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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.
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package org.bouncycastle.pqc.crypto.util;

import java.io.IOException;
import java.io.InputStream;
import java.util.HashMap;
import java.util.Map;

import org.bouncycastle.asn1.ASN1InputStream;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.pqc.crypto.lms.HSSPublicKeyParameters;
import org.bouncycastle.pqc.crypto.lms.LMSPublicKeyParameters;
import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.Pack;

/**
 * Factory to create asymmetric public key parameters for asymmetric ciphers from range of
 * ASN.1 encoded SubjectPublicKeyInfo objects.
 */
public class PublicKeyFactory
{
    private static Map converters = new HashMap();

    static
    {
        converters.put(PKCSObjectIdentifiers.id_alg_hss_lms_hashsig, new LMSConverter());
    }

    /**
     * Create a public key from a SubjectPublicKeyInfo encoding
     *
     * @param keyInfoData the SubjectPublicKeyInfo encoding
     * @return the appropriate key parameter
     * @throws IOException on an error decoding the key
     */
    public static AsymmetricKeyParameter createKey(byte[] keyInfoData)
        throws IOException
    {
        if (keyInfoData == null)
        {
            throw new IllegalArgumentException("keyInfoData array null");
        }
        if (keyInfoData.length == 0)
        {
            throw new IllegalArgumentException("keyInfoData array empty");
        }
        return createKey(SubjectPublicKeyInfo.getInstance(ASN1Primitive.fromByteArray(keyInfoData)));
    }

    /**
     * Create a public key from a SubjectPublicKeyInfo encoding read from a stream
     *
     * @param inStr the stream to read the SubjectPublicKeyInfo encoding from
     * @return the appropriate key parameter
     * @throws IOException on an error decoding the key
     */
    public static AsymmetricKeyParameter createKey(InputStream inStr)
        throws IOException
    {
        return createKey(SubjectPublicKeyInfo.getInstance(new ASN1InputStream(inStr).readObject()));
    }

    /**
     * Create a public key from the passed in SubjectPublicKeyInfo
     *
     * @param keyInfo the SubjectPublicKeyInfo containing the key data
     * @return the appropriate key parameter
     * @throws IOException on an error decoding the key
     */
    public static AsymmetricKeyParameter createKey(SubjectPublicKeyInfo keyInfo)
        throws IOException
    {
        if (keyInfo == null)
        {
            throw new IllegalArgumentException("keyInfo argument null");
        }
        return createKey(keyInfo, null);
    }

    /**
     * Create a public key from the passed in SubjectPublicKeyInfo
     *
     * @param keyInfo       the SubjectPublicKeyInfo containing the key data
     * @param defaultParams default parameters that might be needed.
     * @return the appropriate key parameter
     * @throws IOException on an error decoding the key
     */
    public static AsymmetricKeyParameter createKey(SubjectPublicKeyInfo keyInfo, Object defaultParams)
        throws IOException
    {
        if (keyInfo == null)
        {
            throw new IllegalArgumentException("keyInfo argument null");
        }

        AlgorithmIdentifier algId = keyInfo.getAlgorithm();
        SubjectPublicKeyInfoConverter converter = (SubjectPublicKeyInfoConverter)converters.get(algId.getAlgorithm());

        if (converter != null)
        {
            return converter.getPublicKeyParameters(keyInfo, defaultParams);
        }
        else
        {
            throw new IOException("algorithm identifier in public key not recognised: " + algId.getAlgorithm());
        }
    }

    private static abstract class SubjectPublicKeyInfoConverter
    {
        abstract AsymmetricKeyParameter getPublicKeyParameters(SubjectPublicKeyInfo keyInfo, Object defaultParams)
            throws IOException;
    }

    private static class LMSConverter
        extends SubjectPublicKeyInfoConverter
    {
        AsymmetricKeyParameter getPublicKeyParameters(SubjectPublicKeyInfo keyInfo, Object defaultParams)
            throws IOException
        {
            byte[] keyEnc = ASN1OctetString.getInstance(keyInfo.parsePublicKey()).getOctets();

            if (Pack.bigEndianToInt(keyEnc, 0) == 1)
            {
                return LMSPublicKeyParameters.getInstance(Arrays.copyOfRange(keyEnc, 4, keyEnc.length));
            }
            else
            {
                // public key with extra tree height
                if (keyEnc.length == 64)
                {
                    keyEnc = Arrays.copyOfRange(keyEnc, 4, keyEnc.length);
                }
                return HSSPublicKeyParameters.getInstance(keyEnc);
            }
        }
    }
}