基于BC的SM2公钥压缩与解压缩

ECC的公钥是支持压缩的，SM2椭圆曲线公钥密码算法中也给出了压缩的方法，这里就不讨论如何计算了，直接看看BC是如何实现的。

编码压缩

/**
* 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.");
}

解码解压

/**
* 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);
}

调用的方法在Gist上，给出了一个EC 工具包，包含计算公钥，格式转换等：EC Utils🔚

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