生成密钥安装证书

邮件的加密与解密需要证书。首要任务是生成证书。我们使用.jks生成证书。所以需要创建.jks文件。通过以下命令生成：

keytool -genkey -alias test -keyalg RSA -validity 1825 
-keystore "testmail.jks" 
-storetype JKS -dname "CN=Pavan Solapure,O=Opencodez" 
-ext san=email:user2@testmail.com 
-keypass 123456 -storepass 123456

之后便是生成证书。同时证书需要被本机所认证。我们使用上述生成的.jks文件生成证书并进行安装，使得本机认证生成的证书。这样当发送加密文件的时候，邮件客户端能够正确的证书和密钥。

keytool -importkeystore -srckeystore "testmail.jks" 
-destkeystore "testmail.p12" -srcstoretype JKS 
-deststoretype PKCS12 -srcstorepass 123456 
-deststorepass 123456 -srcalias test -destalias test 
-srckeypass 123456 -destkeypass 123456 -noprompt

生成证书后，双击testmail.p12，你将被引导安装文件。之后生成的证书将被本机所认证。

引入jar包

读取证书（MailEncryptionUtil.class）

首先我们先获取证书。

private static X509Certificate getRecipientPublicCertificate(MimeMessage message) throws Exception{
    ClassLoader classLoader = MailEncryptionUtil.class.getClassLoader();
    File file = new File(classLoader.getResource("testmail.p12").getFile());
    FileInputStream fis = new FileInputStream(file);
    KeyStore ks = KeyStore.getInstance("PKCS12");
    ks.load(fis, "123456".toCharArray());
    X509Certificate recipientCert = (X509Certificate) ks.getCertificate("test");
    return recipientCert;
}

对邮件信息进行加密

public static MimeMessage encryptMessage(MimeMessage message) throws Exception{
    Security.addProvider(new BouncyCastleProvider());
        
    SMIMEEnvelopedGenerator gen = new SMIMEEnvelopedGenerator();
    X509Certificate recipientCert = getRecipientPublicCertificate(message);

    gen.addRecipientInfoGenerator(new JceKeyTransRecipientInfoGenerator(recipientCert).setProvider("BC"));
        
    MimeBodyPart msg = new MimeBodyPart();
    msg.setContent(message.getContent(), message.getContentType());
        
    MimeBodyPart mp = gen.generate(msg, new JceCMSContentEncryptorBuilder(CMSAlgorithm.RC2_CBC).setProvider("BC").build());
    message.setContent(mp.getContent(), mp.getContentType());
    message.saveChanges();
        
    return message;
}

首先我们将BouncyCastle作为安全提供者。之后是获取证书。之后是生成可以用于发送了邮件消息类。

邮件的发送（EmailService.class）

public void sendPlainTextMail(Email email){
    String host = "localhost";
    Properties properties = System.getProperties();
    properties.setProperty("mail.smtp.host", host);
    Session session = Session.getDefaultInstance(properties);
        
    try {
        MimeMessage message = new MimeMessage(session);
        BodyPart messageBodyPart = new MimeBodyPart();
            
        message.setFrom(new InternetAddress(email.getFrom()));
        message.addRecipient(Message.RecipientType.TO, new InternetAddress(email.getToAsList()));
        message.setSubject(email.getSubject());
            
        messageBodyPart.setText(email.getMessage());
        Multipart multipart = new MimeMultipart();
        multipart.addBodyPart(messageBodyPart);
        message.setText(email.getMessage());
        //Transport.send(message);
        Transport.send(MailEncryptionUtil.encryptMessage(message));
        System.out.println("Sent message successfully.");
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

首先我们要在Session服务器中配置邮件服务器的基本信息。之后指定邮件的发送发、接收方、主题和内容等信息。之后是用之前写的MailEncryptionUtil对邮件信息进行加密并发送。

在主类中指定发送邮件的基本信息

public static void main(String[] args) {
    String from = "rhf0410@testmail.com";
    String to = "user2@testmail.com";
    String subject = "Java mail which has been encrypted";
        
    String message = "Hello encrypted mail.";
        
    Email email = new Email(from, to, subject, message);
        
    //Sent email
    emailService.sendPlainTextMail(email);
}