package keygen

import (
	"crypto/hmac"
	"crypto/sha256"
	"strconv"
)

// The legacy C implementation (legacy/keygen.a, no source shipped) derived the
// share token from the video id ALONE -- no key, no salt. Worse, its state
// collapsed to (seed*17+42)%62 after the first round, so the token depended
// only on id%62: 62 distinct tokens for the whole service. Anyone could upload
// 62 private videos, copy the tokens off their own /home and unlock every
// private video on every team's box. Replaced with a keyed HMAC.
//
// Same signatures, same 30-char base62 alphabet, so nothing else changes.
// keygen.h / legacy/keygen.a are no longer part of the build (cgo is gone).
const (
	tokenLen = 30
	alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

	// Server-side secret. Rotating it invalidates every token already handed
	// out, so change it only between rounds.
	secret = "D1gkxNv7_HphLJjNuFag_stjaByXLZ1Y0l12RGppZtw"
)

func GenerateKey(vid int) string {
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write([]byte(strconv.Itoa(vid)))
	sum := mac.Sum(nil)

	out := make([]byte, tokenLen)
	for i := 0; i < tokenLen; i++ {
		out[i] = alphabet[int(sum[i%len(sum)])%len(alphabet)]
	}
	return string(out)
}

func ValidateKey(token string, vid int) bool {
	// Constant-time: never leak how much of the token was correct.
	if hmac.Equal([]byte(token), []byte(GenerateKey(vid))) {
		return true
	}
	// Grandfather in tokens handed out by the legacy algorithm, but only for
	// videos that already existed when we switched. See legacy.go.
	return legacyAccepted(token, vid)
}