2016年5月1日 星期日

Java JCE - AES Encryption & Decryption @2016-05-01 (English Version)

Foreword:

Why I write this article ?  In my original thinking, I believe that there is a lot of blogs talk about Java AES example. However, I still found some sample code is not clearly to explain the issues of these sample. Therefore, I am afraid someone may really adopt these samples in their production system.

In addition, in order to simplify the explanation, the following sample will ignore the exception handle.

Content:

Some similar sample code that you often find on internet is as the following:
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class AES_DEFAULT {  
 public static byte[] Encrypt(SecretKey secretKey, String msg) throws Exception  
 {  
  Cipher cipher = Cipher.getInstance("AES"); //: default is AES/ECB/PKCS5Padding
  cipher.init(Cipher.ENCRYPT_MODE, secretKey); 
  System.out.println("AES_DEFAULT IV:"+cipher.getIV());
  System.out.println("AES_DEFAULT Algoritm:"+cipher.getAlgorithm());
  byte[] byteCipherText = cipher.doFinal(msg.getBytes());  
  System.out.println("Encrypted result and base64 encoded：" + Base64.getEncoder().encodeToString(byteCipherText));
  return byteCipherText;  
 }  

 public static byte[] Decrypt(SecretKey secretKey, byte[] cipherText) throws Exception  
 {  
  Cipher cipher = Cipher.getInstance("AES"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey);  
  byte[] decryptedText = cipher.doFinal(cipherText);  
  String strDecryptedText = new String(decryptedText);
  System.out.println("Decrypted result：" + strDecryptedText);
  return decryptedText;  
 }  

 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(128,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

  byte[] cipher = AES_DEFAULT.Encrypt(secretKey, "I am PlainText!!");
  AES_DEFAULT.Decrypt(secretKey, cipher);  
 }
}

The above sample code can work well for encryption and decryption, but there are some issues:
  1. The first issue is located at the line 4. It uses the ECB cipher mode, however this cipher mode is not a secure cipher mode for AES, because it will cause the cipher block is the same if the input plain-text is the same. You could refer to the Wiki. 
  2. The second issue is located at the line 25. We should not use the 128 bits length as the AES KEY. Now is 2016, the recommended length of AES key is at least 256 bits.
  3. The third issue is the line 8. It use the msg.getBytes( ). This style will be fine if the program is running at the same platform or machine. However, your program will run at different platform/machine, and you will find the default charset may be different for different platform/machine. Therefore, this may cause the decrypted result is not as you expected.  

The recommended implementation is as the following sample:

Important warring: You should select the correct cipher mode,  for example: CCM or GCM mode. (Update 2022/03/04) The reason is the CBC mode is vulnerable to padding oracle attacks.

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class AES_CBC_PKCS5PADDING {
 
 public static byte[] Encrypt(SecretKey secretKey, byte[] iv, String msg) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING"); 
  cipher.init(Cipher.ENCRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  System.out.println("AES_CBC_PKCS5PADDING IV:"+cipher.getIV());
  System.out.println("AES_CBC_PKCS5PADDING Algoritm:"+cipher.getAlgorithm());
  byte[] byteCipherText = cipher.doFinal(msg.getBytes("UTF-8"));
  System.out.println("Encrypted result and base64 encoded：" + Base64.getEncoder().encodeToString(byteCipherText));
  return byteCipherText;
 }
 
 public static void Decrypt(SecretKey secretKey, byte[] cipherText, byte[] iv) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  byte[] decryptedText = cipher.doFinal(cipherText);
  String strDecryptedText = new String(decryptedText);
  System.out.println("Decrypted result：" + strDecryptedText);
 }
 
 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

  byte[] cipher = AES_CBC_PKCS5PADDING.Encrypt(secretKey, iv, "I am PlainText!!");
  AES_CBC_PKCS5PADDING.Decrypt(secretKey, cipher, iv);  
 }
}
As you seeing at line 3,  we adopt the CBC cipher mode with PKCS5 Padding. You could refer to the  Padding for the detail.  At line 8, we directly invoke msg.getBytes("UTF-8") to avoid some charset issues. Of course,  you could specify it use ANSI , and it is still work well if your plain-text contain ANSI only,

Here, allow me to remind you. The recommended length of AES key is 256 bits. In addition, you should always generate a new IV to encrypt if you use the same AES key to encrypt data.

You could refer the below sample code to generate the required Secret Key and IV. In addition, you also need to provide the same IV to the decryption side.

  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

Sometime, you may see the following error message, the reason is the default Oracle's Java Runtime can not generate the AES 256 bits keys because the policy issue.

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Exception in thread "main" java.security.InvalidKeyException: Illegal key size or default parameters
 at javax.crypto.Cipher.checkCryptoPerm(Cipher.java:1026)
 at javax.crypto.Cipher.implInit(Cipher.java:801)
 at javax.crypto.Cipher.chooseProvider(Cipher.java:864)
 at javax.crypto.Cipher.init(Cipher.java:1249)
 at javax.crypto.Cipher.init(Cipher.java:1186)

You need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files if you have the same problem at your environment. One more thing that you may need to check is the policy file version should be same as the JRE version at your environment. (The Android will not have this issue)

Finally,  you may ask common questions as the followings：
  1. How to encrypt a stream file with large file size (EX:Video or Audio)？
  2. How to randomly access specific block data of the encrypted file ?
The possible solution is as below ：
  • The answer for the first one question is easy. Just study the Java Cipher API doc carefully, then you can invoke the update method of Cipher with multiple times , and invoke the doFinal method at the end.
  • The answer for the 2nd question is to change the cipher mode as the CTR cipher mode. In addition, you need to write a calculate IV method for the target block that you want.
You could refer to the following implementation:
(Note: This sample code does not testing well, please don't adopt it on production system.)

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class AES_CTR_PKCS5PADDING {
 private static final int BLOCK_SIZE = 16;
 
 public static void Encrypt(SecretKey secretKey, byte[] iv, File plainTextFile, File encryptedFile) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.ENCRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  System.out.println("AES_CTR_PKCS5PADDING IV:"+cipher.getIV());
  System.out.println("AES_CTR_PKCS5PADDING Algoritm:"+cipher.getAlgorithm());
  byte buf[] = new byte[4096];
  try (InputStream in = new FileInputStream(plainTextFile);
    OutputStream out = new FileOutputStream(encryptedFile);){
   int readBytes = in.read(buf);   
   while(readBytes > 0){
    byte[] cipherBytes = cipher.update(buf, 0 , readBytes);
    out.write(cipherBytes);
    readBytes = in.read(buf);
   }
   cipher.doFinal();
  }
 }
 
 public static void Decrypt(SecretKey secretKey, byte[] iv, File cipherTextFile, File decryptedFile) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  
  if(!decryptedFile.exists()){
   decryptedFile.createNewFile(); //: Here, it may be fail if ...
  }
  
  byte buf[] = new byte[4096];
  try (InputStream in = new FileInputStream(cipherTextFile);
    OutputStream out = new FileOutputStream(decryptedFile);){
   int readBytes = in.read(buf);   
   while(readBytes > 0){
    byte[] decryptedBytes = cipher.update(buf, 0 , readBytes);
    out.write(decryptedBytes);
    readBytes = in.read(buf);
   }
   cipher.doFinal();
  }
 }
  
 public static byte[] DecryptPartial(SecretKey secretKey, byte[] iv, File cipherTextFile, int blockIndex, int blockCount ) throws Exception{
  final int offset = blockIndex * BLOCK_SIZE;
  final int bufSize = blockCount * BLOCK_SIZE;

  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, calculateIVForBlock(new IvParameterSpec(iv), blockIndex ));

  byte[] decryptedBytes = new byte[bufSize];
  try (FileInputStream in = new FileInputStream(cipherTextFile)){
   byte inputBuf[] = new byte[bufSize];
   in.skip(offset);
   int readBytes = in.read(inputBuf);
   decryptedBytes = cipher.update(inputBuf, 0, readBytes);
  }
  return decryptedBytes;
 } 

 private static IvParameterSpec calculateIVForBlock(final IvParameterSpec iv,
         final long blockIndex) {  
     final BigInteger biginIV = new BigInteger(1, iv.getIV());
     final BigInteger blockIV = biginIV.add(BigInteger.valueOf(blockIndex));
     final byte[] blockIVBytes = blockIV.toByteArray();

     // Normalize the blockIVBytes as 16 bytes for IV
     if(blockIVBytes.length == BLOCK_SIZE){
      return new IvParameterSpec(blockIVBytes);
     }
     if(blockIVBytes.length > BLOCK_SIZE ){
      // For example: if the blockIVBytes length is 18, blockIVBytes is [0],[1],...[16],[17]
      // We have to remove [0],[1] , so we change the offset = 2
      int offset = blockIVBytes.length - BLOCK_SIZE;
      return new IvParameterSpec(blockIVBytes, offset, BLOCK_SIZE);
     }
     else{
      // For example: if the blockIVBytes length is 14, blockIVBytes is [0],[1],...[12],[13]
      // We have to insert 2 bytes at head
      final byte[] newBlockIV = new byte[BLOCK_SIZE]; //: default set to 0 for 16 bytes
      int offset = blockIVBytes.length - BLOCK_SIZE;
      System.arraycopy(blockIVBytes, 0, newBlockIV, offset, blockIVBytes.length);
      return new IvParameterSpec(newBlockIV);
     }
 }
 
 private static void createTestFile(String path) throws Exception{
  File test = new File(path);  
  try(FileOutputStream out = new FileOutputStream(test)){

   StringBuffer buf = new StringBuffer(16);

   int blockCount = 100000;
   for(int i = 0 ; i < blockCount ; i ++){
    buf.append(i);
    int size = buf.length();
    for(int j = 0; j < (14-size); j++ ){
     buf.append('#');
    }
    out.write(buf.toString().getBytes());
    out.write("\r\n".getBytes());
    buf.delete(0, 16);
   }   
  }  
 }
 
 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);
  
  {
   String originalFile = "~/PlainText.txt";
   String encryptedFile = "~/CipherText.enc"; 
   String deryptedFile = "~/Decrypted.txt";   

   AES_CTR_PKCS5PADDING.createTestFile(originalFile); //: Create Testing Data
   
   AES_CTR_PKCS5PADDING.Encrypt(secretKey, iv, new File(originalFile), new File(encryptedFile));
   AES_CTR_PKCS5PADDING.Decrypt(secretKey, iv, new File(encryptedFile), new File(deryptedFile));
   byte[] ret = AES_CTR_PKCS5PADDING.DecryptPartial(secretKey, iv, new File(encryptedFile), 100, 10);   
   System.out.println(new String(ret));
  }
 }

Final:

I don't talk about the GCM cipher mode here. In principle, you don't need the GCM if you don't need the authentication

Reference:

* https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_Codebook_.28ECB.29
* https://docs.oracle.com/javase/8/docs/technotes/guides/security/SunProviders.html

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Java JCE - AES 的 Encryption & Decryption @2016-05-01

前言：

都已經2016年了，本來不打算寫這篇的，但偶然發現網路上有多個中文部落格，甚至是論壇上分享或是討論 Java 的 AES 的程式，都沒有討論一些問題，深怕一堆人看到這種範例程式就放到你開發的系統上。另外，為了簡化程式來說明，以下的程式並不考慮Exception處理方式。

內容：

先來看網路上常見的程式的寫法：
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class AES_DEFAULT {  
 public static byte[] Encrypt(SecretKey secretKey, String msg) throws Exception  
 {  
  Cipher cipher = Cipher.getInstance("AES"); //: 等同 AES/ECB/PKCS5Padding
  cipher.init(Cipher.ENCRYPT_MODE, secretKey); 
  System.out.println("AES_DEFAULT IV:"+cipher.getIV());
  System.out.println("AES_DEFAULT Algoritm:"+cipher.getAlgorithm());
  byte[] byteCipherText = cipher.doFinal(msg.getBytes());  
  System.out.println("加密結果的Base64編碼：" + Base64.getEncoder().encodeToString(byteCipherText));
  return byteCipherText;  
 }  

 public static byte[] Decrypt(SecretKey secretKey, byte[] cipherText) throws Exception  
 {  
  Cipher cipher = Cipher.getInstance("AES"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey);  
  byte[] decryptedText = cipher.doFinal(cipherText);  
  String strDecryptedText = new String(decryptedText);
  System.out.println("解密結果：" + strDecryptedText);
  return decryptedText;  
 }  

 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(128,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

  byte[] cipher = AES_DEFAULT.Encrypt(secretKey, "I am PlainText!!");
  AES_DEFAULT.Decrypt(secretKey, cipher);  
 }
}
上面這種寫法，這個程式對於加解密的運作是正常的，但會有潛在的3個問題：


  1. 第 1 個問題在第 4 行 這個用法所使用的 Cipher mode 是 ECB，也就是比較不安全的方式。原因是 ECB 對於相同的資料加密後的結果會是一樣的，有興趣可以參考(Wiki上的那張企鵝圖)。 如果你的應用是每次加密時 secret key 都是重新產生的，而且需要被加密的資料每次都完全不同的時候，各自的資料內容本身也是異質性相當高，如果採用這種做法也沒有太大的問題，但還是不建議。因為以加密的應用來說，常見的對象就是檔案或是運用在傳輸加密。而這兩種方式，大部分都會有相同的資料。以檔案來說，相同類型的檔案你用binary編輯器打開檔案，你就可以觀察到檔案的前面都會有雷同的資料。通訊協定更是如此，例如：HTTP通訊協定。
  2. 第 2 個問題在第 25 行 secret key 長度的問題，不應該使用 128 bits 長度，強度太弱。
  3. 第 3 個問題在第 8 行的 msg.getBytes( )，不同的作業系統所使用的預設 charset 可能是不同的。這樣的做法，有可能會造成不同平台在加密時的行為不如你所預期，也就是解密後的內容可能跟你當初要加密的資料不同。


比較建議的寫法是採用下面這種：

Important warring: You should select the correct cipher mode,  for example: CCM or GCM mode. (Update 2022/03/04) The reason is the CBC mode is vulnerable to padding oracle attacks.

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class AES_CBC_PKCS5PADDING {
 
 public static byte[] Encrypt(SecretKey secretKey, byte[] iv, String msg) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING"); 
  cipher.init(Cipher.ENCRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  System.out.println("AES_CBC_PKCS5PADDING IV:"+cipher.getIV());
  System.out.println("AES_CBC_PKCS5PADDING Algoritm:"+cipher.getAlgorithm());
  byte[] byteCipherText = cipher.doFinal(msg.getBytes("UTF-8"));
  System.out.println("加密結果的Base64編碼：" + Base64.getEncoder().encodeToString(byteCipherText));

  return byteCipherText;
 }
 
 public static void Decrypt(SecretKey secretKey, byte[] cipherText, byte[] iv) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  byte[] decryptedText = cipher.doFinal(cipherText);
  String strDecryptedText = new String(decryptedText);
  System.out.println("解密結果：" + strDecryptedText);
 }
 
 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

  byte[] cipher = AES_CBC_PKCS5PADDING.Encrypt(secretKey, iv, "I am PlainText!!");
  AES_CBC_PKCS5PADDING.Decrypt(secretKey, cipher, iv);  
 }
}
在第 3 行明確指定採用 CBC 的 cipher mode，並且指定 Padding 方式，有興趣可參考這篇 Padding 運作方式。在第 8 行，直接指定用 msg.getBytes("UTF-8")，避免一些問題，如果需要加密的內容都是純ANSI，你可以指定用 ANSI 就可以。

此處，還是要強調 AES的加密 Secret Key 的長度建議至少要 256 bit 以上，用同一把 Secret Key 做加密時候，應該都要產生新的 IV 來加密。可以參考下面這種寫法來產生所需要的 AES Secret Key 和 IV。另外，IV 在解密時候，也要用當初加密使用的相同ＩＶ才可以。

  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);

另外，由於 Oracle 官方預設標準的Java執行環境沒法支援產生 AES 256 bits 長度的 secret key。所以，你會遇到下面這種錯訊息：

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Exception in thread "main" java.security.InvalidKeyException: Illegal key size or default parameters
 at javax.crypto.Cipher.checkCryptoPerm(Cipher.java:1026)
 at javax.crypto.Cipher.implInit(Cipher.java:801)
 at javax.crypto.Cipher.chooseProvider(Cipher.java:864)
 at javax.crypto.Cipher.init(Cipher.java:1249)
 at javax.crypto.Cipher.init(Cipher.java:1186)

這表示你的執行環境需要安裝 Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files。這裡要特別注意，你務必要安裝跟執行環境JVM相同版本的JCE Policy，否則可能會遇到一些怪異的現象。(Android執行環境不在此限)

最後，常問遇到的問題就是：

  1. 如果檔案很大的Stream類型的檔案(如:Video or Audio)要如何做到加密？
  2. 想要隨意位置讀取已經加密的內容要怎麼做？

建議的做法：


  • 第 1 個問題，很簡單請看清楚 Cipher 的 API doc，採用多次呼叫 update 的方法，最後再呼叫 doFinal 方法即可。
  • 第 2 個問題，只要將 Cipher mode 改為 CTR 即可，另外如果是要隨機存取某個 Block 的資料，必須要自己重新計算那個 Block 開始的 IV。
請參考類似下面的寫法：
(Note: This sample code does not testing well, please don't adopt it on production system.)

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class AES_CTR_PKCS5PADDING {
 private static final int BLOCK_SIZE = 16;
 
 public static void Encrypt(SecretKey secretKey, byte[] iv, File plainTextFile, File encryptedFile) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.ENCRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  System.out.println("AES_CTR_PKCS5PADDING IV:"+cipher.getIV());
  System.out.println("AES_CTR_PKCS5PADDING Algoritm:"+cipher.getAlgorithm());
  byte buf[] = new byte[4096];
  try (InputStream in = new FileInputStream(plainTextFile);
    OutputStream out = new FileOutputStream(encryptedFile);){
   int readBytes = in.read(buf);   
   while(readBytes > 0){
    byte[] cipherBytes = cipher.update(buf, 0 , readBytes);
    out.write(cipherBytes);
    readBytes = in.read(buf);
   }
   cipher.doFinal();
  }
 }
 
 public static void Decrypt(SecretKey secretKey, byte[] iv, File cipherTextFile, File decryptedFile) throws Exception{
  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));    
  
  if(!decryptedFile.exists()){
   decryptedFile.createNewFile(); //: Here, it may be fail if ...
  }
  
  byte buf[] = new byte[4096];
  try (InputStream in = new FileInputStream(cipherTextFile);
    OutputStream out = new FileOutputStream(decryptedFile);){
   int readBytes = in.read(buf);   
   while(readBytes > 0){
    byte[] decryptedBytes = cipher.update(buf, 0 , readBytes);
    out.write(decryptedBytes);
    readBytes = in.read(buf);
   }
   cipher.doFinal();
  }
 }
  
 public static byte[] DecryptPartial(SecretKey secretKey, byte[] iv, File cipherTextFile, int blockIndex, int blockCount ) throws Exception{
  final int offset = blockIndex * BLOCK_SIZE;
  final int bufSize = blockCount * BLOCK_SIZE;

  Cipher cipher = Cipher.getInstance("AES/CTR/PKCS5PADDING"); 
  cipher.init(Cipher.DECRYPT_MODE, secretKey, calculateIVForBlock(new IvParameterSpec(iv), blockIndex ));

  byte[] decryptedBytes = new byte[bufSize];
  try (FileInputStream in = new FileInputStream(cipherTextFile)){
   byte inputBuf[] = new byte[bufSize];
   in.skip(offset);
   int readBytes = in.read(inputBuf);
   decryptedBytes = cipher.update(inputBuf, 0, readBytes);
  }
  return decryptedBytes;
 } 

 private static IvParameterSpec calculateIVForBlock(final IvParameterSpec iv,
         final long blockIndex) {  
     final BigInteger biginIV = new BigInteger(1, iv.getIV());
     final BigInteger blockIV = biginIV.add(BigInteger.valueOf(blockIndex));
     final byte[] blockIVBytes = blockIV.toByteArray();

     // Normalize the blockIVBytes as 16 bytes for IV
     if(blockIVBytes.length == BLOCK_SIZE){
      return new IvParameterSpec(blockIVBytes);
     }
     if(blockIVBytes.length > BLOCK_SIZE ){
      // For example: if the blockIVBytes length is 18, blockIVBytes is [0],[1],...[16],[17]
      // We have to remove [0],[1] , so we change the offset = 2
      int offset = blockIVBytes.length - BLOCK_SIZE;
      return new IvParameterSpec(blockIVBytes, offset, BLOCK_SIZE);
     }
     else{
      // For example: if the blockIVBytes length is 14, blockIVBytes is [0],[1],...[12],[13]
      // We have to insert 2 bytes at head
      final byte[] newBlockIV = new byte[BLOCK_SIZE]; //: default set to 0 for 16 bytes
      int offset = blockIVBytes.length - BLOCK_SIZE;
      System.arraycopy(blockIVBytes, 0, newBlockIV, offset, blockIVBytes.length);
      return new IvParameterSpec(newBlockIV);
     }
 }
 
 private static void createTestFile(String path) throws Exception{
  File test = new File(path);  
  try(FileOutputStream out = new FileOutputStream(test)){

   StringBuffer buf = new StringBuffer(16);

   int blockCount = 100000;
   for(int i = 0 ; i < blockCount ; i ++){
    buf.append(i);
    int size = buf.length();
    for(int j = 0; j < (14-size); j++ ){
     buf.append('#');
    }
    out.write(buf.toString().getBytes());
    out.write("\r\n".getBytes());
    buf.delete(0, 16);
   }   
  }  
 }
 
 public static void main(String args[]) throws Exception{
  KeyGenerator keyGen = KeyGenerator.getInstance("AES");
  keyGen.init(256,new SecureRandom( ) );
  SecretKey secretKey = keyGen.generateKey();
  byte[] iv = new byte[16]; 
  SecureRandom prng = new SecureRandom();
  prng.nextBytes(iv);
  
  {
   String originalFile = "~/PlainText.txt";
   String encryptedFile = "~/CipherText.enc"; 
   String deryptedFile = "~/Decrypted.txt";   

   AES_CTR_PKCS5PADDING.createTestFile(originalFile); //: Create Testing Data
   
   AES_CTR_PKCS5PADDING.Encrypt(secretKey, iv, new File(originalFile), new File(encryptedFile));
   AES_CTR_PKCS5PADDING.Decrypt(secretKey, iv, new File(encryptedFile), new File(deryptedFile));
   byte[] ret = AES_CTR_PKCS5PADDING.DecryptPartial(secretKey, iv, new File(encryptedFile), 100, 10);   
   System.out.println(new String(ret));
  }
 }



最後：

這邊我沒有提到另外一種 GCM 的 Cipher Mode，原則上，如果你沒有 Authentication 的需要時候，就不需要用到 GCM。

Reference:

* https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_Codebook_.28ECB.29
* https://docs.oracle.com/javase/8/docs/technotes/guides/security/SunProviders.html

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