Java ECC encoded Key too large

Question

I am new to the EC-encryption and have some struggle with it. I am using Java 8 and the BouncyCatle provider. The Question I have now is: when I generate an EC-KeyPair with

Answer 1

The code (modified from BouncyCastle) below can work with any public key (not only secp521r1)

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.DERNull;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.asn1.x9.X962Parameters;
import org.bouncycastle.asn1.x9.X9ECParameters;
import org.bouncycastle.asn1.x9.X9ECPoint;
import org.bouncycastle.asn1.x9.X9ObjectIdentifiers;
import org.bouncycastle.jcajce.provider.asymmetric.ec.BCECPublicKey;
import org.bouncycastle.jcajce.provider.asymmetric.util.EC5Util;
import org.bouncycastle.jcajce.provider.asymmetric.util.ECUtil;
import org.bouncycastle.jcajce.provider.asymmetric.util.KeyUtil;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.jce.spec.ECNamedCurveSpec;
import org.bouncycastle.math.ec.ECCurve;

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.Security;
import java.security.spec.ECParameterSpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

public class TestCompressionEncoded {

    static X962Parameters getDomainParametersFromName(ECParameterSpec ecSpec, boolean compress) {
        X962Parameters x962Param;
        if (ecSpec instanceof ECNamedCurveSpec) {
            ASN1ObjectIdentifier var3 = ECUtil.getNamedCurveOid(((ECNamedCurveSpec)ecSpec).getName());
            if (var3 == null) {
                var3 = new ASN1ObjectIdentifier(((ECNamedCurveSpec)ecSpec).getName());
            }

            x962Param = new X962Parameters(var3);
        } else if (ecSpec == null) {
            x962Param = new X962Parameters(DERNull.INSTANCE);
        } else {
            ECCurve var5 = EC5Util.convertCurve(ecSpec.getCurve());
            X9ECParameters var4 = new X9ECParameters(var5, new X9ECPoint(EC5Util.convertPoint(var5, ecSpec.getGenerator()), compress), ecSpec.getOrder(), BigInteger.valueOf((long)ecSpec.getCofactor()), ecSpec.getCurve().getSeed());
            x962Param = new X962Parameters(var4);
        }
        return x962Param;
    }

    static byte[] encodeKeyWithCompression(BCECPublicKey x) throws Exception {
        AlgorithmIdentifier var1 = new AlgorithmIdentifier(X9ObjectIdentifiers.id_ecPublicKey, getDomainParametersFromName(x.getParams(), true));
        byte[] var2 = x.getQ().getEncoded(true);
        return KeyUtil.getEncodedSubjectPublicKeyInfo(var1, var2);
    }

    public static void main(String...args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());
        String publicKey = "MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAELPqrW2JAXKTbjfh9M3X3b85Uje7T0r2gu7qKPmmyagGFnfckwVFpKg10+S2ttJYVUB4q+kPpnJg/YHV5xMnSLA==";
        KeyFactory fact = KeyFactory.getInstance("ECDSA", "BC");
        BCECPublicKey bcePubKey = (BCECPublicKey) fact.generatePublic(new X509EncodedKeySpec( Base64.getDecoder().decode(publicKey)));

        System.out.println("Uncompressed encoded value: " + publicKey);
        System.out.println("Compressed encoded value: " + Base64.getEncoder().encodeToString(encodeKeyWithCompression(bcePubKey)));
    }
}

The output (for prime256v1)

Uncompressed encoded value: MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAELPqrW2JAXKTbjfh9M3X3b85Uje7T0r2gu7qKPmmyagGFnfckwVFpKg10+S2ttJYVUB4q+kPpnJg/YHV5xMnSLA==
Compressed encoded value: MDYwEAYHKoZIzj0CAQYFK4EEAAoDIgACLPqrW2JAXKTbjfh9M3X3b85Uje7T0r2gu7qKPmmyagE=