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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 1.8 and up. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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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;
    }
}