Golang AWS Cognito Validate JWT token

auth.go

package auth

import (
	"crypto/rsa"
	"encoding/base64"
	"encoding/binary"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"math/big"
	"net/http"

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

// Auth ...
type Auth struct {
	jwk               *JWK
	jwkURL            string
	cognitoRegion     string
	cognitoUserPoolID string
}

// Config ...
type Config struct {
	CognitoRegion     string
	CognitoUserPoolID string
}

// JWK ...
type JWK struct {
	Keys []struct {
		Alg string `json:"alg"`
		E   string `json:"e"`
		Kid string `json:"kid"`
		Kty string `json:"kty"`
		N   string `json:"n"`
	} `json:"keys"`
}

// NewAuth ...
func NewAuth(config *Config) *Auth {
	a := &Auth{
		cognitoRegion:     config.CognitoRegion,
		cognitoUserPoolID: config.CognitoUserPoolID,
	}

	a.jwkURL = fmt.Sprintf("https://cognito-idp.%s.amazonaws.com/%s/.well-known/jwks.json", a.cognitoRegion, a.cognitoUserPoolID)
	err := a.CacheJWK()
	if err != nil {
		log.Fatal(err)
	}

	return a
}

// CacheJWK ...
func (a *Auth) CacheJWK() error {
	req, err := http.NewRequest("GET", a.jwkURL, nil)
	if err != nil {
		return err
	}

	req.Header.Add("Accept", "application/json")
	client := &http.Client{}
	resp, err := client.Do(req)
	if err != nil {
		return err
	}

	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return err
	}

	jwk := new(JWK)
	err = json.Unmarshal(body, jwk)
	if err != nil {
		return err
	}

	a.jwk = jwk
	return nil
}

// ParseJWT ...
func (a *Auth) ParseJWT(tokenString string) (*jwt.Token, error) {
	token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
		key := convertKey(a.jwk.Keys[1].E, a.jwk.Keys[1].N)
		return key, nil
	})
	if err != nil {
		return token, err
	}

	return token, nil
}

// JWK ...
func (a *Auth) JWK() *JWK {
	return a.jwk
}

// JWKURL ...
func (a *Auth) JWKURL() string {
	return a.jwkURL
}

// https://gist.github.com/MathieuMailhos/361f24316d2de29e8d41e808e0071b13
func convertKey(rawE, rawN string) *rsa.PublicKey {
	decodedE, err := base64.RawURLEncoding.DecodeString(rawE)
	if err != nil {
		panic(err)
	}
	if len(decodedE) < 4 {
		ndata := make([]byte, 4)
		copy(ndata[4-len(decodedE):], decodedE)
		decodedE = ndata
	}
	pubKey := &rsa.PublicKey{
		N: &big.Int{},
		E: int(binary.BigEndian.Uint32(decodedE[:])),
	}
	decodedN, err := base64.RawURLEncoding.DecodeString(rawN)
	if err != nil {
		panic(err)
	}
	pubKey.N.SetBytes(decodedN)
	return pubKey
}

auth_test.go

package auth

import (
	"os"
	"testing"
)

func TestCacheJWT(t *testing.T) {
	if !(os.Getenv("AWS_COGNITO_USER_POOL_ID") != "" && os.Getenv("AWS_COGNITO_REGION") != "") {
		t.Skip("requires AWS Cognito environment variables")
	}

	auth := NewAuth(&Config{
		CognitoRegion:     os.Getenv("AWS_COGNITO_REGION"),
		CognitoUserPoolID: os.Getenv("AWS_COGNITO_USER_POOL_ID"),
	})

	err := auth.CacheJWK()
	if err != nil {
		t.Error(err)
	}

	jwt := "eyJraWQiOiJlS3lvdytnb1wvXC9yWmtkbGFhRFNOM25jTTREd0xTdFhibks4TTB5b211aE09IiwiYWxnIjoiUlMyNTYifQ.eyJzdWIiOiJjMTcxOGY3OC00ODY5LTRmMmEtYTk2ZS1lYmEwYmJkY2RkMjEiLCJldmVudF9pZCI6IjZmYWMyZGNjLTJlMzUtMTFlOS05NDZjLTZiZDI0YmRlZjFiNiIsInRva2VuX3VzZSI6ImFjY2VzcyIsInNjb3BlIjoiYXdzLmNvZ25pdG8uc2lnbmluLnVzZXIuYWRtaW4iLCJhdXRoX3RpbWUiOjE1NDk5MTQyNjUsImlzcyI6Imh0dHBzOlwvXC9jb2duaXRvLWlkcC51cy13ZXN0LTIuYW1hem9uYXdzLmNvbVwvdXMtd2VzdC0yX0wwVldGSEVueSIsImV4cCI6MTU0OTkxNzg2NSwiaWF0IjoxNTQ5OTE0MjY1LCJqdGkiOiIzMTg0MDdkMC0zZDNhLTQ0NDItOTMyYy1lY2I0MjQ2MzRiYjIiLCJjbGllbnRfaWQiOiI2ZjFzcGI2MzZwdG4wNzRvbjBwZGpnbms4bCIsInVzZXJuYW1lIjoiYzE3MThmNzgtNDg2OS00ZjJhLWE5NmUtZWJhMGJiZGNkZDIxIn0.rJl9mdCrw_lertWhC5RiJcfhRP-xwTYkPLPXmi_NQEO-LtIJ-kwVEvUaZsPnBXku3bWBM3V35jdJloiXclbffl4SDLVkkvU9vzXDETAMaZEzOY1gDVcg4YzNNR4H5kHnl-G-XiN5MajgaWbjohDHTvbPnqgW7e_4qNVXueZv2qfQ8hZ_VcyniNxMGaui-C0_YuR6jdH-T14Wl59Cyf-UFEyli1NZFlmpUQ8QODGMUI12PVFOZiHJIOZ3CQM_Xs-TlRy53RlKGFzf6RQfRm57rJw_zLyJHHnB8DZgbdCRfhNsqZka7ZZUUAlS9aMzdmSc3pPFSJ-hH3p8eFAgB4E71g"

	token, err := auth.ParseJWT(jwt)
	if err != nil {
		t.Error(err)
	}

	if !token.Valid {
		t.Fail()
	}
}

@miguelmota Thanks. This has been very helpful.