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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.encoding;

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

/**
 * Input stream that processes quoted-printable data, converting it into what was originally intended.
 */
public class QuotedPrintableInputStream
    extends FilterInputStream
{

    public QuotedPrintableInputStream(InputStream input)
    {
        super(input);
    }

    public int read(byte[] buf, int bufOff, int len) throws IOException
    {
        int i = 0;
        while (i != len)
        {
            int ch = this.read();
            if (ch < 0)
            {
                break;
            }
            buf[i + bufOff] = (byte)ch;
            i++;
        }

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

    public int read()
        throws IOException
    {
        int v = in.read();
        if (v == -1)
        {
            return -1;
        }
 
        // V was the quote '=' character/
        while (v == '=')
        {
            //
            // Get the next character.
            //
            int j = in.read();
            if (j == -1)
            {
                throw new IllegalStateException("Quoted '=' at end of stream");
            }

            // For systems generating CRLF line endings.
            if (j == '\r')
            {
                j = in.read();
                if (j == '\n')
                {
                    //
                    // This was a line break that was not actually a line break in the original information.
                    // So return the next data.
                    //
                    j = in.read();
                }
                v = j;
                continue;
            }
            else if (j == '\n')
            {
                // As above but without preceding CR.
                v = in.read();
                continue;
            }
            else
            {

                int chr = 0;

                if (j >= '0' && j <= '9')
                {
                    chr = j - '0';
                }
                else if (j >= 'A' && j <= 'F')
                {
                    chr = 10 + (j - 'A');
                }
                else
                {
                    throw new IllegalStateException("Expecting '0123456789ABCDEF after quote that was not immediately followed by LF or CRLF");
                }

                chr <<= 4;

                j = in.read();

                if (j >= '0' && j <= '9')
                {
                    chr |= j - '0';
                }
                else if (j >= 'A' && j <= 'F')
                {
                    chr |= 10 + (j - 'A');
                }
                else
                {
                    throw new IllegalStateException("Expecting second '0123456789ABCDEF after quote that was not immediately followed by LF or CRLF");
                }

                return chr;
            }
        }

        return v;
    }
}