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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 1.8. Note: this package includes the NTRU encryption algorithms.

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package org.bouncycastle.pqc.crypto.xmss;

import java.text.ParseException;
import java.util.ArrayList;
import java.util.List;

/**
 * Reduced XMSS Signature.
 *
 */
public class XMSSReducedSignature implements XMSSStoreableObjectInterface {

	private final XMSSParameters params;
	private final WOTSPlusSignature wotsPlusSignature;
	private final List authPath;

	protected XMSSReducedSignature(Builder builder) throws ParseException {
		super();
		params = builder.params;
		if (params == null) {
			throw new NullPointerException("params == null");
		}
		int n = params.getDigestSize();
		int len = params.getWOTSPlus().getParams().getLen();
		int height = params.getHeight();
		byte[] reducedSignature = builder.reducedSignature;
		if (reducedSignature != null) {
			/* import */
			int signatureSize = len * n;
			int authPathSize = height * n;
			int totalSize = signatureSize + authPathSize;
			if (reducedSignature.length != totalSize) {
				throw new ParseException("signature has wrong size", 0);
			}
			int position = 0;
			byte[][] wotsPlusSignature = new byte[len][];
			for (int i = 0; i < wotsPlusSignature.length; i++) {
				wotsPlusSignature[i] = XMSSUtil.extractBytesAtOffset(reducedSignature, position, n);
				position += n;
			}
			this.wotsPlusSignature = new WOTSPlusSignature(params.getWOTSPlus().getParams(), wotsPlusSignature);

			List nodeList = new ArrayList();
			for (int i = 0; i < height; i++) {
				nodeList.add(new XMSSNode(i, XMSSUtil.extractBytesAtOffset(reducedSignature, position, n)));
				position += n;
			}
			authPath = nodeList;
		} else {
			/* set */
			WOTSPlusSignature tmpSignature = builder.wotsPlusSignature;
			if (tmpSignature != null) {
				wotsPlusSignature = tmpSignature;
			} else {
				wotsPlusSignature = new WOTSPlusSignature(params.getWOTSPlus().getParams(), new byte[len][n]);
			}
			List tmpAuthPath = builder.authPath;
			if (tmpAuthPath != null) {
				if (tmpAuthPath.size() != height) {
					throw new IllegalArgumentException("size of authPath needs to be equal to height of tree");
				}
				authPath = tmpAuthPath;
			} else {
				authPath = new ArrayList();
			}
		}
	}
	
	public static class Builder {
		
		/* mandatory */
		private final XMSSParameters params;
		/* optional */
		private WOTSPlusSignature wotsPlusSignature = null;
		private List authPath = null;
		private byte[] reducedSignature = null;
		
		public Builder(XMSSParameters params) {
			super();
			this.params = params;
		}
		
		public Builder withWOTSPlusSignature(WOTSPlusSignature val) {
			wotsPlusSignature = val;
			return this;
		}
		
		public Builder withAuthPath(List val) {
			authPath = val;
			return this;
		}
		
		public Builder withReducedSignature(byte[] val) {
			reducedSignature = XMSSUtil.cloneArray(val);
			return this;
		}
		
		public XMSSReducedSignature build() throws ParseException {
			return new XMSSReducedSignature(this);
		}
	}

	public byte[] toByteArray() {
		/* signature || authentication path */
		int n = params.getDigestSize();
		int signatureSize = params.getWOTSPlus().getParams().getLen() * n;
		int authPathSize = params.getHeight() * n;
		int totalSize = signatureSize + authPathSize;
		byte[] out = new byte[totalSize];
		int position = 0;
		/* copy signature */
		byte[][] signature = this.wotsPlusSignature.toByteArray();
		for (int i = 0; i < signature.length; i++) {
			XMSSUtil.copyBytesAtOffset(out, signature[i], position);
			position += n;
		}
		/* copy authentication path */
		for (int i = 0; i < authPath.size(); i++) {
			byte[] value = authPath.get(i).getValue();
			XMSSUtil.copyBytesAtOffset(out, value, position);
			position += n;
		}
		return out;
	}

	public XMSSParameters getParams() {
		return params;
	}

	public WOTSPlusSignature getWOTSPlusSignature() {
		return wotsPlusSignature;
	}

	public List getAuthPath() {
		return authPath;
	}
}