Java如何将一个证书中的公钥替换成另外一个公钥

要将一个证书中的公钥替换成另外一个公钥，需要使用Java的Bouncy Castle库操作证书文件。以下是Java代码示例：

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.cert.X509CertificateHolder;
import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
import org.bouncycastle.jcajce.provider.asymmetric.ec.BCECPublicKey;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.openssl.PEMParser;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;
import org.bouncycastle.openssl.jcajce.JcaPEMWriter;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;

import java.io.FileReader;
import java.io.FileWriter;
import java.security.KeyFactory;
import java.security.Security;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECParameterSpec;
import java.security.spec.ECPublicKeySpec;

public class SM2CertPublicKeyReplacer {

    public static void main(String[] args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());

        // 读取源证书和目标公钥
        CertificateFactory cf = CertificateFactory.getInstance("X.509", "BC");
        X509Certificate cert = (X509Certificate) cf.generateCertificate(new FileReader("sm2_cert.pem"));
        PemReader pemReader = new PemReader(new FileReader("new_pub_key.pem"));
        PemObject pemObject = pemReader.readPemObject();
        pemReader.close();
        byte[] publicKeyBytes = pemObject.getContent();
        JcaPEMKeyConverter converter = new JcaPEMKeyConverter().setProvider("BC");
        ECPublicKey newPublicKey = (ECPublicKey) converter.getPublicKey(pemObject);

        // 生成新的SubjectPublicKeyInfo
        ECParameterSpec ecSpec = newPublicKey.getParams();
        ECPublicKeySpec pubKeySpec = new ECPublicKeySpec(newPublicKey.getW(), ecSpec);
        KeyFactory keyFact = KeyFactory.getInstance("EC", "BC");
        BCECPublicKey newBCECPublicKey = (BCECPublicKey) keyFact.generatePublic(pubKeySpec);
        SubjectPublicKeyInfo newSpki = SubjectPublicKeyInfo.getInstance(newBCECPublicKey.getEncoded());

        // 替换证书中的公钥
        X509CertificateHolder certHolder = new X509CertificateHolder(cert.getEncoded());
        certHolder = certHolder.replaceSubjectPublicKeyInfo(newSpki);
        JcaX509CertificateConverter certConverter = new JcaX509CertificateConverter().setProvider("BC");
        cert = certConverter.getCertificate(certHolder);

        // 保存新证书
        JcaPEMWriter pemWriter = new JcaPEMWriter(new FileWriter("new_sm2_cert.pem"));
        pemWriter.writeObject(cert);
        pemWriter.close();
    }
}

其中，`sm2_cert.pem`是原证书文件，`new_pub_key.pem`是新的公钥文件。在读取公钥时，需要使用JcaPEMKeyConverter将PEM格式的公钥文件转换为ECPublicKey类型。要替换证书中的公钥，需要使用Bouncy Castle库提供的X509CertificateHolder类的`replaceSubjectPublicKeyInfo()`方法生成新的SubjectPublicKeyInfo，然后使用JcaX509CertificateConverter类将X509CertificateHolder转换为X509Certificate类型。最后，使用JcaPEMWriter将新证书保存到文件中。