// Copyright (C) 2021 Marius Schellenberger

package server

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"

	"git.giftfish.de/ston1th/jwt/v3"
)

const keySize = jwt.KeySize / 2

func genKey(size int) (b []byte, err error) {
	b = make([]byte, size)
	_, err = rand.Reader.Read(b)
	return
}

func newXsrf(secret []byte) (s string, err error) {
	rnd, err := genKey(keySize)
	s = base64.RawURLEncoding.EncodeToString(append(rnd, xor(rnd, secret)...))
	return
}

func checkXsrf(xsrf string, secret []byte) bool {
	t, err := base64.RawURLEncoding.DecodeString(xsrf)
	if err != nil {
		return false
	}
	if len(t) != jwt.KeySize {
		return false
	}
	return subtle.ConstantTimeCompare(secret, xor(t[keySize:], t[:keySize])) == 1
}

func xor(a, b []byte) (c []byte) {
	n := len(a)
	if len(b) < n {
		return nil
	}
	c = make([]byte, n)
	for i := 0; i < n; i++ {
		c[i] = a[i] ^ b[i]
	}
	return
}