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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 and up.

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package org.bouncycastle.pqc.crypto.xmss;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.Xof;

/**
 * Crypto functions for XMSS.
 */
final class KeyedHashFunctions
{
    private final Digest digest;
    private final int digestSize;

    protected KeyedHashFunctions(ASN1ObjectIdentifier treeDigest, int digestSize)
    {
        super();
        if (treeDigest == null)
        {
            throw new NullPointerException("digest == null");
        }
        this.digest = DigestUtil.getDigest(treeDigest);
        this.digestSize = digestSize;
    }

    private byte[] coreDigest(int fixedValue, byte[] key, byte[] index)
    {
        byte[] in = XMSSUtil.toBytesBigEndian(fixedValue, digestSize);
        /* fill first n byte of out buffer */
        digest.update(in, 0, in.length);
		/* add key */
        digest.update(key, 0, key.length);
		/* add index */
        digest.update(index, 0, index.length);

        byte[] out = new byte[digestSize];
        if (digest instanceof Xof)
        {
            ((Xof)digest).doFinal(out, 0, digestSize);
        }
        else
        {
            digest.doFinal(out, 0);
        }
        return out;
    }

    protected byte[] F(byte[] key, byte[] in)
    {
        if (key.length != digestSize)
        {
            throw new IllegalArgumentException("wrong key length");
        }
        if (in.length != digestSize)
        {
            throw new IllegalArgumentException("wrong in length");
        }
        return coreDigest(0, key, in);
    }

    protected byte[] H(byte[] key, byte[] in)
    {
        if (key.length != digestSize)
        {
            throw new IllegalArgumentException("wrong key length");
        }
        if (in.length != (2 * digestSize))
        {
            throw new IllegalArgumentException("wrong in length");
        }
        return coreDigest(1, key, in);
    }

    protected byte[] HMsg(byte[] key, byte[] in)
    {
        if (key.length != (3 * digestSize))
        {
            throw new IllegalArgumentException("wrong key length");
        }
        return coreDigest(2, key, in);
    }

    protected byte[] PRF(byte[] key, byte[] address)
    {
        if (key.length != digestSize)
        {
            throw new IllegalArgumentException("wrong key length");
        }
        if (address.length != 32)
        {
            throw new IllegalArgumentException("wrong address length");
        }
        return coreDigest(3, key, address);
    }
}