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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.pqc.crypto.ntru;

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.AsymmetricCipherKeyPairGenerator;
import org.bouncycastle.crypto.KeyGenerationParameters;
import org.bouncycastle.pqc.math.ntru.polynomial.DenseTernaryPolynomial;
import org.bouncycastle.pqc.math.ntru.polynomial.IntegerPolynomial;
import org.bouncycastle.pqc.math.ntru.polynomial.Polynomial;
import org.bouncycastle.pqc.math.ntru.polynomial.ProductFormPolynomial;
import org.bouncycastle.pqc.math.ntru.util.Util;

/**
 * Generates key pairs.

 * The parameter p is hardcoded to 3.
 */
public class NTRUEncryptionKeyPairGenerator
    implements AsymmetricCipherKeyPairGenerator
{
    private NTRUEncryptionKeyGenerationParameters params;

    /**
     * Constructs a new instance with a set of encryption parameters.
     *
     * @param param encryption parameters
     */
    public void init(KeyGenerationParameters param)
    {
        this.params = (NTRUEncryptionKeyGenerationParameters)param;
    }

    /**
     * Generates a new encryption key pair.
     *
     * @return a key pair
     */
    public AsymmetricCipherKeyPair generateKeyPair()
    {
        int N = params.N;
        int q = params.q;
        int df = params.df;
        int df1 = params.df1;
        int df2 = params.df2;
        int df3 = params.df3;
        int dg = params.dg;
        boolean fastFp = params.fastFp;
        boolean sparse = params.sparse;

        Polynomial t;
        IntegerPolynomial fq;
        IntegerPolynomial fp = null;

        // choose a random f that is invertible mod 3 and q
        while (true)
        {
            IntegerPolynomial f;

            // choose random t, calculate f and fp
            if (fastFp)
            {
                // if fastFp=true, f is always invertible mod 3
                t = params.polyType == NTRUParameters.TERNARY_POLYNOMIAL_TYPE_SIMPLE ? Util.generateRandomTernary(N, df, df, sparse, params.getRandom()) : ProductFormPolynomial.generateRandom(N, df1, df2, df3, df3, params.getRandom());
                f = t.toIntegerPolynomial();
                f.mult(3);
                f.coeffs[0] += 1;
            }
            else
            {
                t = params.polyType == NTRUParameters.TERNARY_POLYNOMIAL_TYPE_SIMPLE ? Util.generateRandomTernary(N, df, df - 1, sparse, params.getRandom()) : ProductFormPolynomial.generateRandom(N, df1, df2, df3, df3 - 1, params.getRandom());
                f = t.toIntegerPolynomial();
                fp = f.invertF3();
                if (fp == null)
                {
                    continue;
                }
            }

            fq = f.invertFq(q);
            if (fq == null)
            {
                continue;
            }
            break;
        }

        // if fastFp=true, fp=1
        if (fastFp)
        {
            fp = new IntegerPolynomial(N);
            fp.coeffs[0] = 1;
        }

        // choose a random g that is invertible mod q
        DenseTernaryPolynomial g;
        while (true)
        {
            g = DenseTernaryPolynomial.generateRandom(N, dg, dg - 1, params.getRandom());
            if (g.invertFq(q) != null)
            {
                break;
            }
        }

        IntegerPolynomial h = g.mult(fq, q);
        h.mult3(q);
        h.ensurePositive(q);
        g.clear();
        fq.clear();

        NTRUEncryptionPrivateKeyParameters priv = new NTRUEncryptionPrivateKeyParameters(h, t, fp, params.getEncryptionParameters());
        NTRUEncryptionPublicKeyParameters pub = new NTRUEncryptionPublicKeyParameters(h, params.getEncryptionParameters());
        return new AsymmetricCipherKeyPair(pub, priv);
    }
}