Get ID token thru OpenID Connect

gistfile1.txt

package main

import (
	"crypto/rand"
	"encoding/base64"
	"io"
	"log"
	"net/http"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
	"golang.org/x/net/context"
	"golang.org/x/oauth2"
)

var (
	clientID     = "xx"
	clientSecret = "xxx"
	issuerURL    = "https://example.com"
)

func randString(nByte int) (string, error) {
	b := make([]byte, nByte)
	if _, err := io.ReadFull(rand.Reader, b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

func setCallbackCookie(w http.ResponseWriter, r *http.Request, name, value string) {
	c := &http.Cookie{
		Name:     name,
		Value:    value,
		MaxAge:   int(time.Hour.Seconds()),
		Secure:   r.TLS != nil,
		HttpOnly: true,
	}
	http.SetCookie(w, c)
}

func main() {
	ctx := context.Background()

	provider, err := oidc.NewProvider(ctx, issuerURL)
	if err != nil {
		log.Fatal(err)
	}

	config := oauth2.Config{
		ClientID:     clientID,
		ClientSecret: clientSecret,
		Endpoint:     provider.Endpoint(),
		RedirectURL:  "http://127.0.0.1:5556/auth/callback",
		Scopes:       []string{oidc.ScopeOpenID},
	}

	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		state, err := randString(16)
		if err != nil {
			http.Error(w, "Internal error", http.StatusInternalServerError)
			return
		}
		nonce, err := randString(16)
		if err != nil {
			http.Error(w, "Internal error", http.StatusInternalServerError)
			return
		}
		setCallbackCookie(w, r, "state", state)
		setCallbackCookie(w, r, "nonce", nonce)

		http.Redirect(w, r, config.AuthCodeURL(state, oidc.Nonce(nonce)), http.StatusFound)
	})

	http.HandleFunc("/auth/callback", func(w http.ResponseWriter, r *http.Request) {
		state, err := r.Cookie("state")
		if err != nil {
			http.Error(w, "state not found", http.StatusBadRequest)
			return
		}
		if r.URL.Query().Get("state") != state.Value {
			http.Error(w, "state did not match", http.StatusBadRequest)
			return
		}

		oauth2Token, err := config.Exchange(ctx, r.URL.Query().Get("code"))
		if err != nil {
			http.Error(w, "Failed to exchange token: "+err.Error(), http.StatusInternalServerError)
			return
		}
		rawIDToken, ok := oauth2Token.Extra("id_token").(string)
		if !ok {
			http.Error(w, "No id_token field in oauth2 token.", http.StatusInternalServerError)
			return
		}

		w.Write([]byte(rawIDToken))
	})

	log.Printf("listening on http://%s/", "127.0.0.1:5556")
	log.Fatal(http.ListenAndServe("127.0.0.1:5556", nil))
}