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using System.IO;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Tls.Crypto;
using Org.BouncyCastle.Utilities;
namespace Org.BouncyCastle.Tls
{
/// <summary>
/// Implements cookie generation/verification for a DTLS server as described in RFC 4347,
/// 4.2.1. Denial of Service Countermeasures.
/// </summary>
/// <remarks>
/// RFC 4347 4.2.1 additionally recommends changing the secret frequently. This class does not handle that
/// internally, so the instance should be replaced instead.
/// </remarks>
public class DtlsVerifier
{
private readonly TlsCrypto m_crypto;
private readonly byte[] m_macKey;
public DtlsVerifier(TlsCrypto crypto)
{
m_crypto = crypto;
m_macKey = SecureRandom.GetNextBytes(crypto.SecureRandom, 32);
}
public virtual DtlsRequest VerifyRequest(byte[] clientID, byte[] data, int dataOff, int dataLen,
DatagramSender sender)
{
try
{
int msgLen = DtlsRecordLayer.ReceiveClientHelloRecord(data, dataOff, dataLen);
if (msgLen < 0)
return null;
int bodyLength = msgLen - DtlsReliableHandshake.MessageHeaderLength;
if (bodyLength < 39) // Minimum (syntactically) valid DTLS ClientHello length
return null;
int msgOff = dataOff + DtlsRecordLayer.RecordHeaderLength;
var buf = DtlsReliableHandshake.ReceiveClientHelloMessage(msg: data, msgOff, msgLen);
if (buf == null)
return null;
var macInput = new MemoryStream(bodyLength);
ClientHello clientHello = ClientHello.Parse(buf, dtlsOutput: macInput);
if (clientHello == null)
return null;
long recordSeq = TlsUtilities.ReadUint48(data, dataOff + 5);
byte[] cookie = clientHello.Cookie;
TlsMac mac = m_crypto.CreateHmac(MacAlgorithm.hmac_sha256);
mac.SetKey(m_macKey, 0, m_macKey.Length);
mac.Update(clientID, 0, clientID.Length);
macInput.WriteTo(new TlsMacSink(mac));
byte[] expectedCookie = mac.CalculateMac();
if (Arrays.FixedTimeEquals(expectedCookie, cookie))
{
byte[] message = TlsUtilities.CopyOfRangeExact(data, msgOff, msgOff + msgLen);
return new DtlsRequest(recordSeq, message, clientHello);
}
DtlsReliableHandshake.SendHelloVerifyRequest(sender, recordSeq, expectedCookie);
}
catch (IOException)
{
// Ignore
}
return null;
}
}
}
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