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package main

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"strings"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
	"github.com/peterbourgon/ff/v3"
	"golang.org/x/oauth2"
)

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)
}

type args struct {
	clientProvider string
	clientID       string
	clientSecret   string
	port           int
	scopes         string
}

func main() {
	var args args
	fs := flag.NewFlagSet("oidc", flag.ExitOnError)
	fs.StringVar(&args.clientProvider, "client-provider", "", "Client provider (e.g. https://accounts.google.com)")
	fs.StringVar(&args.clientID, "client-id", "", "Client ID")
	fs.StringVar(&args.clientSecret, "client-secret", "", "Client secret")
	fs.IntVar(&args.port, "port", 8000, "Port to run on")
	fs.StringVar(&args.scopes, "scopes", "profile,email", "Comma-delimited scopes")
	fs.String("config", ".env", "Env config")
	if err := ff.Parse(fs, os.Args[1:],
		ff.WithEnvVarPrefix("OIDC"),
		ff.WithConfigFileFlag("config"),
		ff.WithAllowMissingConfigFile(true),
		ff.WithConfigFileParser(ff.EnvParser),
	); err != nil {
		log.Fatal(err)
	}
	ctx := context.Background()
	scopes := strings.Split(args.scopes, ",")

	provider, err := oidc.NewProvider(ctx, args.clientProvider)
	if err != nil {
		log.Fatal(err)
	}
	oidcConfig := &oidc.Config{
		ClientID: args.clientID,
	}
	verifier := provider.Verifier(oidcConfig)

	config := oauth2.Config{
		ClientID:     args.clientID,
		ClientSecret: args.clientSecret,
		Endpoint:     provider.Endpoint(),
		RedirectURL:  fmt.Sprintf("http://localhost:%d/callback", args.port),
		Scopes:       append([]string{oidc.ScopeOpenID}, scopes...),
	}

	mux := http.NewServeMux()
	mux.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)
	})

	mux.HandleFunc("/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
		}
		idToken, err := verifier.Verify(ctx, rawIDToken)
		if err != nil {
			http.Error(w, "Failed to verify ID Token: "+err.Error(), http.StatusInternalServerError)
			return
		}

		nonce, err := r.Cookie("nonce")
		if err != nil {
			http.Error(w, "nonce not found", http.StatusBadRequest)
			return
		}
		if idToken.Nonce != nonce.Value {
			http.Error(w, "nonce did not match", http.StatusBadRequest)
			return
		}

		userInfo, err := provider.UserInfo(ctx, oauth2.StaticTokenSource(oauth2Token))
		if err != nil {
			http.Error(w, "Failed to get userinfo: "+err.Error(), http.StatusInternalServerError)
			return
		}

		resp := struct {
			OAuth2Token   *oauth2.Token    `json:"oauth2_token"`
			IDTokenClaims *json.RawMessage `json:"claims"`
			UserInfo      *oidc.UserInfo   `json:"user_info"`
		}{
			OAuth2Token:   oauth2Token,
			IDTokenClaims: new(json.RawMessage),
			UserInfo:      userInfo,
		}

		if err := idToken.Claims(&resp.IDTokenClaims); err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}
		w.Header().Set("Content-Type", "application/json")
		enc := json.NewEncoder(w)
		enc.SetIndent("", "\t")
		if err := enc.Encode(resp); err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
		}
	})

	bind := fmt.Sprintf(":%d", args.port)
	log.Println("listening on http://localhost" + bind)
	log.Fatal(http.ListenAndServe(bind, mux))
}