Golang HS256. An example to encode jwt with HS256 in golang.

hs256.go

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"fmt"
)

func HS256(data, key []byte) []byte {
	h := hmac.New(sha256.New, key)
	h.Write(data)
	return h.Sum(nil)
}

func main() {
	key, _ := hex.DecodeString("412b0d8f10fb2bb46ae06e869caf2c90")
	data := []byte("the message to sign")
	bytes := HS256(data, key)

	fmt.Println(base64.RawURLEncoding.EncodeToString(bytes))
	// gkb5qQ5MyYhNEyfUIsk0PquMRQz7h_wd4u4jpyBoSfg

	// JWT
	rawHeader := `{"alg":"HS256","typ":"JWT"}`
	rawPayload := `{"sub":"1234567890","name":"John Doe","iat":1516239022}`
	textSecret := "your-256-bit-secret"

	header := base64.RawURLEncoding.EncodeToString([]byte(rawHeader))
	payload := base64.RawURLEncoding.EncodeToString([]byte(rawPayload))
	signature := HS256(
		[]byte(fmt.Sprintf("%v.%v", header, payload)),
		[]byte(textSecret),
	)

	jwt := fmt.Sprintf(
		"%v.%v.%v",
		header,
		payload,
		base64.RawURLEncoding.EncodeToString(signature),
	)

	fmt.Println(jwt)
	// eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
}