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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 and up.

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package org.bouncycastle.asn1;

import java.io.IOException;

/**
 * A BIT STRING with DER encoding - the first byte contains the count of padding bits included in the byte array's last byte.
 */
public class DERBitString
    extends ASN1BitString
{
    /**
     * return a Bit String from the passed in object
     *
     * @param obj a DERBitString or an object that can be converted into one.
     * @exception IllegalArgumentException if the object cannot be converted.
     * @return a DERBitString instance, or null.
     */
    public static DERBitString getInstance(
        Object  obj)
    {
        if (obj == null || obj instanceof DERBitString)
        {
            return (DERBitString)obj;
        }
        if (obj instanceof DLBitString)
        {
            return new DERBitString(((DLBitString)obj).data, ((DLBitString)obj).padBits);
        }
        if (obj instanceof byte[])
        {
            try
            {
                return (DERBitString)fromByteArray((byte[])obj);
            }
            catch (Exception e)
            {
                throw new IllegalArgumentException("encoding error in getInstance: " + e.toString());
            }
        }

        throw new IllegalArgumentException("illegal object in getInstance: " + obj.getClass().getName());
    }

    /**
     * return a Bit String from a tagged object.
     *
     * @param obj the tagged object holding the object we want
     * @param explicit true if the object is meant to be explicitly
     *              tagged false otherwise.
     * @exception IllegalArgumentException if the tagged object cannot
     *               be converted.
     * @return a DERBitString instance, or null.
     */
    public static DERBitString getInstance(
        ASN1TaggedObject obj,
        boolean          explicit)
    {
        ASN1Primitive o = obj.getObject();

        if (explicit || o instanceof DERBitString)
        {
            return getInstance(o);
        }
        else
        {
            return fromOctetString(ASN1OctetString.getInstance(o).getOctets());
        }
    }

    protected DERBitString(byte data, int padBits)
    {
        super(data, padBits);
    }

    /**
     * @param data the octets making up the bit string.
     * @param padBits the number of extra bits at the end of the string.
     */
    public DERBitString(
        byte[]  data,
        int     padBits)
    {
        super(data, padBits);
    }

    public DERBitString(
        byte[]  data)
    {
        this(data, 0);
    }

    public DERBitString(
        int value)
    {
        super(getBytes(value), getPadBits(value));
    }

    public DERBitString(
        ASN1Encodable obj)
        throws IOException
    {
        super(obj.toASN1Primitive().getEncoded(ASN1Encoding.DER), 0);
    }

    boolean isConstructed()
    {
        return false;
    }

    int encodedLength()
    {
        return 1 + StreamUtil.calculateBodyLength(data.length + 1) + data.length + 1;
    }

    void encode(ASN1OutputStream out, boolean withTag) throws IOException
    {
        int len = data.length;
        if (0 == len
            || 0 == padBits
            || (data[len - 1] == (byte)(data[len - 1] & (0xFF << padBits))))
        {
            out.writeEncoded(withTag, BERTags.BIT_STRING, (byte)padBits, data);
        }
        else
        {
            byte der = (byte)(data[len - 1] & (0xFF << padBits));
            out.writeEncoded(withTag, BERTags.BIT_STRING, (byte)padBits, data, 0, len - 1, der);
        }
    }

    ASN1Primitive toDERObject()
    {
        return this;
    }

    ASN1Primitive toDLObject()
    {
        return this;
    }

    static DERBitString fromOctetString(byte[] bytes)
    {
        if (bytes.length < 1)
        {
            throw new IllegalArgumentException("truncated BIT STRING detected");
        }

        int padBits = bytes[0];
        byte[] data = new byte[bytes.length - 1];

        if (data.length != 0)
        {
            System.arraycopy(bytes, 1, data, 0, bytes.length - 1);
        }

        return new DERBitString(data, padBits);
    }
}