在 Java 使用加密演算法（二）：使用 RSA 加密與解密（最後修改：2015-10-11）

利用 RSA 加密演算法來加密字串的範例如下～
這裡總共分成兩段程式碼，分別是 Main.java 以及 myEncryption.java
Main.java 是主要程序；myEncryption.java 進行加解密。


Main.java

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.util.Calendar;

import javax.crypto.Cipher;

import org.bouncycastle.util.encoders.Base64;

public class Main {
    private static String ALGORITHM = "RSA/ECB/PKCS1Padding";
    
    public static void main(String[] args) {
        try {
            // Generate key pair, and load it into public key and private key instances.
            KeyPairGenerator keygen = KeyPairGenerator.getInstance("RSA");
            SecureRandom random = new SecureRandom();
            random.setSeed("test".getBytes());
            keygen.initialize(1024, random); // TODO Change length may cause the result incorrect.
            KeyPair keyPair = keygen.generateKeyPair();
            PublicKey publicKey = keyPair.getPublic();
            PrivateKey privateKey = keyPair.getPrivate();
            
            // Tag for timer.
            long start, end;

            // Generate the plain text
            String plainText = "abc123!@#";
    
            System.out.println("Encryption:");
            start = Calendar.getInstance().getTimeInMillis();

            // Initiate the encryptor.
            MyEncryption encryption = new MyEncryption();
            // Initiate a variable for storing the encrypting result.
            byte[] result = null;
            
            try {
                // Encrypt
                result = encryption.cryptByRSA(plainText.getBytes("UTF-8"), publicKey, ALGORITHM, Cipher.ENCRYPT_MODE);
                end = Calendar.getInstance().getTimeInMillis();
                System.out.println("Encrypted result: " + Base64.toBase64String(result));
                System.out.println("Encrypted length: " + result.length);
                System.out.println("Encrypted time: " + (end-start) + "ms\n");
        
                // Decrypt
                System.out.println("Decryption:");
                start = Calendar.getInstance().getTimeInMillis();
                byte[] decryptResult = encryption.cryptByRSA(result, privateKey, ALGORITHM, Cipher.DECRYPT_MODE);
                end = Calendar.getInstance().getTimeInMillis();
                System.out.println("Decrypted string: " + new String(decryptResult, "UTF-8"));
                System.out.println("Decrypted time: " + (end-start) + "ms\n");
            } catch (Exception e) {
                e.printStackTrace();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

上述程式碼中，42 行呼叫 MyEncryption 進行加密、51 行呼叫 MyEncryption 進行解密。
被加密的字串是 30 行所定義的 String 變數 plainText
而中間同時會計算加密和解密所花費的時間。（因為查到的資料常說 RSA 是速度比較慢的加密方法...）

比較特別的地方是 13 行定義的「RSA/ECB/PKCS1Padding」，是包括指定演算法名稱以及 mode、padding 等
可以參考官方的文件 Standard Algorithm Name Documentation 中的 Implementation Requirement 小節。

myEncryption.java

import java.security.InvalidKeyException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

public class MyEncryption0 implements EncryptionItf {
    
    /**
     * Encrypt/Decrypt the data according to the specified way.
     */
    public byte[] cryptByRSA(byte[] data, Key key, String algorithm, int mode) {
        try {
            Cipher cipher = Cipher.getInstance(algorithm);
            
            // Initiate the cipher.
            if (mode == Cipher.ENCRYPT_MODE)
                cipher.init(Cipher.ENCRYPT_MODE, (RSAPublicKey) key);
            else
                cipher.init(Cipher.DECRYPT_MODE, (RSAPrivateKey) key);
            
            // Encrypt/Decrypt the data.
            return cipher.doFinal(data);
        } catch (IllegalBlockSizeException | BadPaddingException | InvalidKeyException | NoSuchAlgorithmException
                | NoSuchPaddingException e) {
            e.printStackTrace();
        }
        return null;
    }
}

另外要注意的是，根據參考連結 2 的描述，因為 RSA 是屬於 block chiper，所以一次只能加密很短的字串
如果加密長度超過 117 bytes，執行時就會丟出 Exception 了。

javax.crypto.IllegalBlockSizeException: Data must not be longer than 117 bytes
      at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:346)
      at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:391)
      at javax.crypto.Cipher.doFinal(Cipher.java:2087)
      at MyEncryption0.cryptByRSA(MyEncryption0.java:28)
      at Main.main(Main.java:40)

PS. 可參考 Provider 原始碼中的註解，部分節錄如下：
We only do one RSA operation per doFinal() call. If the application passes more data via calls to update() or doFinal(), we throw an IllegalBlockSizeException when doFinal() is called (see JCE API spec). Bulk encryption using RSA does not make sense and is not standardized.

要解決此問題，可以透過將資料拆分成很多個小塊來達成，請參閱「在 Java 使用加密演算法（三）：使用 RSA 加解密長資料」。

程式碼的執行結果如下：

Encryption:
Encrypted result: R/5rw6C/d34BBDKHpq+cz100K1xQrXe/bsldRgU/is3IOJz13/p7uOlma5rdqfAGBVaE3BGYhGsRcp+tu0VhA/ze+XosQUM4nSFbFI74oxxUVhl35yTmPV/e+CU9ugzi42kSYX/YXbzl702nB/t8/Ub66KiTqu2oQXlpcP9kCw0=
Encrypted length: 128
Encrypted time: 443ms

Decryption:
Decrypted string: abc123!@#
Decrypted time: 11ms

參考資料：
1、使用Java進行RSA加解密
2、JAVA里面RSA加密算法的使用
3、有關印出加密後的資料的討論可參考 Java RSA Encryption (stackoverflow)