加解密：签名算法、数字证书

• 签名算法

# 如果小明用自己的私钥加密了一条消息，比如小明喜欢小红，然后他公开了加密消息，由于任何人都可以用小明的公钥解密，从而使得任何人都可以确认小明喜欢小红这条消息肯定是小明发出的，其他人不能伪造这个消息，小明也不能抵赖这条消息不是自己写的
# 私钥加密得到的密文实际上就是数字签名，要验证这个签名是否正确，只能用私钥持有者的公钥进行解密验证。使用数字签名的目的是为了确认某个信息确实是由某个发送方发送的，任何人都不可能伪造消息，并且，发送方也不能抵赖

import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.security.*;

public class Main {
	public static void main(String[] args) throws GeneralSecurityException {
		// 生成RSA公钥/私钥:
		KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA");
		kpGen.initialize(1024);
		KeyPair kp = kpGen.generateKeyPair();
		PrivateKey sk = kp.getPrivate();
		PublicKey pk = kp.getPublic();

		// 待签名的消息:
		byte[] message = "Hello, I am Bob!".getBytes(StandardCharsets.UTF_8);

		// 用私钥签名:
		Signature s = Signature.getInstance("SHA1withRSA");
		s.initSign(sk);
		s.update(message);
		byte[] signed = s.sign();
		System.out.println(String.format("signature: %x", new BigInteger(1, signed)));

		// 用公钥验证:
		Signature v = Signature.getInstance("SHA1withRSA");
		v.initVerify(pk);
		v.update(message);
		boolean valid = v.verify(signed);
		System.out.println("valid? " + valid);
	}
}

# 控制台
signature: 2ae58de22ae68ec52bb5440895bc132b162ef3de8ac487128aaf5f4553d2696f85091ce27eb22505ebd03ecdd32e24ee51f925dcf097dd5360a875a406e96011ae9c336bd9b140f2be48b7f09d23f443ff469c6db12935e046ca668e8c9e1eb03db9327261d71274d4a7977aa68b0fcf03c5a1d653cc8d630d40051ff23e0127
valid? true

• 数字证书

# 创建证书
keytool -storepass 123456 -genkeypair -keyalg RSA -keysize 1024 -sigalg SHA1withRSA -validity 3650 -alias mycert -keystore my.keystore -dname "CN=www.sample.com, OU=sample, O=sample, L=BJ, ST=BJ, C=CN"

keyalg：指定RSA加密算法；
sigalg：指定SHA1withRSA签名算法；
validity：指定证书有效期3650天；
alias：指定证书在程序中引用的名称；
dname：最重要的CN=www.sample.com指定了Common Name，如果证书用在HTTPS中，这个名称必须与域名完全一致

执行上述命令，JDK会在当前目录创建一个my.keystore文件，并存储创建成功的一个私钥和一个证书，它的别名是mycert

# 将数字证书放到项目的src路径下

import java.io.InputStream;
import java.math.BigInteger;
import java.security.*;
import java.security.cert.*;
import javax.crypto.Cipher;

public class Main {
    public static void main(String[] args) throws Exception {
        byte[] message = "Hello, use X.509 cert!".getBytes("UTF-8");
        // 读取KeyStore:
        KeyStore ks = loadKeyStore("/my.keystore", "123456");
        // 读取私钥:
        PrivateKey privateKey = (PrivateKey) ks.getKey("mycert", "123456".toCharArray());
        // 读取证书:
        X509Certificate certificate = (X509Certificate) ks.getCertificate("mycert");
        // 加密:
        byte[] encrypted = encrypt(certificate, message);
        System.out.println(String.format("encrypted: %x", new BigInteger(1, encrypted)));
        // 解密:
        byte[] decrypted = decrypt(privateKey, encrypted);
        System.out.println("decrypted: " + new String(decrypted, "UTF-8"));
        // 签名:
        byte[] sign = sign(privateKey, certificate, message);
        System.out.println(String.format("signature: %x", new BigInteger(1, sign)));
        // 验证签名:
        boolean verified = verify(certificate, message, sign);
        System.out.println("verify: " + verified);
    }

    static KeyStore loadKeyStore(String keyStoreFile, String password) {
        try (InputStream input = Main.class.getResourceAsStream(keyStoreFile)) {
            if (input == null) {
                throw new RuntimeException("file not found in classpath: " + keyStoreFile);
            }
            KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
            ks.load(input, password.toCharArray());
            return ks;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    static byte[] encrypt(X509Certificate certificate, byte[] message) throws GeneralSecurityException {
        Cipher cipher = Cipher.getInstance(certificate.getPublicKey().getAlgorithm());
        cipher.init(Cipher.ENCRYPT_MODE, certificate.getPublicKey());
        return cipher.doFinal(message);
    }

    static byte[] decrypt(PrivateKey privateKey, byte[] data) throws GeneralSecurityException {
        Cipher cipher = Cipher.getInstance(privateKey.getAlgorithm());
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        return cipher.doFinal(data);
    }

    static byte[] sign(PrivateKey privateKey, X509Certificate certificate, byte[] message)
            throws GeneralSecurityException {
        Signature signature = Signature.getInstance(certificate.getSigAlgName());
        signature.initSign(privateKey);
        signature.update(message);
        return signature.sign();
    }

    static boolean verify(X509Certificate certificate, byte[] message, byte[] sig) throws GeneralSecurityException {
        Signature signature = Signature.getInstance(certificate.getSigAlgName());
        signature.initVerify(certificate);
        signature.update(message);
        return signature.verify(sig);
    }
}

# 控制台
encrypted: 84331667f208bb711abe72d00fa1a9af4b03393cc0bcd53902731c9159cfd2c59f297433d845fb46ff2674064e58b7101502adcc181551f952db973fc6a69d9b214cdb5e443cdac9f1e9a36aa37c5051a9b6f7f929fd6a9aa3c0d4f90046dcf2aed20d8c9aa7efbd8dcfedd960628025ba5587d7dbc39650b75b3b22203ab555
decrypted: Hello, use X.509 cert!
signature: 43a8a2cf36a0e398a829806d235a192ff6f6634219d661e87539d365876ab7d14e39534bf3a834b06790c04a34f5d6b0e6476c8ddea54c04b380449f1efa3c71aaccfba7ba7241f91bcf81881a95affa9bd098fa15044da4e95d452914976b2b8be7825491404905ba1a4128cb72ec7611750b7d6b7fd99118bc7aba98bb3beb
verify: true

浏览器和服务器建立安全连接：
    浏览器向服务器发起请求，服务器向浏览器发送自己的数字证书；
    浏览器用操作系统内置的Root CA来验证服务器的证书是否有效，如果有效，就使用该证书加密一个随机的AES口令并发送给服务器；
    服务器用自己的私钥解密获得AES口令，并在后续通讯中使用AES加密。