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A GWT cryptography library ported from Legion of the Bouncy Castle.
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package org.bouncycastle.crypto.tls;

import java.io.IOException;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.util.Arrays;

/**
 * A NULL CipherSuite with optional MAC
 */
public class TlsNullCipher
    implements TlsCipher
{
    protected TlsContext context;

    protected TlsMac writeMac;
    protected TlsMac readMac;

    public TlsNullCipher(TlsContext context)
    {
        this.context = context;
        this.writeMac = null;
        this.readMac = null;
    }

    public TlsNullCipher(TlsContext context, Digest clientWriteDigest, Digest serverWriteDigest)
        throws IOException
    {
        if ((clientWriteDigest == null) != (serverWriteDigest == null))
        {
            throw new TlsFatalAlert(AlertDescription.internal_error);
        }

        this.context = context;

        TlsMac clientWriteMac = null, serverWriteMac = null;

        if (clientWriteDigest != null)
        {
            int key_block_size = clientWriteDigest.getDigestSize()
                + serverWriteDigest.getDigestSize();
            byte[] key_block = TlsUtils.calculateKeyBlock(context, key_block_size);

            int offset = 0;

            clientWriteMac = new TlsMac(context, clientWriteDigest, key_block, offset,
                clientWriteDigest.getDigestSize());
            offset += clientWriteDigest.getDigestSize();

            serverWriteMac = new TlsMac(context, serverWriteDigest, key_block, offset,
                serverWriteDigest.getDigestSize());
            offset += serverWriteDigest.getDigestSize();

            if (offset != key_block_size)
            {
                throw new TlsFatalAlert(AlertDescription.internal_error);
            }
        }

        if (context.isServer())
        {
            writeMac = serverWriteMac;
            readMac = clientWriteMac;
        }
        else
        {
            writeMac = clientWriteMac;
            readMac = serverWriteMac;
        }
    }

    public int getPlaintextLimit(int ciphertextLimit)
    {
        int result = ciphertextLimit;
        if (writeMac != null)
        {
            result -= writeMac.getSize();
        }
        return result;
    }

    public byte[] encodePlaintext(long seqNo, short type, byte[] plaintext, int offset, int len)
        throws IOException
    {
        if (writeMac == null)
        {
            return Arrays.copyOfRange(plaintext, offset, offset + len);
        }

        byte[] mac = writeMac.calculateMac(seqNo, type, plaintext, offset, len);
        byte[] ciphertext = new byte[len + mac.length];
        System.arraycopy(plaintext, offset, ciphertext, 0, len);
        System.arraycopy(mac, 0, ciphertext, len, mac.length);
        return ciphertext;
    }

    public byte[] decodeCiphertext(long seqNo, short type, byte[] ciphertext, int offset, int len)
        throws IOException
    {
        if (readMac == null)
        {
            return Arrays.copyOfRange(ciphertext, offset, offset + len);
        }

        int macSize = readMac.getSize();
        if (len < macSize)
        {
            throw new TlsFatalAlert(AlertDescription.decode_error);
        }

        int macInputLen = len - macSize;

        byte[] receivedMac = Arrays.copyOfRange(ciphertext, offset + macInputLen, offset + len);
        byte[] computedMac = readMac.calculateMac(seqNo, type, ciphertext, offset, macInputLen);

        if (!Arrays.constantTimeAreEqual(receivedMac, computedMac))
        {
            throw new TlsFatalAlert(AlertDescription.bad_record_mac);
        }

        return Arrays.copyOfRange(ciphertext, offset, offset + macInputLen);
    }
}