EC秘钥对工具，计算公钥，压缩公钥，解压缩，转换为ASN.1结构

CalculatePubKey.java

import java.io.IOException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECPublicKeySpec;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

import org.bouncycastle.asn1.ASN1InputStream;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.asn1.x9.ECNamedCurveTable;
import org.bouncycastle.asn1.x9.X9ECParameters;
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.jce.provider.BouncyCastleProvider;
import org.bouncycastle.jce.spec.ECNamedCurveSpec;
import org.bouncycastle.jce.spec.ECParameterSpec;
import org.bouncycastle.jce.spec.ECPrivateKeySpec;
import org.bouncycastle.math.ec.ECPoint;
import org.bouncycastle.util.encoders.Hex;

public class CalculatePubKey {

	private static final String privKey = "308193020100301306072a8648ce3d020106082a8648ce3d0301070479307702010104204833b1bae3464a7495896b9a332cdaa31f4b3b072cc79336d6b6270604d2f7aca00a06082a8648ce3d030107a144034200042f2cc7e74cdd56bfb6b30b92ec59eea14011973ebadb31cc04b151f9eb15d5ebc5d28fd940889c3b0eb234c1bd7d5fbf4c6d2f1692cfd41c37a361a19e6f32b9";

	private static final String pubKey = "3059301306072a8648ce3d020106082a8648ce3d030107034200042f2cc7e74cdd56bfb6b30b92ec59eea14011973ebadb31cc04b151f9eb15d5ebc5d28fd940889c3b0eb234c1bd7d5fbf4c6d2f1692cfd41c37a361a19e6f32b9";

	private static PrivateKey priv;

	private static PublicKey pub;

	static {
		try {
			instance();
		} catch (Exception e) {
		}
	}

	public static void main(String[] args) throws Exception {
		PublicKey pub = getPublicKey(priv);
		System.out.println("original ---> " + pubKey);
		System.out.println("calculate --> " + Hex.toHexString(pub.getEncoded()));
		byte[] hex = encodePoint(pub);
		System.out.println("encode -----> " + Hex.toHexString(hex));
		PublicKey pubkey = decodePoint(Hex.toHexString(hex), "prime256v1");
		System.out.println("decode -----> " + Hex.toHexString(pubkey.getEncoded()));
	}

	/**
	 * initial private key and public key
	 * 
	 * @throws Exception
	 */
	public static void instance() throws Exception {
		KeyFactory keyFactory = KeyFactory.getInstance("EC", new BouncyCastleProvider());
		PKCS8EncodedKeySpec encodedPrivateKey = new PKCS8EncodedKeySpec(Hex.decode(privKey));
		priv = keyFactory.generatePrivate(encodedPrivateKey);
		X509EncodedKeySpec encodedPublicKey = new X509EncodedKeySpec(Hex.decode(pubKey));
		pub = keyFactory.generatePublic(encodedPublicKey);
	}

	/**
	 * generate public from private key
	 * 
	 * @param privateKey
	 * @return
	 * @throws Exception
	 */
	public static PublicKey getPublicKey(PrivateKey privateKey) throws Exception {
		KeyFactory keyFactory = KeyFactory.getInstance("EC", new BouncyCastleProvider());
		ECPrivateKeySpec privSpec = keyFactory.getKeySpec(privateKey, ECPrivateKeySpec.class);
		ECParameterSpec privParams = privSpec.getParams();
		// calculate public key
		ECPoint Q = privParams.getG().multiply(privSpec.getD());
		return toAsn1PublicKey(Q, getPrivateKeyOID(privateKey));
	}

	/**
	 * 从EC私钥中获取OID
	 * 
	 * @param privateKey
	 * @return
	 */
	public static ASN1Primitive getPrivateKeyOID(PrivateKey privateKey) {
		try (ASN1InputStream aIn = new ASN1InputStream(privateKey.getEncoded())) {
			PrivateKeyInfo info = PrivateKeyInfo.getInstance(aIn.readObject());
			return info.getPrivateKeyAlgorithm().getParameters().toASN1Primitive();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return null;
	}

	/**
	 * return ASN1Primitive OID from publickey
	 * 
	 * @param publicKey
	 * @return oid
	 */
	public static ASN1Primitive getPublicKeyOID(PublicKey publicKey) {
		try (ASN1InputStream aIn = new ASN1InputStream(publicKey.getEncoded())) {
			SubjectPublicKeyInfo info = SubjectPublicKeyInfo.getInstance(aIn.readObject());
			return info.getAlgorithm().getParameters().toASN1Primitive();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return null;
	}

	/**
	 * encode EC PublicKey
	 * 
	 * @param publicKey EC公钥
	 * @return 压缩的公钥
	 * @throws InvalidKeySpecException 不是EC公钥，抛出异常
	 */
	public static byte[] encodePoint(PublicKey publicKey) throws InvalidKeySpecException {
		if (publicKey instanceof ECPublicKey) {
			BCECPublicKey bcPubKey = (BCECPublicKey) publicKey;
			return bcPubKey.getQ().getEncoded(true);
		}
		throw new InvalidKeySpecException("cannot identify EC public key.");
	}

	/**
	 * decode 16进制的Point X
	 * 
	 * @param compressedKey
	 * @param curveName
	 * @throws InvalidKeySpecException
	 * @throws NoSuchAlgorithmException
	 */
	public static PublicKey decodePoint(String compressedKey, String curveName)
			throws NoSuchAlgorithmException, InvalidKeySpecException {
		X9ECParameters spec = ECNamedCurveTable.getByName(curveName);
		// 根据X恢复点Y
		ECPoint W = spec.getCurve().decodePoint(Hex.decode(compressedKey));
		return toAsn1PublicKey(W, curveName);
	}

	public static PublicKey toAsn1PublicKey(ECPoint W, ASN1Primitive oid)
			throws NoSuchAlgorithmException, InvalidKeySpecException {
		ASN1ObjectIdentifier asn1Oid = new ASN1ObjectIdentifier(oid.toString());
		String curvename = ECNamedCurveTable.getName(asn1Oid);
		return toAsn1PublicKey(W, curvename);
	}

	/**
	 * ECPoint 构建ASN.1结构公钥
	 * 
	 * @param W
	 * @param curveName
	 * @return
	 * @throws NoSuchAlgorithmException
	 * @throws InvalidKeySpecException
	 */
	public static PublicKey toAsn1PublicKey(ECPoint W, String curveName)
			throws NoSuchAlgorithmException, InvalidKeySpecException {
		KeyFactory keyFactory = KeyFactory.getInstance("EC", new BouncyCastleProvider());
		X9ECParameters spec = ECUtil.getNamedCurveByName(curveName);
		java.security.spec.ECParameterSpec params = new ECNamedCurveSpec(curveName, spec.getCurve(), spec.getG(),
				spec.getN());
		ECPublicKeySpec pubKeySpec = new ECPublicKeySpec(EC5Util.convertPoint(W), params);
		return keyFactory.generatePublic(pubKeySpec);
	}

}