Golang JWT token verification with jose-jwt

main.go

package main

import (
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"io/ioutil"
	"log"
	"os"

	"gopkg.in/square/go-jose.v2"
	"gopkg.in/square/go-jose.v2/jwt"
)

func main() {

	tokenString, err := ioutil.ReadFile("accesstoken.txt")
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error opening accesstoken.txt\n")
	}

	certBuffer, err := ioutil.ReadFile("cert.pem")
	if err != nil {
		fmt.Fprintf(os.Stderr, "Error opening cert.pem\n")
	}

	publicKey, err := LoadPublicKey(certBuffer)
	if err != nil {
		log.Fatalf("failed to load PEM :%+v", err)
	}

	parsed, err := jwt.ParseSigned(string(tokenString))
	if err != nil {
		log.Fatalf("failed to parse JWT:%+v", err)
	}

	allClaims := make(map[string]interface{})
	err = parsed.Claims(publicKey, &allClaims)
	if err != nil {
		log.Fatalf("Failed to get claims JWT: %+v", err)
	}

	fmt.Print(allClaims["exp"])

}

// LoadPublicKey is a function from jose-util
func LoadPublicKey(data []byte) (interface{}, error) {
	input := data

	block, _ := pem.Decode(data)
	if block != nil {
		input = block.Bytes
	}

	// Try to load SubjectPublicKeyInfo
	pub, err0 := x509.ParsePKIXPublicKey(input)
	if err0 == nil {
		return pub, nil
	}

	cert, err1 := x509.ParseCertificate(input)
	if err1 == nil {
		return cert.PublicKey, nil
	}

	jwk, err2 := LoadJSONWebKey(data, true)
	if err2 == nil {
		return jwk, nil
	}

	return nil, fmt.Errorf("square/go-jose: parse error, got '%s', '%s' and '%s'", err0, err1, err2)
}

// LoadJSONWebKey is another function from jose-util
func LoadJSONWebKey(json []byte, pub bool) (*jose.JSONWebKey, error) {
	var jwk jose.JSONWebKey
	err := jwk.UnmarshalJSON(json)
	if err != nil {
		return nil, err
	}
	if !jwk.Valid() {
		return nil, errors.New("invalid JWK key")
	}
	if jwk.IsPublic() != pub {
		return nil, errors.New("priv/pub JWK key mismatch")
	}
	return &jwk, nil
}