err != nil {
		return nil, err
	}

	nonce, err := randutil.Hex(64) // 256 bits
	if err != nil {
		return nil, err
	}

	switch provider {
	case "google":
		return &oauth{
			provider:         provider,
			clientID:         clientID,
			clientSecret:     clientSecret,
			scope:            scope,
			authzEndpoint:    "https://accounts.google.com/o/oauth2/v2/auth",
			tokenEndpoint:    "https://www.googleapis.com/oauth2/v4/token",
			userInfoEndpoint: "https://www.googleapis.com/oauth2/v3/userinfo",
			loginHint:        opts.Email,
			state:            state,
			codeChallenge:    challenge,
			nonce:            nonce,
			implicit:         opts.Implicit,
			errCh:            make(chan error),
			tokCh:            make(chan *token),
		}, nil
	default:
		userinfoEp := ""
		if authzEp == "" && tokenEp == "" {
			d, err := disco(provider)
			if err != nil {
				return nil, err
			}

			if _, ok := d["authorization_endpoint"]; !ok {
				return nil, errors.New("missing 'authorization_endpoint' in provider metadata")
			}
			if _, ok := d["token_endpoint"]; !ok {
				return nil, errors.New("missing 'token_endpoint' in provider metadata")
			}
			authzEp = d["authorization_endpoint"].(string)
			tokenEp = d["token_endpoint"].(string)
			userinfoEp = d["token_endpoint"].(string)
		}
		return &oauth{
			provider:         provider,
			clientID:         clientID,
			clientSecret:     clientSecret,
			scope:            scope,
			authzEndpoint:    authzEp,
			tokenEndpoint:    tokenEp,
			userInfoEndpoint: userinfoEp,
			loginHint:        opts.Email,
			state:            state,
			codeChallenge:    challenge,
			nonce:            nonce,
			implicit:         opts.Implicit,
			errCh:            make(chan error),
			tokCh:            make(chan *token),
		}, nil
	}
}

func disco(provider string) (map[string]interface{}, error) {
	url, err := url.Parse(provider)
	if err != nil {
		return nil, err
	}
	// TODO: OIDC and OAuth specify two different ways of constructing this
	// URL. This is the OIDC way. Probably want to try both. See
	// https://tools.ietf.org/html/rfc8414#section-5
	if strings.Index(url.Path, "/.well-known/openid-configuration") == -1 {
		url.Path = path.Join(url.Path, "/.well-known/openid-configuration")
	}
	resp, err := http.Get(url.String())
	if err != nil {
		return nil, errors.Wrapf(err, "error retrieving %s", url.String())
	}
	defer resp.Body.Close()
	b, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, errors.Wrapf(err, "error retrieving %s", url.String())
	}
	details := make(map[string]interface{})
	if err := json.Unmarshal(b, &details); err != nil {
		return nil, errors.Wrapf(err, "error reading %s: unsupported format", url.String())
	}
	return details, err
}

// DoLoopbackAuthorization performs the log in into the identity provider
// opening a browser and using a redirect_uri in a loopback IP address
// (http://127.0.0.1:port or http://[::1]:port).
func (o *oauth) DoLoopbackAuthorization() (*token, error) {
	srv := httptest.NewServer(o)
	o.redirectURI = srv.URL
	defer srv.Close()

	// Get auth url and open it in a browser
	authURL, err := o.Auth()
	if err != nil {
		return nil, err
	}

	if err := exec.OpenInBrowser(authURL); err != nil {
		fmt.Fprintln(os.Stderr, "Cannot open a web browser on your platform.")
		fmt.Fprintln(os.Stderr)
		fmt.Fprintln(os.Stderr, "Open a local web browser and visit:")
		fmt.Fprintln(os.Stderr)
		fmt.Fprintln(os.Stderr, authURL)
		fmt.Fprintln(os.Stderr)
	} else {
		fmt.Fprintln(os.Stderr, "Your default web browser has been opened to visit:")
		fmt.Fprintln(os.Stderr)
		fmt.Fprintln(os.Stderr, authURL)
		fmt.Fprintln(os.Stderr)
	}

	// Wait for response and return the token
	select {
	case tok := <-o.tokCh:
		return tok, nil
	case err := <-o.errCh:
		return nil, err
	case <-time.After(2 * time.Minute):
		return nil, errors.New("oauth command timed out, please try again")
	}
}

// DoManualAuthorization performs the log in into the identity provider
// allowing the user to open a browser on a different system and then entering
// the authorization code on the Step CLI.
func (o *oauth) DoManualAuthorization() (*token, error) {
	o.redirectURI = oobCallbackUrn
	authURL, err := o.Auth()
	if err != nil {
		return nil, err
	}

	fmt.Fprintln(os.Stderr, "Open a local web browser and visit:")
	fmt.Fprintln(os.Stderr)
	fmt.Fprintln(os.Stderr, authURL)
	fmt.Fprintln(os.Stderr)

	// Read from the command line
	fmt.Fprint(os.Stderr, "Enter verification code: ")
	reader := bufio.NewReader(os.Stdin)
	code, err := reader.ReadString('\n')
	if err != nil {
		return nil, errors.WithStack(err)
	}

	tok, err := o.Exchange(o.tokenEndpoint, code)
	if err != nil {
		return nil, err
	}
	if tok.Err != "" || tok.ErrDesc != "" {
		return nil, errors.Errorf("Error exchanging authorization code: %s. %s", tok.Err, tok.ErrDesc)
	}
	return tok, nil
}

// DoTwoLeggedAuthorization performs two-legged OAuth using the jwt-bearer
// grant type.
func (o *oauth) DoTwoLeggedAuthorization(issuer string) (*token, error) {
	pemBytes := []byte(o.clientSecret)
	block, _ := pem.Decode(pemBytes)
	if block == nil {
		return nil, fmt.Errorf("failed to read private key pem block")
	}
	priv, err := x509.ParsePKCS8PrivateKey(block.Bytes)
	if err != nil {
		return nil, errors.Wrap(err, "error parsing private key")
	}

	// Add claims
	now := int(time.Now().Unix())
	c := map[string]interface{}{
		"aud":   o.tokenEndpoint,
		"nbf":   now,
		"iat":   now,
		"exp":   now + 3600,
		"iss":   issuer,
		"scope": o.scope,
	}

	so := new(jose.SignerOptions)
	so.WithType("JWT")
	so.WithHeader("kid", o.clientID)

	// Sign JWT
	signer, err := jose.NewSigner(jose.SigningKey{
		Algorithm: "RS256",
		Key:       priv,
	}, so)
	if err != nil {
		return nil, errors.Wrapf(err, "error creating JWT signer")
	}

	raw, err := jose.Signed(signer).Claims(c).CompactSerialize()
	if err != nil {
		return nil, errors.Wrapf(err, "error serializing JWT")
	}

	// Construct the POST request to fetch the OAuth token.
	params := url.Values{
		"assertion":  []string{string(raw)},
		"grant_type": []string{jwtBearerUrn},
	}

	// Send the POST request and return token.
	resp, err := http.PostForm(o.tokenEndpoint, params)
	if err != nil {
		return nil, errors.Wrapf(err, "error from token endpoint")
	}
	defer resp.Body.Close()

	var tok token
	if err := json.NewDecoder(resp.Body).Decode(&tok); err != nil {
		return nil, errors.WithStack(err)
	}

	return &tok, nil
}

// DoJWTAuthorization generates a JWT instead of an OAuth token. Only works for
// certain APIs. See https://developers.google.com/identity/protocols/OAuth2ServiceAccount#jwt-auth.
func (o *oauth) DoJWTAuthorization(issuer, aud string) (*token, error) {
	pemBytes := []byte(o.clientSecret)
	block, _ := pem.Decode(pemBytes)
	if block == nil {
		return nil, fmt.Errorf("failed to read private key pem block")
	}
	priv, err := x509.ParsePKCS8PrivateKey(block.Bytes)
	if err != nil {
		return nil, errors.Wrap(err, "error parsing private key")
	}

	// Add claims
	now := int(time.Now().Unix())
	c := map[string]interface{}{
		"aud": aud,
		"nbf": now,
		"iat": now,
		"exp": now + 3600,
		"iss": issuer,
		"sub": issuer,
	}

	so := new(jose.SignerOptions)
	so.WithType("JWT")
	so.WithHeader("kid", o.clientID)

	// Sign JWT
	signer, err := jose.NewSigner(jose.SigningKey{
		Algorithm: "RS256",
		Key:       priv,
	}, so)
	if err != nil {
		return nil, errors.Wrapf(err, "error creating JWT signer")
	}

	raw, err := jose.Signed(signer).Claims(c).CompactSerialize()
	if err != nil {
		return nil, errors.Wrapf(err, "error serializing JWT")
	}

	tok := &token{string(raw), "", "", 3600, "Bearer", "", ""}
	return tok, nil
}

// ServeHTTP is the handler that performs the OAuth 2.0 dance and returns the
// tokens using channels.
func (o *oauth) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if req.URL.Path != "/" {
		http.NotFound(w, req)
		return
	}

	q := req.URL.Query()
	errStr := q.Get("error")
	if errStr != "" {
		o.badRequest(w, "Failed to authenticate: "+errStr)
		return
	}

	if o.implicit {
		o.implicitHandler(w, req)
		return
	}

	code, state := q.Get("code"), q.Get("state")
	if code == "" || state == "" {
		fmt.Fprintf(os.Stderr, "Invalid request received: http://%s%s\n", req.RemoteAddr, req.URL.String())
		fmt.Fprintf(os.Stderr, "You may have an app or browser plugin that needs to be turned off\n")
		http.Error(w, "400 bad request", http.StatusBadRequest)
		return
	}

	if code == "" {
		o.badRequest(w, "Failed to authenticate: missing or invalid code")
		return
	}

	if state == "" || state != o.state {
		o.badRequest(w, "Failed to authenticate: missing or invalid state")
		return
	}

	tok, err := o.Exchange(o.tokenEndpoint, code)
	if err != nil {
		o.badRequest(w, "Failed exchanging authorization code: "+err.Error())
		return
	}
	if tok.Err != "" || tok.ErrDesc != "" {
		o.badRequest(w, fmt.Sprintf("Failed exchanging authorization code: %s. %s", tok.Err, tok.ErrDesc))
		return
	}

	w.WriteHeader(http.StatusOK)
	w.Header().Add("Content-Type", "text/plain; charset=utf-8")
	w.Write([]byte(`<html><head><title>OAuth Request Successful</title>`))
	w.Write([]byte(`</head><body><p style='font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"; font-size: 22px; color: #333; width: 400px; margin: 0 auto; text-align: center; line-height: 1.7; padding: 20px;'>`))
	w.Write([]byte(`<strong style='font-size: 28px; color: #000;'>Success</strong><br />Look for the token on the command line`))
	w.Write([]byte(`</p></body></html>`))
	o.tokCh <- tok
}

func (o *oauth) implicitHandler(w http.ResponseWriter, req *http.Request) {
	q := req.URL.Query()
	if hash := q.Get("urlhash"); hash == "true" {
		state := q.Get("state")
		if state == "" || state != o.state {
			o.badRequest(w, "Failed to authenticate: missing or invalid state")
			return
		}
		accessToken := q.Get("access_token")
		if accessToken == "" {
			o.badRequest(w, "Failed to authenticate: missing access token")
			return
		}

		w.WriteHeader(http.StatusOK)
		w.Header().Add("Content-Type", "text/plain; charset=utf-8")
		w.Write([]byte(`<html><head><title>OAuth Request Successful</title>`))
		w.Write([]byte(`</head><body><p style='font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"; font-size: 22px; color: #333; width: 400px; margin: 0 auto; text-align: center; line-height: 1.7; padding: 20px;'>`))
		w.Write([]byte(`<strong style='font-size: 28px; color: #000;'>Success</strong><br />Look for the token on the command line`))
		w.Write([]byte(`</p></body></html>`))

		expiresIn, _ := strconv.Atoi(q.Get("expires_in"))
		o.tokCh <- &token{
			AccessToken:  accessToken,
			IDToken:      q.Get("id_token"),
			RefreshToken: q.Get("refresh_token"),
			ExpiresIn:    expiresIn,
			TokenType:    q.Get("token_type"),
		}
		return
	}

	w.WriteHeader(http.StatusOK)
	w.Header().Add("Content-Type", "text/html; charset=utf-8")
	w.Write([]byte(`<html><head><title>Processing OAuth Request</title>`))
	w.Write([]byte(`</head>`))
	w.Write([]byte(`<script type="text/javascript">`))
	w.Write([]byte(fmt.Sprintf(`function redirect(){var hash = window.location.hash.substr(1); document.location.href = "%s?urlhash=true&"+hash;}`, o.redirectURI)))
	w.Write([]byte(`if (window.addEventListener) window.addEventListener("load", redirect, false); else if (window.attachEvent) window.attachEvent("onload", redirect); else window.onload = redirect;`))
	w.Write([]byte("</script>"))
	w.Write([]byte(`<body><p style='font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"; font-size: 22px; color: #333; width: 400px; margin: 0 auto; text-align: center; line-height: 1.7; padding: 20px;'>`))
	w.Write([]byte(`<strong style='font-size: 28px; color: #000;'>Success</strong><br />`))
	w.Write([]byte(`Click <a href="javascript:redirect();">here</a> if your browser does not automatically redirect you`))
	w.Write([]byte(`</p></body></html>`))
	return
}

// Auth returns the OAuth 2.0 authentication url.
func (o *oauth) Auth() (string, error) {
	u, err := url.Parse(o.authzEndpoint)
	if err != nil {
		return "", errors.WithStack(err)
	}

	q := u.Query()
	q.Add("client_id", o.clientID)
	q.Add("redirect_uri", o.redirectURI)
	if o.implicit {
		q.Add("response_type", "id_token token")
	} else {
		q.Add("response_type", "code")
		q.Add("code_challenge_method", "S256")
		s256 := sha256.Sum256([]byte(o.codeChallenge))
		q.Add("code_challenge", base64.RawURLEncoding.EncodeToString(s256[:]))
	}
	q.Add("scope", o.scope)
	q.Add("state", o.state)
	q.Add("nonce", o.nonce)
	if o.loginHint != "" {
		q.Add("login_hint", o.loginHint)
	}
	u.RawQuery = q.Encode()
	return u.String(), nil
}

// Exchange exchanges the authorization code for refresh and access tokens.
func (o *oauth) Exchange(tokenEndpoint, code string) (*token, error) {
	data := url.Values{}
	data.Set("code", code)
	data.Set("client_id", o.clientID)
	data.Set("client_secret", o.clientSecret)
	data.Set("redirect_uri", o.redirectURI)
	data.Set("grant_type", "authorization_code")
	data.Set("code_verifier", o.codeChallenge)

	resp, err := http.PostForm(tokenEndpoint, data)
	if err != nil {
		return nil, errors.WithStack(err)
	}
	defer resp.Body.Close()

	var tok token
	if err := json.NewDecoder(resp.Body).Decode(&tok); err != nil {
		return nil, errors.WithStack(err)
	}

	return &tok, nil
}

func (o *oauth) badRequest(w http.ResponseWriter, msg string) {
	w.WriteHeader(http.StatusBadRequest)
	w.Header().Add("Content-Type", "text/plain; charset=utf-8")
	w.Write([]byte(`<html><head><title>OAuth Request Unsuccessful</title>`))
	w.Write([]byte(`</head><body><p style='font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"; font-size: 22px; color: #333; width: 400px; margin: 0 auto; text-align: center; line-height: 1.7; padding: 20px;'>`))
	w.Write([]byte(`<strong style='font-size: 28px; color: red;'>Failure</strong><br />`))
	w.Write([]byte(msg))
	w.Write([]byte(`</p></body></html>`))
	o.errCh <- errors.New(msg)
}