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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.asn1;

import java.io.IOException;

/**
 * A BIT STRING with DER encoding.
 */
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);
        }

        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)o).getOctets());
        }
    }
    
    protected DERBitString(
        byte    data,
        int     padBits)
    {
        this(toByteArray(data), padBits);
    }

    private static byte[] toByteArray(byte data)
    {
        byte[] rv = new byte[1];

        rv[0] = data;

        return rv;
    }

    /**
     * @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)
        throws IOException
    {
        byte[] string = derForm(data, padBits);
        byte[] bytes = new byte[string.length + 1];

        bytes[0] = (byte)getPadBits();
        System.arraycopy(string, 0, bytes, 1, bytes.length - 1);

        out.writeEncoded(BERTags.BIT_STRING, bytes);
    }

    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);
    }
}