在传输过程中，即使攻击者截获了传输的密文，并得到了乙的公钥，也无法破解密文，因为只有乙的私钥才能解密密文。同样，如果乙要回复加密信息给甲，那么需要甲先公布甲的公钥给乙用于加密，甲自己保存甲的私钥用于解密。
非对称加密鼻祖：RSA
RSA算法基于一个十分简单的数论事实：将两个大质数（素数）相乘十分容易，但是想要对其乘积进行因式分解却极其困难，因此可以将乘积公开作为加密密钥。比如：取两个简单的质数：67，73，得到两者乘积很简单4891；但是要想对4891进行因式分解，其工作量成几何增加。
应用场景：
HTTPS请求的SSL层。
在JDK中也提供了RSA的实现，下面给出示例：
/** * 创建密匙对 * * @return */ private KeyPair genKeyPair() { //创建 RSA Key 的生产者。 KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("RSA"); //利用用户密码作为随机数初始化出 1024 比特 Key 的生产者。 //SecureRandom 是生成安全随机数序列，password.getBytes() 是种子，只要种子相同，序列就一样。 keyPairGen.initialize(1024, new SecureRandom("password".getBytes())); //创建密钥对 return keyPairGen.generateKeyPair(); } /** * 生成公匙 * * @return */ public PublicKey genPublicKey() { try { //创建密钥对 KeyPair keyPair = genKeyPair(); //生成公钥 PublicKey publicKey = keyPair.getPublic(); X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicKey.getEncoded()); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); publicKey = keyFactory.generatePublic(keySpec); return publicKey; } catch (Exception e) { e.printStackTrace(); } return null; } /** * 生成私匙 * * @return */ public PrivateKey genPrivateKey() { try { //创建密钥对 KeyPair keyPair = genKeyPair(); //生成私匙 PrivateKey privateKey = keyPair.getPrivate(); X509EncodedKeySpec keySpec = new X509EncodedKeySpec(privateKey.getEncoded()); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); return keyFactory.generatePrivate(keySpec); } catch (Exception e) { e.printStackTrace(); } return null; } /** * 公钥加密 * * @param data * @param publicKey * @return * @throws Exception */ public static byte[] encryptByPublicKey(byte[] data, String publicKey) throws Exception { X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(publicKey.getBytes()); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); Key publicK = keyFactory.generatePublic(x509KeySpec); // 对数据加密 Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm()); cipher.init(Cipher.ENCRYPT_MODE, publicK); int inputLen = data.length; ByteArrayOutputStream out = new ByteArrayOutputStream(); int offSet = 0; byte[] cache; int i = 0; // 对数据分段加密 while (inputLen - offSet > 0) { if (inputLen - offSet > 117) { cache = cipher.doFinal(data, offSet, 117); } else { cache = cipher.doFinal(data, offSet, inputLen - offSet); } out.write(cache, 0, cache.length); i++; offSet = i * 117; } byte[] encryptedData = out.toByteArray(); out.close(); return encryptedData; } /** * 私钥解密 * * @param encryptedData * @param privateKey * @return * @throws Exception */ public static byte[] decryptByPrivateKey(byte[] encryptedData, String privateKey) throws Exception { PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(privateKey.getBytes()); KeyFactory keyFactory = KeyFactory.getInstance(KEY_ALGORITHM); Key privateK = keyFactory.generatePrivate(pkcs8KeySpec); Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm()); cipher.init(Cipher.DECRYPT_MODE, privateK); int inputLen = encryptedData.length; ByteArrayOutputStream out = new ByteArrayOutputStream(); int offSet = 0; byte[] cache; int i = 0; // 对数据分段解密 while (inputLen - offSet > 0) { if (inputLen - offSet > 118) { cache = cipher.doFinal(encryptedData, offSet, 118); } else { cache = cipher.doFinal(encryptedData, offSet, inputLen - offSet); } out.write(cache, 0, cache.length); i++; offSet = i * 118; } byte[] decryptedData = out.toByteArray(); out.close(); return decryptedData; } /** * 私钥加密 * * @param data * @param privateKey * @return * @throws Exception */ public static byte[] encryptByPrivateKey(byte[] data, String privateKey) throws Exception { PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(publicKey.getBytes()); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); Key privateK = keyFactory.generatePrivate(pkcs8KeySpec); Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm()); cipher.init(Cipher.ENCRYPT_MODE, privateK); int inputLen = data.length; ByteArrayOutputStream out = new ByteArrayOutputStream(); int offSet = 0; byte[] cache; int i = 0; // 对数据分段加密 while (inputLen - offSet > 0) { if (inputLen - offSet > 117) { cache = cipher.doFinal(data, offSet, 117); } else { cache = cipher.doFinal(data, offSet, inputLen - offSet); } out.write(cache, 0, cache.length); i++; offSet = i * 117; } byte[] encryptedData = out.toByteArray(); out.close(); return encryptedData; } /** * 公钥解密 * * @param encryptedData * @param publicKey * @return * @throws Exception */ public static byte[] decryptByPublicKey(byte[] encryptedData, String publicKey) throws Exception { X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(publicKey.getBytes()); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); Key publicK = keyFactory.generatePublic(x509KeySpec); Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm()); cipher.init(Cipher.DECRYPT_MODE, publicK); int inputLen = encryptedData.length; ByteArrayOutputStream out = new ByteArrayOutputStream(); int offSet = 0; byte[] cache; int i = 0; // 对数据分段解密 while (inputLen - offSet > 0) { if (inputLen - offSet > 118) { cache = cipher.doFinal(encryptedData, offSet, 118); } else { cache = cipher.doFinal(encryptedData, offSet, inputLen - offSet); } out.write(cache, 0, cache.length); i++; offSet = i * 118; } byte[] decryptedData = out.toByteArray(); out.close(); return decryptedData; }
4. 组合加密