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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.lms;

import org.bouncycastle.crypto.Digest;

class SeedDerive
{
    private final byte[] I;
    private final byte[] masterSeed;
    private final Digest digest;
    private int q;
    private int j;


    public SeedDerive(byte[] I, byte[] masterSeed, Digest digest)
    {
        this.I = I;
        this.masterSeed = masterSeed;
        this.digest = digest;
    }

    public int getQ()
    {
        return q;
    }

    public void setQ(int q)
    {
        this.q = q;
    }

    public int getJ()
    {
        return j;
    }

    public void setJ(int j)
    {
        this.j = j;
    }

    public byte[] getI()
    {
        return I;
    }

    public byte[] getMasterSeed()
    {
        return masterSeed;
    }


    public byte[] deriveSeed(byte[] target, int offset)
    {
        if (target.length - offset < digest.getDigestSize())
        {
            throw new IllegalArgumentException("target length is less than digest size.");
        }

        digest.update(I, 0, I.length);
        digest.update((byte)(q >>> 24));
        digest.update((byte)(q >>> 16));
        digest.update((byte)(q >>> 8));
        digest.update((byte)(q));

        digest.update((byte)(j >>> 8));
        digest.update((byte)(j));
        digest.update((byte)0xFF);
        digest.update(masterSeed, 0, masterSeed.length);

        digest.doFinal(target, offset); // Digest resets here.

        return target;
    }

    public void deriveSeed(byte[] target, boolean incJ)
    {
        deriveSeed(target, incJ, 0);
    }


    public void deriveSeed(byte[] target, boolean incJ, int offset)
    {

        deriveSeed(target, offset);

        if (incJ)
        {
            j++;
        }

    }
}