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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.ntru;

import java.nio.ByteBuffer;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.pqc.math.ntru.polynomial.IntegerPolynomial;
import org.bouncycastle.pqc.math.ntru.polynomial.Polynomial;

/**
* Signs, verifies data and generates key pairs.
* @deprecated the NTRUSigner algorithm was broken in 2012 by Ducas and Nguyen. See
* 
* https://www.di.ens.fr/~ducas/NTRUSign_Cryptanalysis/DucasNguyen_Learning.pdf
* for details.
*/
public class NTRUSigner
{
    private NTRUSigningParameters params;
    private Digest hashAlg;
    private NTRUSigningPrivateKeyParameters signingKeyPair;
    private NTRUSigningPublicKeyParameters verificationKey;

    /**
     * Constructs a new instance with a set of signature parameters.
     *
     * @param params signature parameters
     */
    public NTRUSigner(NTRUSigningParameters params)
    {
        this.params = params;
    }

    /**
     * Resets the engine for signing a message.
     *
     * @param forSigning
     * @param params
     */
    public void init(boolean forSigning, CipherParameters params)
    {
        if (forSigning)
        {
            this.signingKeyPair = (NTRUSigningPrivateKeyParameters)params;
        }
        else
        {
            this.verificationKey = (NTRUSigningPublicKeyParameters)params;
        }
        hashAlg = this.params.hashAlg;
        hashAlg.reset();
    }

    /**
      * Adds data to sign or verify.
      *
      * @param b data
      */
     public void update(byte b)
     {
         if (hashAlg == null)
         {
             throw new IllegalStateException("Call initSign or initVerify first!");
         }

         hashAlg.update(b);
     }

    /**
     * Adds data to sign or verify.
     *
     * @param m data
     * @param off offset
     * @param length number of bytes
     */
    public void update(byte[] m, int off, int length)
    {
        if (hashAlg == null)
        {
            throw new IllegalStateException("Call initSign or initVerify first!");
        }

        hashAlg.update(m, off, length);
    }

    /**
     * Adds data to sign and computes a signature over this data and any data previously added via {@link #update(byte[], int, int)}.
     *
     * @return a signature
     * @throws IllegalStateException if initSign was not called
     */
    public byte[] generateSignature()
    {
        if (hashAlg == null || signingKeyPair == null)
        {
            throw new IllegalStateException("Call initSign first!");
        }

        byte[] msgHash = new byte[hashAlg.getDigestSize()];

        hashAlg.doFinal(msgHash, 0);
        return signHash(msgHash, signingKeyPair);
    }

    private byte[] signHash(byte[] msgHash, NTRUSigningPrivateKeyParameters kp)
    {
        int r = 0;
        IntegerPolynomial s;
        IntegerPolynomial i;

        NTRUSigningPublicKeyParameters kPub = kp.getPublicKey();
        do
        {
            r++;
            if (r > params.signFailTolerance)
            {
                throw new IllegalStateException("Signing failed: too many retries (max=" + params.signFailTolerance + ")");
            }
            i = createMsgRep(msgHash, r);
            s = sign(i, kp);
        }
        while (!verify(i, s, kPub.h));

        byte[] rawSig = s.toBinary(params.q);
        ByteBuffer sbuf = ByteBuffer.allocate(rawSig.length + 4);
        sbuf.put(rawSig);
        sbuf.putInt(r);
        return sbuf.array();
    }

    private IntegerPolynomial sign(IntegerPolynomial i, NTRUSigningPrivateKeyParameters kp)
    {
        int N = params.N;
        int q = params.q;
        int perturbationBases = params.B;

        NTRUSigningPrivateKeyParameters kPriv = kp;
        NTRUSigningPublicKeyParameters kPub = kp.getPublicKey();

        IntegerPolynomial s = new IntegerPolynomial(N);
        int iLoop = perturbationBases;
        while (iLoop >= 1)
        {
            Polynomial f = kPriv.getBasis(iLoop).f;
            Polynomial fPrime = kPriv.getBasis(iLoop).fPrime;

            IntegerPolynomial y = f.mult(i);
            y.div(q);
            y = fPrime.mult(y);

            IntegerPolynomial x = fPrime.mult(i);
            x.div(q);
            x = f.mult(x);

            IntegerPolynomial si = y;
            si.sub(x);
            s.add(si);

            IntegerPolynomial hi = (IntegerPolynomial)kPriv.getBasis(iLoop).h.clone();
            if (iLoop > 1)
            {
                hi.sub(kPriv.getBasis(iLoop - 1).h);
            }
            else
            {
                hi.sub(kPub.h);
            }
            i = si.mult(hi, q);

            iLoop--;
        }

        Polynomial f = kPriv.getBasis(0).f;
        Polynomial fPrime = kPriv.getBasis(0).fPrime;

        IntegerPolynomial y = f.mult(i);
        y.div(q);
        y = fPrime.mult(y);

        IntegerPolynomial x = fPrime.mult(i);
        x.div(q);
        x = f.mult(x);

        y.sub(x);
        s.add(y);
        s.modPositive(q);
        return s;
    }

    /**
     * Verifies a signature for any data previously added via {@link #update(byte[], int, int)}.
     *
     * @param sig a signature
     * @return whether the signature is valid
     * @throws IllegalStateException if initVerify was not called
     */
    public boolean verifySignature(byte[] sig)
    {
        if (hashAlg == null || verificationKey == null)
        {
            throw new IllegalStateException("Call initVerify first!");
        }

        byte[] msgHash = new byte[hashAlg.getDigestSize()];

        hashAlg.doFinal(msgHash, 0);

        return verifyHash(msgHash, sig, verificationKey);
    }

    private boolean verifyHash(byte[] msgHash, byte[] sig, NTRUSigningPublicKeyParameters pub)
    {
        ByteBuffer sbuf = ByteBuffer.wrap(sig);
        byte[] rawSig = new byte[sig.length - 4];
        sbuf.get(rawSig);
        IntegerPolynomial s = IntegerPolynomial.fromBinary(rawSig, params.N, params.q);
        int r = sbuf.getInt();
        return verify(createMsgRep(msgHash, r), s, pub.h);
    }

    private boolean verify(IntegerPolynomial i, IntegerPolynomial s, IntegerPolynomial h)
    {
        int q = params.q;
        double normBoundSq = params.normBoundSq;
        double betaSq = params.betaSq;

        IntegerPolynomial t = h.mult(s, q);
        t.sub(i);
        long centeredNormSq = (long)(s.centeredNormSq(q) + betaSq * t.centeredNormSq(q));
        return centeredNormSq <= normBoundSq;
    }

    protected IntegerPolynomial createMsgRep(byte[] msgHash, int r)
    {
        int N = params.N;
        int q = params.q;

        int c = 31 - Integer.numberOfLeadingZeros(q);
        int B = (c + 7) / 8;
        IntegerPolynomial i = new IntegerPolynomial(N);

        ByteBuffer cbuf = ByteBuffer.allocate(msgHash.length + 4);
        cbuf.put(msgHash);
        cbuf.putInt(r);
        NTRUSignerPrng prng = new NTRUSignerPrng(cbuf.array(), params.hashAlg);

        for (int t = 0; t < N; t++)
        {
            byte[] o = prng.nextBytes(B);
            int hi = o[o.length - 1];
            hi >>= 8 * B - c;
            hi <<= 8 * B - c;
            o[o.length - 1] = (byte)hi;

            ByteBuffer obuf = ByteBuffer.allocate(4);
            obuf.put(o);
            obuf.rewind();
            // reverse byte order so it matches the endianness of java ints
            i.coeffs[t] = Integer.reverseBytes(obuf.getInt());
        }
        return i;
    }
}