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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.7. Note: this package includes the IDEA and NTRU encryption algorithms.
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package org.bouncycastle.asn1;

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

import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.io.Streams;

/**
 * Base class for BIT STRING objects
 */
public abstract class ASN1BitString
    extends ASN1Primitive
    implements ASN1String
{
    private static final char[]  table = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };

    protected final byte[]      data;
    protected final int         padBits;

    /**
     * @param bitString an int containing the BIT STRING
     * @return the correct number of pad bits for a bit string defined in
     * a 32 bit constant
     */
    static protected int getPadBits(
        int bitString)
    {
        int val = 0;
        for (int i = 3; i >= 0; i--)
        {
            //
            // this may look a little odd, but if it isn't done like this pre jdk1.2
            // JVM's break!
            //
            if (i != 0)
            {
                if ((bitString >> (i * 8)) != 0)
                {
                    val = (bitString >> (i * 8)) & 0xFF;
                    break;
                }
            }
            else
            {
                if (bitString != 0)
                {
                    val = bitString & 0xFF;
                    break;
                }
            }
        }

        if (val == 0)
        {
            return 0;
        }

        int bits = 1;

        while (((val <<= 1) & 0xFF) != 0)
        {
            bits++;
        }

        return 8 - bits;
    }

    /**
     * @param bitString an int containing the BIT STRING
     * @return the correct number of bytes for a bit string defined in
     * a 32 bit constant
     */
    static protected byte[] getBytes(int bitString)
    {
        if (bitString == 0)
        {
            return new byte[0];
        }

        int bytes = 4;
        for (int i = 3; i >= 1; i--)
        {
            if ((bitString & (0xFF << (i * 8))) != 0)
            {
                break;
            }
            bytes--;
        }

        byte[] result = new byte[bytes];
        for (int i = 0; i < bytes; i++)
        {
            result[i] = (byte) ((bitString >> (i * 8)) & 0xFF);
        }

        return result;
    }

    /**
     * Base constructor.
     *
     * @param data the octets making up the bit string.
     * @param padBits the number of extra bits at the end of the string.
     */
    public ASN1BitString(
        byte[]  data,
        int     padBits)
    {
        if (data == null)
        {
            throw new NullPointerException("data cannot be null");
        }
        if (data.length == 0 && padBits != 0)
        {
            throw new IllegalArgumentException("zero length data with non-zero pad bits");
        }
        if (padBits > 7 || padBits < 0)
        {
            throw new IllegalArgumentException("pad bits cannot be greater than 7 or less than 0");
        }

        this.data = Arrays.clone(data);
        this.padBits = padBits;
    }

    /**
     * Return a String representation of this BIT STRING
     *
     * @return a String representation.
     */
    public String getString()
    {
        StringBuffer          buf = new StringBuffer("#");
        ByteArrayOutputStream bOut = new ByteArrayOutputStream();
        ASN1OutputStream      aOut = new ASN1OutputStream(bOut);

        try
        {
            aOut.writeObject(this);
        }
        catch (IOException e)
        {
            throw new ASN1ParsingException("Internal error encoding BitString: " + e.getMessage(), e);
        }

        byte[]    string = bOut.toByteArray();

        for (int i = 0; i != string.length; i++)
        {
            buf.append(table[(string[i] >>> 4) & 0xf]);
            buf.append(table[string[i] & 0xf]);
        }

        return buf.toString();
    }

    /**
     * @return the value of the bit string as an int (truncating if necessary)
     */
    public int intValue()
    {
        int value = 0;
        byte[] string = data;

        if (padBits > 0 && data.length <= 4)
        {
            string = derForm(data, padBits);
        }

        for (int i = 0; i != string.length && i != 4; i++)
        {
            value |= (string[i] & 0xff) << (8 * i);
        }

        return value;
    }

    /**
     * Return the octets contained in this BIT STRING, checking that this BIT STRING really
     * does represent an octet aligned string. Only use this method when the standard you are
     * following dictates that the BIT STRING will be octet aligned.
     *
     * @return a copy of the octet aligned data.
     */
    public byte[] getOctets()
    {
        if (padBits != 0)
        {
            throw new IllegalStateException("attempt to get non-octet aligned data from BIT STRING");
        }

        return Arrays.clone(data);
    }

    public byte[] getBytes()
    {
        return derForm(data, padBits);
    }

    public int getPadBits()
    {
        return padBits;
    }

    public String toString()
    {
        return getString();
    }

    public int hashCode()
    {
        return padBits ^ Arrays.hashCode(this.getBytes());
    }

    protected boolean asn1Equals(
        ASN1Primitive  o)
    {
        if (!(o instanceof ASN1BitString))
        {
            return false;
        }

        ASN1BitString other = (ASN1BitString)o;

        return this.padBits == other.padBits
            && Arrays.areEqual(this.getBytes(), other.getBytes());
    }

    protected static byte[] derForm(byte[] data, int padBits)
    {
        byte[] rv = Arrays.clone(data);
        // DER requires pad bits be zero
        if (padBits > 0)
        {
            rv[data.length - 1] &= 0xff << padBits;
        }

        return rv;
    }

    static ASN1BitString fromInputStream(int length, InputStream stream)
        throws IOException
    {
        if (length < 1)
        {
            throw new IllegalArgumentException("truncated BIT STRING detected");
        }

        int padBits = stream.read();
        byte[] data = new byte[length - 1];

        if (data.length != 0)
        {
            if (Streams.readFully(stream, data) != data.length)
            {
                throw new EOFException("EOF encountered in middle of BIT STRING");
            }

            if (padBits > 0 && padBits < 8)
            {
                if (data[data.length - 1] != (byte)(data[data.length - 1] & (0xff << padBits)))
                {
                    return new DLBitString(data, padBits);
                }
            }
        }

        return new DERBitString(data, padBits);
    }

    public ASN1Primitive getLoadedObject()
    {
        return this.toASN1Primitive();
    }

    ASN1Primitive toDERObject()
    {
        return new DERBitString(data, padBits);
    }

    ASN1Primitive toDLObject()
    {
        return new DLBitString(data, padBits);
    }

    abstract void encode(ASN1OutputStream out)
        throws IOException;
}