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The Bouncy Castle Java APIs for the OpenPGP Protocol. 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.gpg;

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

import org.bouncycastle.openpgp.OpenedPGPKeyData;
import org.bouncycastle.openpgp.PGPExtendedKeyHeader;
import org.bouncycastle.util.Strings;

public class PGPSecretKeyParser
{


    /**
     * Test if the stream supports extended
     *
     * @param inputStream
     * @return
     * @throws IOException
     */
    public static boolean isExtendedSExpression(InputStream inputStream)
        throws IOException
    {
        if (!inputStream.markSupported())
        {
            throw new IOException("input stream must support mark");
        }
        inputStream.mark(1);
        int c = inputStream.read();
        inputStream.reset();

        return c != '(';
    }

    private static int lastIndexOfWhitespace(String str)
    {
        if (str.length() == 0)
        {
            return -1;
        }
        for (int t = str.length() - 1; t >= 0; t--)
        {
            char c = str.charAt(t);
            if (c <= 32)
            {
                return t;
            }
        }
        return -1;
    }

    private static void consumeUntil(InputStream src, char item, ByteArrayOutputStream accumulator)
        throws IOException
    {
        accumulator.reset();
        int c;
        while ((c = src.read()) > -1)
        {
            if (c == item)
            {
                return;
            }
            accumulator.write(c);
        }
    }


    public static OpenedPGPKeyData parse(InputStream src, int maxExpressionDepth)
        throws IOException
    {

        OpenedPGPKeyData.Builder builder = OpenedPGPKeyData.builder();

        if (PGPSecretKeyParser.isExtendedSExpression(src))
        {

            ByteArrayOutputStream accumulator = new ByteArrayOutputStream();

            String key = null;

            for (; ; )
            {
                consumeUntil(src, ':', accumulator);
                String hunk = Strings.fromByteArray(accumulator.toByteArray()).trim();
                int ws = lastIndexOfWhitespace(hunk);
                if (ws == -1)
                {
                    key = hunk;
                }
                else
                {
                    builder.add(new PGPExtendedKeyHeader(key, hunk.substring(0, ws)));
                    key = hunk.substring(ws).trim();
                }

                if (key.equalsIgnoreCase("Key"))
                {
                    break;
                }

            }
        }

        builder.setKeyExpression(SExpression.parse(src, maxExpressionDepth));
        return builder.build();
    }

}