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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.asn1.x9;

import java.math.BigInteger;

import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Object;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERBitString;
import org.bouncycastle.asn1.DERSequence;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECCurve;
import org.bouncycastle.util.Arrays;

/**
 * ASN.1 def for Elliptic-Curve Curve structure. See
 * X9.62, for further details.
 */
public class X9Curve
    extends ASN1Object
    implements X9ObjectIdentifiers
{
    private ECCurve     curve;
    private byte[]      seed;
    private ASN1ObjectIdentifier fieldIdentifier = null;

    public X9Curve(
        ECCurve     curve)
    {
        this(curve, null);
    }

    public X9Curve(
        ECCurve     curve,
        byte[]      seed)
    {
        this.curve = curve;
        this.seed = Arrays.clone(seed);
        setFieldIdentifier();
    }

    public X9Curve(
        X9FieldID     fieldID,
        BigInteger    order,
        BigInteger    cofactor,
        ASN1Sequence  seq)
    {   
        fieldIdentifier = fieldID.getIdentifier();
        if (fieldIdentifier.equals(prime_field))
        {   
            BigInteger p = ((ASN1Integer)fieldID.getParameters()).getValue();
            BigInteger A = new BigInteger(1, ASN1OctetString.getInstance(seq.getObjectAt(0)).getOctets());      
            BigInteger B = new BigInteger(1, ASN1OctetString.getInstance(seq.getObjectAt(1)).getOctets());      
            curve = new ECCurve.Fp(p, A, B, order, cofactor);
        }
        else if (fieldIdentifier.equals(characteristic_two_field))
        {
            // Characteristic two field
            ASN1Sequence parameters = ASN1Sequence.getInstance(fieldID.getParameters());
            int m = ((ASN1Integer)parameters.getObjectAt(0)).intValueExact();
            ASN1ObjectIdentifier representation = (ASN1ObjectIdentifier)parameters.getObjectAt(1);

            int k1 = 0;
            int k2 = 0;
            int k3 = 0;

            if (representation.equals(tpBasis))
            {
                // Trinomial basis representation
                k1 = ASN1Integer.getInstance(parameters.getObjectAt(2)).intValueExact();
            }   
            else if (representation.equals(ppBasis))
            {
                // Pentanomial basis representation
                ASN1Sequence pentanomial = ASN1Sequence.getInstance(parameters.getObjectAt(2));
                k1 = ASN1Integer.getInstance(pentanomial.getObjectAt(0)).intValueExact();
                k2 = ASN1Integer.getInstance(pentanomial.getObjectAt(1)).intValueExact();
                k3 = ASN1Integer.getInstance(pentanomial.getObjectAt(2)).intValueExact();
            }   
            else
            {
                throw new IllegalArgumentException("This type of EC basis is not implemented");
            }   
            BigInteger A = new BigInteger(1, ASN1OctetString.getInstance(seq.getObjectAt(0)).getOctets());      
            BigInteger B = new BigInteger(1, ASN1OctetString.getInstance(seq.getObjectAt(1)).getOctets());      
            curve = new ECCurve.F2m(m, k1, k2, k3, A, B, order, cofactor);
        }   
        else
        {
            throw new IllegalArgumentException("This type of ECCurve is not implemented");
        }   

        if (seq.size() == 3)
        {
            seed = ((DERBitString)seq.getObjectAt(2)).getBytes();
        }   
    }   

    private void setFieldIdentifier()
    {
        if (ECAlgorithms.isFpCurve(curve))
        {
            fieldIdentifier = prime_field;
        }
        else if (ECAlgorithms.isF2mCurve(curve))
        {
            fieldIdentifier = characteristic_two_field;
        }
        else
        {
            throw new IllegalArgumentException("This type of ECCurve is not implemented");
        }
    }

    public ECCurve  getCurve()
    {
        return curve;
    }

    public byte[]   getSeed()
    {
        return Arrays.clone(seed);
    }

    /**
     * Produce an object suitable for an ASN1OutputStream.
     *      *  Curve ::= SEQUENCE {
     *      a               FieldElement,
     *      b               FieldElement,
     *      seed            BIT STRING      OPTIONAL
     *  }
     * 
     */
    public ASN1Primitive toASN1Primitive()
    {
        ASN1EncodableVector v = new ASN1EncodableVector(3);

        if (fieldIdentifier.equals(prime_field)) 
        { 
            v.add(new X9FieldElement(curve.getA()).toASN1Primitive());
            v.add(new X9FieldElement(curve.getB()).toASN1Primitive());
        } 
        else if (fieldIdentifier.equals(characteristic_two_field)) 
        {
            v.add(new X9FieldElement(curve.getA()).toASN1Primitive());
            v.add(new X9FieldElement(curve.getB()).toASN1Primitive());
        }

        if (seed != null)
        {
            v.add(new DERBitString(seed));
        }

        return new DERSequence(v);
    }
}