example of using JWT for http authentication in go

asymmetric.go

// SPDX-FileCopyrightText: 2021 Henry Bubert <[email protected]>
//
// SPDX-License-Identifier: MIT

package main

// using asymmetric crypto/RSA keys

import (
	"crypto/rsa"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"

	jwt "github.com/dgrijalva/jwt-go"
)

// location of the files used for signing and verification
const (
	privKeyPath = "keys/app.rsa"     // openssl genrsa -out app.rsa keysize
	pubKeyPath  = "keys/app.rsa.pub" // openssl rsa -in app.rsa -pubout > app.rsa.pub
)

// keys are held in global variables
// i havn't seen a memory corruption/info leakage in go yet
// but maybe it's a better idea, just to store the public key in ram?
// and load the signKey on every signing request? depends on  your usage i guess
var (
	verifyKey *rsa.PublicKey
	signKey   *rsa.PrivateKey
)

// read the key files before starting http handlers
func init() {
	signBytes, err := ioutil.ReadFile(privKeyPath)
	fatal(err)

	signKey, err = jwt.ParseRSAPrivateKeyFromPEM(signBytes)
	fatal(err)

	verifyBytes, err := ioutil.ReadFile(pubKeyPath)
	fatal(err)

	verifyKey, err = jwt.ParseRSAPublicKeyFromPEM(verifyBytes)
	fatal(err)
}

func fatal(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

// just some html, to lazy for http.FileServer()
const (
	tokenName = "AccessToken"

	landingHtml = `<h2>Welcome to the JWT Test</h2>

<a href="/restricted">fun area</a>

<form action="/authenticate" method="POST">
	<input type="text" name="user">
	<input type="password" name="pass">
	<input type="submit">
</form>`

	successHtml    = `<h2>Token Set - have fun!</h2><p>Go <a href="/">Back...</a></p>`
	restrictedHtml = `<h1>Welcome!!</h1><img src="https://httpcats.herokuapp.com/200" alt="" />`
)

// serves the form and restricted link
func landingHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	w.WriteHeader(http.StatusOK)
	fmt.Fprint(w, landingHtml)
}

type UserInfo struct {
	Name, Kind string
}

type AccessClaims struct {
	Access   string
	UserInfo UserInfo
	jwt.StandardClaims
}

// reads the form values, checks them and creates the token
func authHandler(w http.ResponseWriter, r *http.Request) {
	// make sure its post
	if r.Method != "POST" {
		w.WriteHeader(http.StatusBadRequest)
		fmt.Fprintln(w, "No POST", r.Method)
		return
	}

	user := r.FormValue("user")
	pass := r.FormValue("pass")

	log.Printf("Authenticate: user[%s] pass[%s]\n", user, pass)

	// check values
	if user != "test" || pass != "known" {
		w.WriteHeader(http.StatusForbidden)
		fmt.Fprintln(w, "Wrong info")
		return
	}

	// set our claims
	claims := AccessClaims{
		Access:   "level1",
		UserInfo: UserInfo{user, "human"},
		StandardClaims: jwt.StandardClaims{
			ExpiresAt: 15000,
			Issuer:    "asym-example",
		},
	}

	// create a signer for rsa 256
	t := jwt.NewWithClaims(jwt.GetSigningMethod("RS256"), claims)

	tokenString, err := t.SignedString(signKey)
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		fmt.Fprintln(w, "Sorry, error while Signing Token!")
		log.Printf("Token Signing error: %v\n", err)
		return
	}

	// i know using cookies to store the token isn't really helpfull for cross domain api usage
	// but it's just an example and i did not want to involve javascript
	http.SetCookie(w, &http.Cookie{
		Name:       tokenName,
		Value:      tokenString,
		Path:       "/",
		RawExpires: "0",
	})

	w.Header().Set("Content-Type", "text/html")
	w.WriteHeader(http.StatusOK)
	fmt.Fprintln(w, successHtml)
}

// only accessible with a valid token
func restrictedHandler(w http.ResponseWriter, r *http.Request) {
	// check if we have a cookie with out tokenName
	tokenCookie, err := r.Cookie(tokenName)
	switch {
	case err == http.ErrNoCookie:
		w.WriteHeader(http.StatusUnauthorized)
		fmt.Fprintln(w, "No Token, no fun!")
		return
	case err != nil:
		w.WriteHeader(http.StatusInternalServerError)
		fmt.Fprintln(w, "Error while Parsing cookie!")
		log.Printf("Cookie parse error: %v\n", err)
		return
	}

	// just for the lulz, check if it is empty.. should fail on Parse anyway..
	if tokenCookie.Value == "" {
		w.WriteHeader(http.StatusUnauthorized)
		fmt.Fprintln(w, "No Token, no fun!")
		return
	}

	// validate the token
	token, err := jwt.Parse(tokenCookie.Value, func(token *jwt.Token) (interface{}, error) {
		// since we only use the one private key to sign the tokens,
		// we also only use its public counter part to verify
		return verifyKey, nil
	})

	// branch out into the possible error from signing
	switch err.(type) {

	case nil: // no error

		if !token.Valid { // but may still be invalid
			w.WriteHeader(http.StatusUnauthorized)
			fmt.Fprintln(w, "WHAT? Invalid Token? F*** off!")
			return
		}

		// see stdout and watch for the CustomUserInfo, nicely unmarshalled
		log.Printf("Someone accessed resricted area! Token:%+v\n", token)
		w.Header().Set("Content-Type", "text/html")
		w.WriteHeader(http.StatusOK)
		fmt.Fprintln(w, restrictedHtml)

	case *jwt.ValidationError: // something was wrong during the validation
		vErr := err.(*jwt.ValidationError)

		switch vErr.Errors {
		case jwt.ValidationErrorExpired:
			w.WriteHeader(http.StatusUnauthorized)
			fmt.Fprintln(w, "Token Expired, get a new one.")
			return

		default:
			w.WriteHeader(http.StatusInternalServerError)
			fmt.Fprintln(w, "Error while Parsing Token!")
			log.Printf("ValidationError error: %+v\n", vErr.Errors)
			return
		}

	default: // something else went wrong
		w.WriteHeader(http.StatusInternalServerError)
		fmt.Fprintln(w, "Error while Parsing Token!")
		log.Printf("Token parse error: %v\n", err)
		return
	}

}

// setup the handlers and start listening to requests
func main() {

	http.HandleFunc("/", landingHandler)
	http.HandleFunc("/authenticate", authHandler)
	http.HandleFunc("/restricted", restrictedHandler)

	log.Println("Listening...")
	fatal(http.ListenAndServe(":8080", nil))
}

go.mod

// SPDX-FileCopyrightText: 2021 Henry Bubert <[email protected]>
//
// SPDX-License-Identifier: MIT

module gist.github.com/cryptix/45c33ecf0ae54828e63b

go 1.17

require github.com/dgrijalva/jwt-go v3.2.0+incompatible

go.sum

github.com/dgrijalva/jwt-go v3.2.0+incompatible h1:7qlOGliEKZXTDg6OTjfoBKDXWrumCAMpl/TFQ4/5kLM=
github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=

LICENSE

MIT License

Copyright (c) <year> <copyright holders>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

First of all, thank you very much this helped me a lot.
Just wanna share about a problem that I had trying to use the package on windows. I got stuck in an error "Invalid Key: Key must be PEM encoded PKCS1 vs PKCS8 private key". I solved by using git bash instead of PowerShell to generate do keys.

Hey thanks! gotta try this one

Thanks for this, although am having issues using the SignedKey to create a token string, its failing fo a reason i cant determine for now, but then, How do u validate the expiration of the token??? Thanks again.

the comment specifying how to generate rsa keys:
openssl rsa -in app.rsa -pubout > app.rsa.pub - i believe should be openssl rsa -in app.rsa -pubout -out app.rsa.pub (according to openssl.org)

Great stuff and thank you for posting this.
Do you have a license that you wish to apply to this code?
Thank you

@hrieke updated

ps: also fixed the claims creation via NewWithClaims and jwt.StandardClaims