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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.pqc.crypto.xmss;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.Encodable;

/**
 * XMSS^MT Signature.
 */
public final class XMSSMTSignature
    implements XMSSStoreableObjectInterface, Encodable
{

    private final XMSSMTParameters params;
    private final long index;
    private final byte[] random;
    private final List reducedSignatures;

    private XMSSMTSignature(Builder builder)
    {
        super();
        params = builder.params;
        if (params == null)
        {
            throw new NullPointerException("params == null");
        }
        int n = params.getTreeDigestSize();
        byte[] signature = builder.signature;
        if (signature != null)
        {
            /* import */
            int len = params.getWOTSPlus().getParams().getLen();
            int indexSize = (int)Math.ceil(params.getHeight() / (double)8);
            int randomSize = n;
            int reducedSignatureSizeSingle = ((params.getHeight() / params.getLayers()) + len) * n;
            int reducedSignaturesSizeTotal = reducedSignatureSizeSingle * params.getLayers();
            int totalSize = indexSize + randomSize + reducedSignaturesSizeTotal;
            if (signature.length != totalSize)
            {
                throw new IllegalArgumentException("signature has wrong size");
            }
            int position = 0;
            index = XMSSUtil.bytesToXBigEndian(signature, position, indexSize);

            if (!XMSSUtil.isIndexValid(params.getHeight(), index))
            {
                throw new IllegalArgumentException("index out of bounds");
            }
            position += indexSize;
            random = XMSSUtil.extractBytesAtOffset(signature, position, randomSize);
            position += randomSize;
            reducedSignatures = new ArrayList();
            while (position < signature.length)
            {
                XMSSReducedSignature xmssSig = new XMSSReducedSignature.Builder(params.getXMSSParameters())
                    .withReducedSignature(XMSSUtil.extractBytesAtOffset(signature, position, reducedSignatureSizeSingle))
                    .build();
                reducedSignatures.add(xmssSig);
                position += reducedSignatureSizeSingle;
            }
        }
        else
        {
            /* set */
            index = builder.index;
            byte[] tmpRandom = builder.random;
            if (tmpRandom != null)
            {
                if (tmpRandom.length != n)
                {
                    throw new IllegalArgumentException("size of random needs to be equal to size of digest");
                }
                random = tmpRandom;
            }
            else
            {
                random = new byte[n];
            }
            List tmpReducedSignatures = builder.reducedSignatures;
            if (tmpReducedSignatures != null)
            {
                reducedSignatures = tmpReducedSignatures;
            }
            else
            {
                reducedSignatures = new ArrayList();
            }
        }
    }

    public byte[] getEncoded()
        throws IOException
    {
        return toByteArray();
    }

    public static class Builder
    {

        /* mandatory */
        private final XMSSMTParameters params;
        /* optional */
        private long index = 0L;
        private byte[] random = null;
        private List reducedSignatures = null;
        private byte[] signature = null;

        public Builder(XMSSMTParameters params)
        {
            super();
            this.params = params;
        }

        public Builder withIndex(long val)
        {
            index = val;
            return this;
        }

        public Builder withRandom(byte[] val)
        {
            random = XMSSUtil.cloneArray(val);
            return this;
        }

        public Builder withReducedSignatures(List val)
        {
            reducedSignatures = val;
            return this;
        }

        public Builder withSignature(byte[] val)
        {
            signature = Arrays.clone(val);
            return this;
        }

        public XMSSMTSignature build()
        {
            return new XMSSMTSignature(this);
        }
    }

    public byte[] toByteArray()
    {
        /* index || random || reduced signatures */
        int n = params.getTreeDigestSize();
        int len = params.getWOTSPlus().getParams().getLen();
        int indexSize = (int)Math.ceil(params.getHeight() / (double)8);
        int randomSize = n;
        int reducedSignatureSizeSingle = ((params.getHeight() / params.getLayers()) + len) * n;
        int reducedSignaturesSizeTotal = reducedSignatureSizeSingle * params.getLayers();
        int totalSize = indexSize + randomSize + reducedSignaturesSizeTotal;
        byte[] out = new byte[totalSize];
        int position = 0;
        /* copy index */
        byte[] indexBytes = XMSSUtil.toBytesBigEndian(index, indexSize);
        XMSSUtil.copyBytesAtOffset(out, indexBytes, position);
        position += indexSize;
        /* copy random */
        XMSSUtil.copyBytesAtOffset(out, random, position);
        position += randomSize;
        /* copy reduced signatures */
        for (XMSSReducedSignature reducedSignature : reducedSignatures)
        {
            byte[] signature = reducedSignature.toByteArray();
            XMSSUtil.copyBytesAtOffset(out, signature, position);
            position += reducedSignatureSizeSingle;
        }
        return out;
    }

    public long getIndex()
    {
        return index;
    }

    public byte[] getRandom()
    {
        return XMSSUtil.cloneArray(random);
    }

    public List getReducedSignatures()
    {
        return reducedSignatures;
    }
}