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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified.
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package org.bouncycastle.eac.jcajce;

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.Provider;
import java.security.PublicKey;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECField;
import java.security.spec.ECFieldF2m;
import java.security.spec.ECFieldFp;
import java.security.spec.ECParameterSpec;
import java.security.spec.ECPublicKeySpec;
import java.security.spec.EllipticCurve;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.eac.EACObjectIdentifiers;
import org.bouncycastle.asn1.eac.ECDSAPublicKey;
import org.bouncycastle.asn1.eac.PublicKeyDataObject;
import org.bouncycastle.asn1.eac.RSAPublicKey;
import org.bouncycastle.eac.EACException;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECCurve;
import org.bouncycastle.math.ec.ECPoint;
import org.bouncycastle.math.field.FiniteField;
import org.bouncycastle.math.field.Polynomial;
import org.bouncycastle.math.field.PolynomialExtensionField;
import org.bouncycastle.util.Arrays;

public class JcaPublicKeyConverter
{
    private EACHelper helper = new DefaultEACHelper();

    public JcaPublicKeyConverter setProvider(String providerName)
    {
        this.helper = new NamedEACHelper(providerName);

        return this;
    }

    public JcaPublicKeyConverter setProvider(Provider provider)
    {
        this.helper = new ProviderEACHelper(provider);

        return this;
    }

    public PublicKey getKey(PublicKeyDataObject publicKeyDataObject)
        throws EACException, InvalidKeySpecException
    {
        if (publicKeyDataObject.getUsage().on(EACObjectIdentifiers.id_TA_ECDSA))
        {
            return getECPublicKeyPublicKey((ECDSAPublicKey)publicKeyDataObject);
        }
        else
        {
            RSAPublicKey pubKey = (RSAPublicKey)publicKeyDataObject;
            RSAPublicKeySpec pubKeySpec = new RSAPublicKeySpec(pubKey.getModulus(), pubKey.getPublicExponent());

            try
            {
                KeyFactory factk = helper.createKeyFactory("RSA");

                return factk.generatePublic(pubKeySpec);
            }
            catch (NoSuchProviderException e)
            {
                throw new EACException("cannot find provider: " + e.getMessage(), e);
            }
            catch (NoSuchAlgorithmException e)
            {
                throw new EACException("cannot find algorithm ECDSA: " + e.getMessage(), e);
            }
        }
    }

    private PublicKey getECPublicKeyPublicKey(ECDSAPublicKey key)
        throws EACException, InvalidKeySpecException
    {
        ECParameterSpec spec = getParams(key);
        java.security.spec.ECPoint publicPoint = getPublicPoint(key);
        ECPublicKeySpec pubKeySpec = new ECPublicKeySpec(publicPoint, spec);

        KeyFactory factk;
        try
        {
            factk = helper.createKeyFactory("ECDSA");
        }
        catch (NoSuchProviderException e)
        {
            throw new EACException("cannot find provider: " + e.getMessage(), e);
        }
        catch (NoSuchAlgorithmException e)
        {
            throw new EACException("cannot find algorithm ECDSA: " + e.getMessage(), e);
        }

        return factk.generatePublic(pubKeySpec);
    }

    private java.security.spec.ECPoint getPublicPoint(ECDSAPublicKey key)
    {
        if (!key.hasParameters())
        {
            throw new IllegalArgumentException("Public key does not contains EC Params");
        }

        BigInteger p = key.getPrimeModulusP();
        ECCurve.Fp curve = new ECCurve.Fp(p, key.getFirstCoefA(), key.getSecondCoefB(), key.getOrderOfBasePointR(), key.getCofactorF());

        ECPoint.Fp pubY = (ECPoint.Fp)curve.decodePoint(key.getPublicPointY());

        return new java.security.spec.ECPoint(pubY.getAffineXCoord().toBigInteger(), pubY.getAffineYCoord().toBigInteger());
    }

    private ECParameterSpec getParams(ECDSAPublicKey key)
    {
        if (!key.hasParameters())
        {
            throw new IllegalArgumentException("Public key does not contains EC Params");
        }

        BigInteger p = key.getPrimeModulusP();
        ECCurve.Fp curve = new ECCurve.Fp(p, key.getFirstCoefA(), key.getSecondCoefB(), key.getOrderOfBasePointR(), key.getCofactorF());

        ECPoint G = curve.decodePoint(key.getBasePointG());

        BigInteger order = key.getOrderOfBasePointR();
        BigInteger coFactor = key.getCofactorF();

        EllipticCurve jcaCurve = convertCurve(curve);

        return new ECParameterSpec(jcaCurve, new java.security.spec.ECPoint(G.getAffineXCoord().toBigInteger(), G.getAffineYCoord().toBigInteger()), order, coFactor.intValue());
    }

    public PublicKeyDataObject getPublicKeyDataObject(ASN1ObjectIdentifier usage, PublicKey publicKey)
    {
        if (publicKey instanceof java.security.interfaces.RSAPublicKey)
        {
            java.security.interfaces.RSAPublicKey pubKey = (java.security.interfaces.RSAPublicKey)publicKey;

            return new RSAPublicKey(usage, pubKey.getModulus(), pubKey.getPublicExponent());
        }
        else
        {
            ECPublicKey pubKey = (ECPublicKey)publicKey;
            java.security.spec.ECParameterSpec params = pubKey.getParams();

            EllipticCurve c1 = params.getCurve();
            ECCurve c2 = convertCurve(c1, params.getOrder(), params.getCofactor());

            ECPoint basePoint = convertPoint(c2, params.getGenerator());
            ECPoint publicPoint = convertPoint(c2, pubKey.getW());

            return new ECDSAPublicKey(usage, ((ECFieldFp)c1.getField()).getP(), c1.getA(), c1.getB(),
                basePoint.getEncoded(false), params.getOrder(), publicPoint.getEncoded(false), params.getCofactor());
        }
    }

    private static org.bouncycastle.math.ec.ECPoint convertPoint(
        ECCurve curve,
        java.security.spec.ECPoint point)
    {
        return curve.createPoint(point.getAffineX(), point.getAffineY());
    }

    private static ECCurve convertCurve(
        EllipticCurve ec, BigInteger order, int coFactor)
    {
        ECField field = ec.getField();
        BigInteger a = ec.getA();
        BigInteger b = ec.getB();

        if (field instanceof ECFieldFp)
        {
            return new ECCurve.Fp(((ECFieldFp)field).getP(), a, b, order, BigInteger.valueOf(coFactor));
        }
        else
        {
            throw new IllegalStateException("not implemented yet!!!");
        }
    }

    private static EllipticCurve convertCurve(
        ECCurve curve)
    {
        ECField field = convertField(curve.getField());
        BigInteger a = curve.getA().toBigInteger(), b = curve.getB().toBigInteger();

        // TODO: the Sun EC implementation doesn't currently handle the seed properly
        // so at the moment it's set to null. Should probably look at making this configurable
        return new EllipticCurve(field, a, b, null);
    }

    private static ECField convertField(FiniteField field)
    {
        if (ECAlgorithms.isFpField(field))
        {
            return new ECFieldFp(field.getCharacteristic());
        }
        else //if (ECAlgorithms.isF2mField(curveField))
        {
            Polynomial poly = ((PolynomialExtensionField)field).getMinimalPolynomial();
            int[] exponents = poly.getExponentsPresent();
            int[] ks = Arrays.reverse(Arrays.copyOfRange(exponents, 1, exponents.length - 1));
            return new ECFieldF2m(poly.getDegree(), ks);
        }
    }
}