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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 to JDK 1.8. 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.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());

        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());

        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();

            return new ECDSAPublicKey(
                usage,
                ((ECFieldFp)params.getCurve().getField()).getP(),
                params.getCurve().getA(), params.getCurve().getB(),
                convertPoint(convertCurve(params.getCurve()), params.getGenerator()).getEncoded(),
                params.getOrder(),
                convertPoint(convertCurve(params.getCurve()), pubKey.getW()).getEncoded(),
                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)
    {
        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);
        }
        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);
        }
    }
}