decode_jwt.go

package main

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
	"os"
	"strings"
)

func main() {
	if len(os.Args) != 2 {
		reportError(fmt.Sprintf("Usage: %s TOKEN", os.Args[0]))
	}

	jwt := os.Args[1]
	parts := strings.Split(jwt, ".")
	if len(parts) != 3 {
		reportError("not JWT string")
	}

	rawHeader := parts[0]
	rawBody := parts[1]

	headerJson, err := base64.RawURLEncoding.DecodeString(rawHeader)
	if err != nil {
		reportError(err.Error())
	}

	bodyJson, err := base64.RawURLEncoding.DecodeString(rawBody)
	if err != nil {
		reportError(err.Error())
	}

	var header interface{}
	err = json.Unmarshal(headerJson, &header)
	if err != nil {
		reportError(err.Error())
	}
	headerStr, err := json.MarshalIndent(header, "", "  ")
	if err != nil {
		reportError(err.Error())
	}

	var body interface{}
	err = json.Unmarshal(bodyJson, &body)
	if err != nil {
		reportError(err.Error())
	}
	bodyStr, err := json.MarshalIndent(body, "", "  ")
	if err != nil {
		reportError(err.Error())
	}

	fmt.Println(string(headerStr))
	fmt.Println(string(bodyStr))
}

func reportError(msg string) {
	fmt.Printf("Invalid JWT: %s\n", msg)
	os.Exit(-1)
}