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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.operator;

import java.io.IOException;
import java.io.OutputStream;

import org.bouncycastle.util.Arrays;

/**
 * A generic class for capturing the mac data at the end of a encrypted data stream.
 * 
 * Note: this class will not close the underlying stream.
 * 
 */
public class MacCaptureStream
    extends OutputStream
{
    private final OutputStream cOut;
    private final byte[] mac;

    int macIndex = 0;

    public MacCaptureStream(OutputStream cOut, int macLength)
    {
        this.cOut = cOut;
        this.mac = new byte[macLength];
    }

    public void write(byte[] buf, int off, int len)
        throws IOException
    {
        if (len >= mac.length)
        {
            cOut.write(mac, 0, macIndex);
            macIndex = mac.length;
            System.arraycopy(buf, off + len - mac.length, mac, 0, mac.length);
            cOut.write(buf, off, len - mac.length);
        }
        else
        {
            for (int i = 0; i != len; i++)
            {
                write(buf[off + i]);
            }
        }
    }

    public void write(int b)
        throws IOException
    {
        if (macIndex == mac.length)
        {
             byte b1 = mac[0];
             System.arraycopy(mac, 1, mac, 0, mac.length - 1);
             mac[mac.length - 1] = (byte)b;
             cOut.write(b1);
        }
        else
        {
            mac[macIndex++] = (byte)b;
        }
    }

    public byte[] getMac()
    {
        return Arrays.clone(mac);
    }
}