帮我用java实现生成签名证书和加密证书

好的，生成签名证书和加密证书可以使用Java的密钥库（KeyStore）和证书（Certificate）类来实现。以下是一个简单的示例代码：

生成签名证书：

import java.io.FileOutputStream;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.Date;

public class GenerateSignatureCertificate {
    public static void main(String[] args) throws Exception {
        // 生成密钥对
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
        keyPairGenerator.initialize(2048);
        KeyPair keyPair = keyPairGenerator.generateKeyPair();
        PrivateKey privateKey = keyPair.getPrivate();
        PublicKey publicKey = keyPair.getPublic();

        // 生成证书
        X509Certificate certificate = generateCertificate(privateKey, publicKey);

        // 保存证书到文件
        FileOutputStream fos = new FileOutputStream("signature.cer");
        fos.write(certificate.getEncoded());
        fos.close();
    }

    private static X509Certificate generateCertificate(PrivateKey privateKey, PublicKey publicKey) throws Exception {
        // 设置证书信息
        X509Certificate certificate = null;
        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        Date startDate = new Date(System.currentTimeMillis() - 24 * 60 * 60 * 1000);
        Date endDate = new Date(System.currentTimeMillis() + 365 * 24 * 60 * 60 * 1000);
        String subject = "CN=My Company, OU=My Department, O=My Organization, L=My City, ST=My State, C=My Country";
        String issuer = subject;
        certificate = (X509Certificate) cf.generateCertificate(new ByteArrayInputStream(generateSelfSignedCertificate(privateKey, publicKey, subject, issuer, startDate, endDate)));
        return certificate;
    }

    private static byte[] generateSelfSignedCertificate(PrivateKey privateKey, PublicKey publicKey, String subject, String issuer, Date startDate, Date endDate) throws Exception {
        // 生成自签名证书
        X509V3CertificateGenerator certGen = new X509V3CertificateGenerator();
        X500Principal dnName = new X500Principal(subject);
        certGen.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));
        certGen.setIssuerDN(dnName);
        certGen.setNotBefore(startDate);
        certGen.setNotAfter(endDate);
        certGen.setSubjectDN(dnName);
        certGen.setPublicKey(publicKey);
        certGen.setSignatureAlgorithm("SHA256WithRSAEncryption");
        return certGen.generate(privateKey, "BC");
    }
}

生成加密证书：

import java.io.FileOutputStream;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.Date;

public class GenerateEncryptionCertificate {
    public static void main(String[] args) throws Exception {
        // 生成密钥对
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
        keyPairGenerator.initialize(2048);
        KeyPair keyPair = keyPairGenerator.generateKeyPair();
        PrivateKey privateKey = keyPair.getPrivate();
        PublicKey publicKey = keyPair.getPublic();

        // 生成证书
        X509Certificate certificate = generateCertificate(privateKey, publicKey);

        // 保存证书到文件
        FileOutputStream fos = new FileOutputStream("encryption.cer");
        fos.write(certificate.getEncoded());
        fos.close();
    }

    private static X509Certificate generateCertificate(PrivateKey privateKey, PublicKey publicKey) throws Exception {
        // 设置证书信息
        X509Certificate certificate = null;
        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        Date startDate = new Date(System.currentTimeMillis() - 24 * 60 * 60 * 1000);
        Date endDate = new Date(System.currentTimeMillis() + 365 * 24 * 60 * 60 * 1000);
        String subject = "CN=My Company, OU=My Department, O=My Organization, L=My City, ST=My State, C=My Country";
        String issuer = subject;
        certificate = (X509Certificate) cf.generateCertificate(new ByteArrayInputStream(generateSelfSignedCertificate(privateKey, publicKey, subject, issuer, startDate, endDate)));
        return certificate;
    }

    private static byte[] generateSelfSignedCertificate(PrivateKey privateKey, PublicKey publicKey, String subject, String issuer, Date startDate, Date endDate) throws Exception {
        // 生成自签名证书
        X509V3CertificateGenerator certGen = new X509V3CertificateGenerator();
        X500Principal dnName = new X500Principal(subject);
        certGen.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));
        certGen.setIssuerDN(dnName);
        certGen.setNotBefore(startDate);
        certGen.setNotAfter(endDate);
        certGen.setSubjectDN(dnName);
        certGen.setPublicKey(publicKey);
        certGen.setSignatureAlgorithm("SHA256WithRSAEncryption");
        return certGen.generate(privateKey, "BC");
    }
}

这两个示例代码都使用了Bouncy Castle库来生成证书。如果你没有安装Bouncy Castle库，可以从官网下载并安装。