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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.its.jcajce;

import java.math.BigInteger;
import java.security.spec.ECField;
import java.security.spec.ECFieldF2m;
import java.security.spec.ECFieldFp;
import java.security.spec.ECParameterSpec;
import java.security.spec.ECPoint;
import java.security.spec.EllipticCurve;

import org.bouncycastle.asn1.x9.X9ECParameters;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECCurve;
import org.bouncycastle.math.field.FiniteField;
import org.bouncycastle.math.field.Polynomial;
import org.bouncycastle.math.field.PolynomialExtensionField;
import org.bouncycastle.util.Arrays;

class ECUtil
{
    static ECPoint convertPoint(org.bouncycastle.math.ec.ECPoint point)
    {
        point = point.normalize();

        return new ECPoint(
            point.getAffineXCoord().toBigInteger(),
            point.getAffineYCoord().toBigInteger());
    }

    public static EllipticCurve convertCurve(
        ECCurve curve,
        byte[] seed)
    {
        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);
    }

    public static ECParameterSpec convertToSpec(
        X9ECParameters domainParameters)
    {
        return new ECParameterSpec(
            convertCurve(domainParameters.getCurve(), null),  // JDK 1.5 has trouble with this if it's not null...
            convertPoint(domainParameters.getG()),
            domainParameters.getN(),
            domainParameters.getH().intValue());
    }

    public 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.reverseInPlace(Arrays.copyOfRange(exponents, 1, exponents.length - 1));
            return new ECFieldF2m(poly.getDegree(), ks);
        }
    }
}