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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.IOException;

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

public class DTLSVerifier
{
    private static TlsMAC createCookieMAC(TlsCrypto crypto)
    {
        TlsMAC mac = crypto.createHMAC(MACAlgorithm.hmac_sha256);

        byte[] secret = new byte[mac.getMacLength()];
        crypto.getSecureRandom().nextBytes(secret);

        mac.setKey(secret, 0, secret.length);

        return mac;
    }

    private final TlsMAC cookieMAC;
    private final TlsMACOutputStream cookieMACOutputStream;

    public DTLSVerifier(TlsCrypto crypto)
    {
        this.cookieMAC = createCookieMAC(crypto);
        this.cookieMACOutputStream = new TlsMACOutputStream(cookieMAC);
    }

    public synchronized DTLSRequest verifyRequest(byte[] clientID, byte[] data, int dataOff, int dataLen,
        DatagramSender sender)
    {
        boolean resetCookieMAC = true;

        try
        {
            cookieMAC.update(clientID, 0, clientID.length);

            DTLSRequest request = DTLSReliableHandshake.readClientRequest(data, dataOff, dataLen, cookieMACOutputStream);
            if (null != request)
            {
                byte[] expectedCookie = cookieMAC.calculateMAC();
                resetCookieMAC = false;

                // TODO Consider stricter HelloVerifyRequest protocol
//                switch (request.getMessageSeq())
//                {
//                case 0:
//                {
//                    DTLSReliableHandshake.sendHelloVerifyRequest(sender, request.getRecordSeq(), expectedCookie);
//                    break;
//                }
//                case 1:
//                {
//                    if (Arrays.constantTimeAreEqual(expectedCookie, request.getClientHello().getCookie()))
//                    {
//                        return request;
//                    }
//                    break;
//                }
//                }

                if (Arrays.constantTimeAreEqual(expectedCookie, request.getClientHello().getCookie()))
                {
                    return request;
                }

                DTLSReliableHandshake.sendHelloVerifyRequest(sender, request.getRecordSeq(), expectedCookie);
            }
        }
        catch (IOException e)
        {
            // Ignore
        }
        finally
        {
            if (resetCookieMAC)
            {
                cookieMAC.reset();
            }
        }

        return null;
    }
}