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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 1.8 and up.
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package org.bouncycastle.asn1;

import java.io.IOException;

/**
 * ASN.1 OctetStrings, with indefinite length rules, and constructed form support.
 * 
 * The Basic Encoding Rules (BER) format allows encoding using so called "constructed form",
 * which DER and CER formats forbid allowing only "primitive form".
 * 

 * This class always produces the constructed form with underlying segments
 * in an indefinite length array.  If the input wasn't the same, then this output
 * is not faithful reproduction.
 * 
 * 
 * See {@link ASN1OctetString} for X.690 encoding rules of OCTET-STRING objects.
 * 
 */
public class BEROctetString
    extends ASN1OctetString
{
    private static final int DEFAULT_SEGMENT_LIMIT = 1000;

    private final int segmentLimit;
    private final ASN1OctetString[] elements;

    /**
     * Convert a vector of octet strings into a single byte string
     */
    static byte[] flattenOctetStrings(ASN1OctetString[] octetStrings)
    {
        int count = octetStrings.length;
        switch (count)
        {
        case 0:
            return EMPTY_OCTETS;
        case 1:
            return octetStrings[0].string;
        default:
        {
            int totalOctets = 0;
            for (int i = 0; i < count; ++i)
            {
                totalOctets += octetStrings[i].string.length;
            }

            byte[] string = new byte[totalOctets];
            for (int i = 0, pos = 0; i < count; ++i)
            {
                byte[] octets = octetStrings[i].string;
                System.arraycopy(octets, 0, string, pos, octets.length);
                pos += octets.length;
            }

//            assert pos == totalOctets;
            return string;
        }
        }
    }

    /**
     * Create an OCTET-STRING object from a byte[]
     * @param string the octets making up the octet string.
     */
    public BEROctetString(byte[] string)
    {
        this(string, DEFAULT_SEGMENT_LIMIT);
    }

    /**
     * Multiple {@link ASN1OctetString} data blocks are input,
     * the result is constructed form.
     *
     * @param elements an array of OCTET STRING to construct the BER OCTET STRING from.
     */
    public BEROctetString(ASN1OctetString[] elements)
    {
        this(elements, DEFAULT_SEGMENT_LIMIT);
    }

    /**
     * Create an OCTET-STRING object from a byte[]
     * @param string the octets making up the octet string.
     * @param segmentLimit the number of octets stored in each DER encoded component OCTET STRING.
     */
    public BEROctetString(byte[] string, int segmentLimit)
    {
        this(string, null, segmentLimit);
    }

    /**
     * Multiple {@link ASN1OctetString} data blocks are input,
     * the result is constructed form.
     *
     * @param elements an array of OCTET STRING to construct the BER OCTET STRING from.
     * @param segmentLimit the number of octets stored in each DER encoded component OCTET STRING.
     */
    public BEROctetString(ASN1OctetString[] elements, int segmentLimit)
    {
        this(flattenOctetStrings(elements), elements, segmentLimit);
    }

    private BEROctetString(byte[] string, ASN1OctetString[] elements, int segmentLimit)
    {
        super(string);
        this.elements = elements;
        this.segmentLimit = segmentLimit;
    }

    boolean encodeConstructed()
    {
        return true;
    }

    int encodedLength(boolean withTag)
        throws IOException
    {
        int totalLength = withTag ? 4 : 3;

        if (null != elements)
        {
            for (int i = 0; i < elements.length; ++i)
            {
                totalLength += elements[i].encodedLength(true);
            }
        }
        else
        {
            int fullSegments = string.length / segmentLimit;
            totalLength += fullSegments * DEROctetString.encodedLength(true, segmentLimit);

            int lastSegmentLength = string.length - (fullSegments * segmentLimit);
            if (lastSegmentLength > 0)
            {
                totalLength += DEROctetString.encodedLength(true, lastSegmentLength);
            }
        }

        return totalLength;
    }

    void encode(ASN1OutputStream out, boolean withTag) throws IOException
    {
        out.writeIdentifier(withTag, BERTags.CONSTRUCTED | BERTags.OCTET_STRING);
        out.write(0x80);

        if (null != elements)
        {
            out.writePrimitives(elements);
        }
        else
        {
            int pos = 0;
            while (pos < string.length)
            {
                int segmentLength = Math.min(string.length - pos, segmentLimit);
                DEROctetString.encode(out, true, string, pos, segmentLength);
                pos += segmentLength;
            }
        }

        out.write(0x00);
        out.write(0x00);
    }
}