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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.
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package org.bouncycastle.pqc.crypto.lms;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.Pack;

class LM_OTS
{

    private static final short D_PBLC = (short)0x8080;
    private static final int ITER_K = 20;
    private static final int ITER_PREV = 23;
    private static final int ITER_J = 22;
    static final int SEED_RANDOMISER_INDEX = ~2;
    static final int MAX_HASH = 32;

    static final short D_MESG = (short)0x8181;


    public static int coef(byte[] S, int i, int w)
    {
        int index = (i * w) / 8;
        int digits_per_byte = 8 / w;
        int shift = w * (~i & (digits_per_byte - 1));
        int mask = (1 << w) - 1;

        return (S[index] >>> shift) & mask;
    }


    public static int cksm(byte[] S, int sLen, LMOtsParameters parameters)
    {
        int sum = 0;

        int w = parameters.getW();

        // NB assumption about size of "w" not overflowing integer.
        int twoWpow = (1 << w) - 1;

        for (int i = 0; i < (sLen * 8 / parameters.getW()); i++)
        {
            sum = sum + twoWpow - coef(S, i, parameters.getW());
        }
        return sum << parameters.getLs();
    }


    public static LMOtsPublicKey lms_ots_generatePublicKey(LMOtsPrivateKey privateKey)
    {
        byte[] K = lms_ots_generatePublicKey(privateKey.getParameter(), privateKey.getI(), privateKey.getQ(), privateKey.getMasterSecret());
        return new LMOtsPublicKey(privateKey.getParameter(), privateKey.getI(), privateKey.getQ(), K);
    }

    static byte[] lms_ots_generatePublicKey(LMOtsParameters parameter, byte[] I, int q, byte[] masterSecret)
    {
        //
        // Start hash that computes the final value.
        //
        Digest publicContext = DigestUtil.getDigest(parameter);
        byte[] prehashPrefix = Composer.compose()
            .bytes(I)
            .u32str(q)
            .u16str(D_PBLC)
            .padUntil(0, 22)
            .build();
        publicContext.update(prehashPrefix, 0, prehashPrefix.length);

        Digest ctx = DigestUtil.getDigest(parameter);

        byte[] buf = Composer.compose()
            .bytes(I)
            .u32str(q)
            .padUntil(0, 23 + ctx.getDigestSize())
            .build();


        SeedDerive derive = new SeedDerive(I, masterSecret, DigestUtil.getDigest(parameter));
        derive.setQ(q);
        derive.setJ(0);

        int p = parameter.getP();
        int n = parameter.getN();
        final int twoToWminus1 = (1 << parameter.getW()) - 1;


        for (int i = 0; i < p; i++)
        {
            derive.deriveSeed(buf, i < p - 1, ITER_PREV); // Private Key!
            Pack.shortToBigEndian((short)i, buf, ITER_K);
            for (int j = 0; j < twoToWminus1; j++)
            {
                buf[ITER_J] = (byte)j;
                ctx.update(buf, 0, buf.length);
                ctx.doFinal(buf, ITER_PREV);
            }
            publicContext.update(buf, ITER_PREV, n);
        }

        byte[] K = new byte[publicContext.getDigestSize()];
        publicContext.doFinal(K, 0);

        return K;

    }

    public static LMOtsSignature lm_ots_generate_signature(LMSigParameters sigParams, LMOtsPrivateKey privateKey, byte[][] path, byte[] message, boolean preHashed)
    {
        //
        // Add the randomizer.
        //

        byte[] C;
        byte[] Q = new byte[MAX_HASH + 2];

        if (!preHashed)
        {
            LMSContext qCtx = privateKey.getSignatureContext(sigParams, path);

            LmsUtils.byteArray(message, 0, message.length, qCtx);

            C = qCtx.getC();
            Q = qCtx.getQ();
        }
        else
        {
            int n = privateKey.getParameter().getN();
            
            C = new byte[n];
            System.arraycopy(message, 0, Q, 0, n);
        }

        return lm_ots_generate_signature(privateKey, Q, C);
    }

    public static LMOtsSignature lm_ots_generate_signature(LMOtsPrivateKey privateKey, byte[] Q, byte[] C)
    {
        LMOtsParameters parameter = privateKey.getParameter();

        int n = parameter.getN();
        int p = parameter.getP();
        int w = parameter.getW();

        byte[] sigComposer = new byte[p * n];

        Digest ctx = DigestUtil.getDigest(parameter);

        SeedDerive derive = privateKey.getDerivationFunction();

        int cs = cksm(Q, n, parameter);
        Q[n] = (byte)((cs >>> 8) & 0xFF);
        Q[n + 1] = (byte)cs;

        byte[] tmp = Composer.compose().bytes(privateKey.getI()).u32str(privateKey.getQ()).padUntil(0, ITER_PREV + n).build();

        derive.setJ(0);
        for (int i = 0; i < p; i++)
        {
            Pack.shortToBigEndian((short)i, tmp, ITER_K);
            derive.deriveSeed(tmp, i < p - 1, ITER_PREV);
            int a = coef(Q, i, w);
            for (int j = 0; j < a; j++)
            {
                tmp[ITER_J] = (byte)j;
                ctx.update(tmp, 0, ITER_PREV + n);
                ctx.doFinal(tmp, ITER_PREV);
            }
            System.arraycopy(tmp, ITER_PREV, sigComposer, n * i, n);
        }

        return new LMOtsSignature(parameter, C, sigComposer);
    }

    public static boolean lm_ots_validate_signature(LMOtsPublicKey publicKey, LMOtsSignature signature, byte[] message, boolean prehashed)
        throws LMSException
    {
        if (!signature.getType().equals(publicKey.getParameter()))
        {
            throw new LMSException("public key and signature ots types do not match");
        }
        return Arrays.areEqual(lm_ots_validate_signature_calculate(publicKey, signature, message), publicKey.getK());
    }

    public static byte[] lm_ots_validate_signature_calculate(LMOtsPublicKey publicKey, LMOtsSignature signature, byte[] message)
    {
        LMSContext ctx = publicKey.createOtsContext(signature);

        LmsUtils.byteArray(message, ctx);

        return lm_ots_validate_signature_calculate(ctx);
    }

    public static byte[] lm_ots_validate_signature_calculate(LMSContext context)
    {
        LMOtsPublicKey publicKey = context.getPublicKey();
        LMOtsParameters parameter = publicKey.getParameter();
        Object sig = context.getSignature();
        LMOtsSignature signature;
        if (sig instanceof LMSSignature)
        {
            signature = ((LMSSignature)sig).getOtsSignature();
        }
        else
        {
            signature = (LMOtsSignature)sig;
        }

        int n = parameter.getN();
        int w = parameter.getW();
        int p = parameter.getP();
        byte[] Q = context.getQ();

        int cs = cksm(Q, n, parameter);
        Q[n] = (byte)((cs >>> 8) & 0xFF);
        Q[n + 1] = (byte)cs;

        byte[] I = publicKey.getI();
        int    q = publicKey.getQ();

        Digest finalContext = DigestUtil.getDigest(parameter);
        LmsUtils.byteArray(I, finalContext);
        LmsUtils.u32str(q, finalContext);
        LmsUtils.u16str(D_PBLC, finalContext);

        byte[] tmp = Composer.compose()
            .bytes(I)
            .u32str(q)
            .padUntil(0, ITER_PREV + n).build();

        int max_digit = (1 << w) - 1;

        byte[] y = signature.getY();

        Digest ctx = DigestUtil.getDigest(parameter);
        for (int i = 0; i < p; i++)
        {
            Pack.shortToBigEndian((short)i, tmp, ITER_K);
            System.arraycopy(y, i * n, tmp, ITER_PREV, n);
            int a = coef(Q, i, w);

            for (int j = a; j < max_digit; j++)
            {
                tmp[ITER_J] = (byte)j;
                ctx.update(tmp, 0, ITER_PREV + n);
                ctx.doFinal(tmp, ITER_PREV);
            }

            finalContext.update(tmp, ITER_PREV, n);
        }

        byte[] K = new byte[n];
        finalContext.doFinal(K, 0);

        return K;
    }
}