Validate Hubspot V3 signature in C#

ValidateSignature.cs

private async Task<bool> ValidateSignature(HttpRequestData req, string privateAppValidationSecret)
{
    var signatureHeader = req.Headers.GetValues("X-HubSpot-Signature-v3").FirstOrDefault();
    var timestampHeader = req.Headers.GetValues("X-HubSpot-Request-Timestamp").FirstOrDefault();

    if (string.IsNullOrWhiteSpace(signatureHeader) || string.IsNullOrWhiteSpace(timestampHeader))
    {
        return false;
    }

    var timestamp = Convert.ToInt64(timestampHeader);
    if (DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() - timestamp > TimeSpan.FromMinutes(5).TotalMilliseconds)
    {
        return false;  // Timestamp is too old
    }

    var requestUri = System.Net.WebUtility.UrlDecode(req.Url.ToString());
    var requestBody = await req.ReadAsStringAsync();
    var utf8String = $"{req.Method}{requestUri}{requestBody}{timestampHeader}";

    using var hmacsha256 = new HMACSHA256(Encoding.UTF8.GetBytes(privateAppValidationSecret ?? throw new ArgumentNullException()));
    var hash = hmacsha256.ComputeHash(Encoding.UTF8.GetBytes(utf8String));
    var computedSignature = Convert.ToBase64String(hash);

    return computedSignature == signatureHeader;
}