gorilla/securecookie

// Hash keys should be at least 32 bytes long
var hashKey = []byte("very-secret")
// Block keys should be 16 bytes (AES-128) or 32 bytes (AES-256) long.
// Shorter keys may weaken the encryption used.
var blockKey = []byte("a-lot-secret")
var s = securecookie.New(hashKey, blockKey)

func SetCookieHandler(w http.ResponseWriter, r *http.Request) {
	value := map[string]string{
		"foo": "bar",
	}
	if encoded, err := s.Encode("cookie-name", value); err == nil {
		cookie := &http.Cookie{
			Name:  "cookie-name",
			Value: encoded,
			Path:  "/",
			Secure: true,
			HttpOnly: true,
		}
		http.SetCookie(w, cookie)
	}
}

func ReadCookieHandler(w http.ResponseWriter, r *http.Request) {
	if cookie, err := r.Cookie("cookie-name"); err == nil {
		value := make(map[string]string)
		if err = s2.Decode("cookie-name", cookie.Value, &value); err == nil {
			fmt.Fprintf(w, "The value of foo is %q", value["foo"])
		}
	}
}

// keys stored in a map will not be persisted between restarts
// a more persistent storage should be considered for production applications.
var cookies = map[string]*securecookie.SecureCookie{
	"previous": securecookie.New(
		securecookie.GenerateRandomKey(64),
		securecookie.GenerateRandomKey(32),
	),
	"current": securecookie.New(
		securecookie.GenerateRandomKey(64),
		securecookie.GenerateRandomKey(32),
	),
}

func SetCookieHandler(w http.ResponseWriter, r *http.Request) {
	value := map[string]string{
		"foo": "bar",
	}
	if encoded, err := securecookie.EncodeMulti("cookie-name", value, cookies["current"]); err == nil {
		cookie := &http.Cookie{
			Name:  "cookie-name",
			Value: encoded,
			Path:  "/",
		}
		http.SetCookie(w, cookie)
	}
}

func ReadCookieHandler(w http.ResponseWriter, r *http.Request) {
	if cookie, err := r.Cookie("cookie-name"); err == nil {
		value := make(map[string]string)
		err = securecookie.DecodeMulti("cookie-name", cookie.Value, &value, cookies["current"], cookies["previous"])
		if err == nil {
			fmt.Fprintf(w, "The value of foo is %q", value["foo"])
		}
	}
}

func Rotate(newCookie *securecookie.SecureCookie) {
	cookies["previous"] = cookies["current"]
	cookies["current"] = newCookie
}