jwt.go

package main

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
)

// JWTHeader ...
type JWTHeader struct {
	Algorithm string `json:"alg"`
	Type      string `json:"typ"`
}

// JWTPayload ...
type JWTPayload struct {
	Subject string `json:"sub"`
	Email   string `json:"email"`
}

func main() {
	key := []byte("123456789")
	jwt := JWTEncode(&JWTPayload{"ABCDEFGHIJKLMOPQRSTUVWXYZ", "[email protected]"}, key)
	fmt.Println(jwt)
	payload := JWTPayload{}
	JWTDecode(jwt, key, &payload)
}

var header, _ = json.Marshal(JWTHeader{"HS256", "JWT"})

// JWTEncode ...
func JWTEncode(payload interface{}, key []byte) []byte {
	marshalledPayload, _ := json.Marshal(payload)

	headerSize := base64.RawURLEncoding.EncodedLen(len(header))
	payloadSize := base64.RawURLEncoding.EncodedLen(len(marshalledPayload))
	signatureSize := base64.RawURLEncoding.EncodedLen(sha256.Size)

	jwt := make([]byte, headerSize+1+payloadSize+1+signatureSize)
	base64.RawURLEncoding.Encode(jwt, header)
	jwt[headerSize] = byte('.')
	base64.RawURLEncoding.Encode(jwt[headerSize+1:], marshalledPayload)
	jwt[headerSize+1+payloadSize] = byte('.')

	mac := hmac.New(sha256.New, key)
	mac.Write(jwt[:headerSize+1+payloadSize])
	base64.RawURLEncoding.Encode(jwt[headerSize+1+payloadSize+1:], mac.Sum(nil))

	return jwt
}

// JWTDecode ...
func JWTDecode(token, key []byte, object interface{}) error {
	headerSize := bytes.IndexRune(token, '.')
	payloadSize := bytes.IndexRune(token[headerSize+1:], '.')

	signature := make([]byte, sha256.Size)
	base64.RawURLEncoding.Decode(signature, token[headerSize+1+payloadSize+1:])

	mac := hmac.New(sha256.New, key)
	mac.Write(token[:headerSize+1+payloadSize])
	if !hmac.Equal(signature, mac.Sum(nil)) {
		return errors.New("invalid signature")
	}

	header := make([]byte, base64.RawURLEncoding.DecodedLen(headerSize))
	base64.RawURLEncoding.Decode(header, token[:headerSize])
	headerStruct := JWTHeader{}
	if err := json.Unmarshal(header, &headerStruct); err != nil {
		return err
	}

	payload := make([]byte, base64.RawURLEncoding.DecodedLen(payloadSize))
	base64.RawURLEncoding.Decode(payload, token[headerSize+1:headerSize+1+payloadSize])
	if err := json.Unmarshal(payload, object); err != nil {
		return err
	}

	return nil
}