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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 to JDK 1.8. 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.mime;

import java.io.IOException;
import java.io.InputStream;

import org.bouncycastle.util.Strings;

public class BoundaryLimitedInputStream
    extends InputStream
{
    final private InputStream src;
    final private byte[] boundary;
    final private byte[] buf;

    private int bufOff = 0;
    private int index = 0;
    private boolean ended = false;

    private int lastI;

    public BoundaryLimitedInputStream(InputStream src, String startBoundary)
    {
        this.src = src;
        boundary = Strings.toByteArray(startBoundary);
        this.buf = new byte[startBoundary.length() + 3];
        this.bufOff = 0;
    }


    public int read()
        throws IOException
    {
        if (ended)
        {
            return -1;
        }

        int i;
        if (index < bufOff)
        {
            i = buf[index++] & 0xff;
            // if this happens we know we've already failed on a "\r\n"
            if (index < bufOff)
            {
                return i;
            }
            index = bufOff = 0;
        }
        else
        {
            i = src.read();
        }

        lastI = i;

        if (i < 0)
        {
            return -1;
        }

        if (i == '\r' || i == '\n') // check for start of boundary
        {
            int ch;
            index = 0;
            if (i == '\r')
            {
                ch = src.read();
                if (ch == '\n')
                {
                    buf[bufOff++] = '\n';
                    ch = src.read();
                }
            }
            else
            {
                ch = src.read();
            }

            if (ch == '-')
            {
                buf[bufOff++] = '-';
                ch = src.read();
            }

            if (ch == '-')
            {
                buf[bufOff++] = '-';

                int base = bufOff;
                int c;

                while ((bufOff - base) != boundary.length && (c = src.read()) >= 0)
                {
                    buf[bufOff] = (byte)c;
                    if (buf[bufOff] != boundary[bufOff - base])
                    {
                        bufOff++;
                        break;
                    }
                    bufOff++;
                }
                
                // we have a match
                if (bufOff - base == boundary.length)
                {
                    ended = true;
                    return -1;
                }
            }
            else
            {
                if (ch >= 0)
                {
                    buf[bufOff++] = (byte)ch;
                }
            }
        }

        return i;
    }
}