package stripeapi

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"strconv"
	"strings"
	"time"
)

// webhookTolerance is how old a signed delivery may be: Stripe's own
// libraries' five minutes, which is what stops a captured one replaying.
const webhookTolerance = 5 * time.Minute

// ErrBadSignature is every way a delivery can fail the check: the header
// is malformed, stale, or signed by none of the secrets.
var ErrBadSignature = errors.New("stripeapi: webhook signature does not check")

// VerifySignature checks a Stripe-Signature header ("t=…,v1=…[,v1=…]")
// over body against any of secrets (more than one while rotating).
func VerifySignature(header string, body []byte, secrets []string, now time.Time) error {
	var ts string
	var sigs [][]byte
	for _, part := range strings.Split(header, ",") {
		k, v, _ := strings.Cut(strings.TrimSpace(part), "=")
		switch k {
		case "t":
			ts = v
		case "v1":
			if b, err := hex.DecodeString(v); err == nil {
				sigs = append(sigs, b)
			}
		}
	}
	t, err := strconv.ParseInt(ts, 10, 64)
	if err != nil || len(sigs) == 0 {
		return ErrBadSignature
	}
	if d := now.Sub(time.Unix(t, 0)); d > webhookTolerance || d < -webhookTolerance {
		return ErrBadSignature
	}
	for _, secret := range secrets {
		mac := hmac.New(sha256.New, []byte(secret))
		mac.Write([]byte(ts))
		mac.Write([]byte("."))
		mac.Write(body)
		want := mac.Sum(nil)
		for _, s := range sigs {
			if hmac.Equal(s, want) {
				return nil
			}
		}
	}
	return ErrBadSignature
}