package web

import (
	"crypto/hkdf"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"strings"
)

var errSeal = errors.New("sealed value rejected")

func deriveKey(master []byte, info string) (*[]byte, error) {
	k, err := hkdf.Key(sha256.New, master, nil, info, 32)
	if err != nil {
		return nil, err
	}
	return &k, nil
}

func mac(key *[]byte, parts ...[]byte) []byte {
	m := hmac.New(sha256.New, *key)
	for _, p := range parts {
		m.Write(p)
	}
	return m.Sum(nil)
}

func seal(key *[]byte, payload []byte) string {
	enc := base64.RawURLEncoding
	return enc.EncodeToString(payload) + "." + enc.EncodeToString(mac(key, payload))
}

func unseal(key *[]byte, v string) ([]byte, error) {
	p64, t64, ok := strings.Cut(v, ".")
	if !ok {
		return nil, errSeal
	}
	enc := base64.RawURLEncoding.Strict()
	payload, err1 := enc.DecodeString(p64)
	tag, err2 := enc.DecodeString(t64)
	if err1 != nil || err2 != nil || !hmac.Equal(mac(key, payload), tag) {
		return nil, errSeal
	}
	return payload, nil
}