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The Bouncy Castle Java APIs for the TLS, including a JSSE provider. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.tls;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

import org.bouncycastle.tls.crypto.TlsCrypto;
import org.bouncycastle.tls.crypto.TlsMAC;
import org.bouncycastle.tls.crypto.TlsMACOutputStream;
import org.bouncycastle.util.Arrays;

/**
 * Implements cookie generation/verification for a DTLS server as described in RFC 4347,
 * 4.2.1. Denial of Service Countermeasures.
 * 
 * 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.
 */
public class DTLSVerifier
{
    private final TlsCrypto crypto;
    private final byte[] macKey;

    public DTLSVerifier(TlsCrypto crypto)
    {
        this.crypto = crypto;
        this.macKey = new byte[32];
        crypto.getSecureRandom().nextBytes(macKey);
    }

    public 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.MESSAGE_HEADER_LENGTH;
            if (bodyLength < 39) // Minimum (syntactically) valid DTLS ClientHello length
            {
                return null;
            }

            int msgOff = dataOff + DTLSRecordLayer.RECORD_HEADER_LENGTH;

            ByteArrayInputStream buf = DTLSReliableHandshake.receiveClientHelloMessage(data, msgOff, msgLen);
            if (buf == null)
            {
                return null;
            }

            ByteArrayOutputStream macInput = new ByteArrayOutputStream(bodyLength);
            ClientHello clientHello = ClientHello.parse(buf, macInput);
            if (clientHello == null)
            {
                return null;
            }

            long recordSeq = TlsUtils.readUint48(data, dataOff + 5);

            byte[] cookie = clientHello.getCookie();

            TlsMAC mac = crypto.createHMAC(MACAlgorithm.hmac_sha256);
            mac.setKey(macKey, 0, macKey.length);
            mac.update(clientID, 0, clientID.length);
            macInput.writeTo(new TlsMACOutputStream(mac));
            byte[] expectedCookie = mac.calculateMAC();

            if (Arrays.constantTimeAreEqual(expectedCookie, cookie))
            {
                byte[] message = TlsUtils.copyOfRangeExact(data, msgOff, msgOff + msgLen);

                return new DTLSRequest(recordSeq, message, clientHello);
            }

            DTLSReliableHandshake.sendHelloVerifyRequest(sender, recordSeq, expectedCookie);
        }
        catch (IOException e)
        {
            // Ignore
        }

        return null;
    }
}