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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.its.operator;

import java.io.IOException;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.DERSequence;
import org.bouncycastle.asn1.sec.SECObjectIdentifiers;
import org.bouncycastle.asn1.teletrust.TeleTrusTObjectIdentifiers;
import org.bouncycastle.oer.its.EccP256CurvePoint;
import org.bouncycastle.oer.its.EccP384CurvePoint;
import org.bouncycastle.oer.its.EcdsaP256Signature;
import org.bouncycastle.oer.its.EcdsaP384Signature;
import org.bouncycastle.oer.its.Signature;
import org.bouncycastle.util.BigIntegers;

public class ECDSAEncoder
{
    public static byte[] toX962(Signature signature)
    {
        byte[] r;
        byte[] s;
        if (signature.getChoice() == Signature.ecdsaNistP256Signature || signature.getChoice() == Signature.ecdsaBrainpoolP256r1Signature)
        {
            EcdsaP256Signature sig = EcdsaP256Signature.getInstance(signature.getValue());
            r = ASN1OctetString.getInstance(sig.getrSig().getValue()).getOctets();
            s = sig.getsSig().getOctets();
        }
        else
        {
            EcdsaP384Signature sig = EcdsaP384Signature.getInstance(signature.getValue());
            r = ASN1OctetString.getInstance(sig.getrSig().getValue()).getOctets();
            s = sig.getsSig().getOctets();
        }

        try
        {
            return new DERSequence(new ASN1Encodable[]{new ASN1Integer(BigIntegers.fromUnsignedByteArray(r)),
                new ASN1Integer(BigIntegers.fromUnsignedByteArray(s))}).getEncoded();
        }
        catch (IOException ioException)
        {
            throw new RuntimeException("der encoding r & s");
        }
    }

    public static Signature toITS(ASN1ObjectIdentifier curveID, byte[] dsaEncoding)
    {
        ASN1Sequence asn1Sig = ASN1Sequence.getInstance(dsaEncoding);

        if (curveID.equals(SECObjectIdentifiers.secp256r1))
        {
            return new Signature(Signature.ecdsaNistP256Signature, new EcdsaP256Signature(
                new EccP256CurvePoint(EccP256CurvePoint.xOnly, new DEROctetString(BigIntegers.asUnsignedByteArray(32, ASN1Integer.getInstance(asn1Sig.getObjectAt(0)).getValue()))),
                new DEROctetString(BigIntegers.asUnsignedByteArray(32, ASN1Integer.getInstance(asn1Sig.getObjectAt(1)).getValue()))));
        }
        if (curveID.equals(TeleTrusTObjectIdentifiers.brainpoolP256r1))
        {
            return new Signature(Signature.ecdsaBrainpoolP256r1Signature, new EcdsaP256Signature(
                new EccP256CurvePoint(EccP256CurvePoint.xOnly, new DEROctetString(BigIntegers.asUnsignedByteArray(32, ASN1Integer.getInstance(asn1Sig.getObjectAt(0)).getValue()))),
                new DEROctetString(BigIntegers.asUnsignedByteArray(32, ASN1Integer.getInstance(asn1Sig.getObjectAt(1)).getValue()))));
        }
        if (curveID.equals(TeleTrusTObjectIdentifiers.brainpoolP384r1))
        {
            return new Signature(Signature.ecdsaBrainpoolP384r1Signature, new EcdsaP384Signature(
                new EccP384CurvePoint(EccP384CurvePoint.xOnly, new DEROctetString(BigIntegers.asUnsignedByteArray(48, ASN1Integer.getInstance(asn1Sig.getObjectAt(0)).getValue()))),
                new DEROctetString(BigIntegers.asUnsignedByteArray(48, ASN1Integer.getInstance(asn1Sig.getObjectAt(1)).getValue()))));
        }

        throw new IllegalArgumentException("unknown curveID");
    }
}