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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.6.

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package org.bouncycastle.asn1.x9;

import java.math.BigInteger;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.DERObject;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.math.ec.ECFieldElement;

/**
 * class for processing an FieldElement as a DER object.
 */
public class X9FieldElement
    extends ASN1Encodable
{
    protected ECFieldElement  f;
    
    private static X9IntegerConverter converter = new X9IntegerConverter();

    public X9FieldElement(ECFieldElement f)
    {
        this.f = f;
    }
    
    public X9FieldElement(BigInteger p, ASN1OctetString s)
    {
        this(new ECFieldElement.Fp(p, new BigInteger(1, s.getOctets())));
    }
    
    public X9FieldElement(int m, int k1, int k2, int k3, ASN1OctetString s)
    {
        this(new ECFieldElement.F2m(m, k1, k2, k3, new BigInteger(1, s.getOctets())));
    }
    
    public ECFieldElement getValue()
    {
        return f;
    }
    
    /**
     * Produce an object suitable for an ASN1OutputStream.
     *      *  FieldElement ::= OCTET STRING
     * 
     * 
     * 

     *  if q is an odd prime then the field element is
     * processed as an Integer and converted to an octet string
     * according to x 9.62 4.3.1.
     *  if q is 2^m then the bit string
     * contained in the field element is converted into an octet
     * string with the same ordering padded at the front if necessary.
     * 
     * 
     */
    public DERObject toASN1Object()
    {
        int byteCount = converter.getByteLength(f);
        byte[] paddedBigInteger = converter.integerToBytes(f.toBigInteger(), byteCount);

        return new DEROctetString(paddedBigInteger);
    }
}