private void ExtractNonce(WebResponse resp)
{
var nonceHeader = resp.Headers.AllKeys.FirstOrDefault(x =>
x.Equals(AcmeProtocol.HEADER_REPLAY_NONCE, StringComparison.OrdinalIgnoreCase));
if (string.IsNullOrEmpty(nonceHeader))
throw new AcmeException("Missing initial replay-nonce header");
NextNonce = resp.Headers[nonceHeader];
if (string.IsNullOrEmpty(NextNonce))
throw new AcmeException("Missing initial replay-nonce header value");
}