// Vérifier la signature d'un webhook Tauraco (Standard Webhooks) — .NET 6+, sans dépendance.
// Verify a Tauraco webhook signature (Standard Webhooks) — .NET 6+, no dependency.
// Vecteur de référence (page « Guide des webhooks ») :
//   secret            whsec_dGF1cmFjby1leGVtcGxlLWRlLXZlY3RldXItMjAyNiE=
//   webhook-id        evt_exemple_0001
//   webhook-timestamp 1793750400
//   corps             {"id":"evt_exemple_0001","type":"webhook.test","data":{"message":"ok"}}
//   signature         v1,z25573sJuRUxUk3JkjiZN3OW2KihQFtLvIGhZTHUnuM=
// Un secret vide ou illisible est refusé ; une signature candidate illisible est ignorée
// (on essaie la suivante : l'en-tête peut en porter plusieurs).
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;

public static class VerifierWebhook
{
    private const long ToleranceSeconds = 300;

    /// <summary><paramref name="headers"/> : noms en minuscules ; <paramref name="body"/> : corps BRUT.</summary>
    public static bool VerifySignature(string secret, IReadOnlyDictionary<string, string> headers, byte[] body, long? now = null)
    {
        if (!headers.TryGetValue("webhook-id", out var id) ||
            !headers.TryGetValue("webhook-timestamp", out var rawTimestamp) ||
            !headers.TryGetValue("webhook-signature", out var signatures) ||
            !long.TryParse(rawTimestamp.Trim(), out var timestamp))
        {
            return false;
        }
        var current = now ?? DateTimeOffset.UtcNow.ToUnixTimeSeconds();
        if (Math.Abs(current - timestamp) > ToleranceSeconds) return false;
        byte[] key;
        try
        {
            key = Convert.FromBase64String(secret.StartsWith("whsec_") ? secret.Substring(6) : secret);
        }
        catch (FormatException)
        {
            return false; // secret illisible
        }
        if (key.Length == 0) return false; // clé vide : n'importe qui pourrait signer
        var prefix = Encoding.UTF8.GetBytes($"{id}.{timestamp}.");
        var signed = new byte[prefix.Length + body.Length];
        Buffer.BlockCopy(prefix, 0, signed, 0, prefix.Length);
        Buffer.BlockCopy(body, 0, signed, prefix.Length, body.Length);
        using var hmac = new HMACSHA256(key);
        var expected = hmac.ComputeHash(signed);
        foreach (var candidate in signatures.Split(' ', StringSplitOptions.RemoveEmptyEntries))
        {
            var parts = candidate.Split(',', 2);
            if (parts.Length != 2 || parts[0] != "v1") continue;
            byte[] received;
            try
            {
                received = Convert.FromBase64String(parts[1]);
            }
            catch (FormatException)
            {
                continue; // candidate illisible : essayer la suivante
            }
            if (CryptographicOperations.FixedTimeEquals(received, expected)) return true;
        }
        return false;
    }
}