图像文件的加密和解密 [英] Encryption and decryption of image file

问题描述

结合我的另一个问题，并在更改了这部分代码之后

In conjunction from my another question, and after changing this small part of the codes

FileOutputStream output = new FileOutputStream("sheepTest.png");
    CipherOutputStream cos = new CipherOutputStream(output, pbeCipher);
    ImageIO.write(input, "PNG", cos);
    cos.close();

我遇到了另一个错误，就是这个

from the decrypt part, I've faced another error which is this

Exception in thread "main" java.lang.IllegalArgumentException: image == null!
at javax.imageio.ImageTypeSpecifier.createFromRenderedImage(Unknown Source)
at javax.imageio.ImageIO.getWriter(Unknown Source)
at javax.imageio.ImageIO.write(Unknown Source)
at encypt.com.trial.main(trial.java:82)

当我点击sheepTest.png，文件为空。错误在哪里？任何人都可以帮我解决错误吗？谢谢。

and when i click the sheepTest.png, the file is empty. Where is the mistake(s)? Can anyone help me to solve the error? Thank you.

public class trial {
public static void main(String[] arg) throws Exception {

   // Scanner to read the user's password. The Java cryptography
   // architecture points out that strong passwords in strings is a
   // bad idea, but we'll let it go for this assignment.
   Scanner scanner = new Scanner(System.in);
   // Arbitrary salt data, used to make guessing attacks against the
   // password more difficult to pull off.
   byte[] salt = { (byte) 0xc7, (byte) 0x73, (byte) 0x21, (byte) 0x8c,
           (byte) 0x7e, (byte) 0xc8, (byte) 0xee, (byte) 0x99 };

   {
     File inputFile = new File("sheep.png");
      BufferedImage input = ImageIO.read(inputFile);
      Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
      SecretKeyFactory keyFac = SecretKeyFactory.getInstance("PBEWithMD5AndDES");
     // Get a password from the user.
     System.out.print("Password: ");
     System.out.flush();
     PBEKeySpec pbeKeySpec = new PBEKeySpec(scanner.nextLine().toCharArray());          
     // Set up other parameters to be used by the password-based
     // encryption.
     PBEParameterSpec pbeParamSpec = new PBEParameterSpec(salt, 20);
     SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);
     // Make a PBE Cyhper object and initialize it to encrypt using
     // the given password.
     Cipher pbeCipher = Cipher.getInstance("PBEWithMD5AndDES");
     pbeCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);
     FileOutputStream output = new FileOutputStream("sheepTest.png");
     CipherOutputStream cos = new CipherOutputStream(
            output, pbeCipher);
       //File outputFile = new File("image.png");
         ImageIO.write(input,"PNG",cos);
      cos.close();          

   }
   // Now, create a Cipher object to decrypt for us. We are repeating
   // some of the same code here to illustrate how java applications on
   // two different hosts could set up compatible encryption/decryption
   // mechanisms.
  {
       File inputFile = new File("sheepTest.png");
         BufferedImage input = ImageIO.read(inputFile);
       // Get another (hopefully the same) password from the user.
      System.out.print("Decryption Password: ");
       System.out.flush();
       PBEKeySpec pbeKeySpec = new PBEKeySpec(scanner.next().toCharArray());
       // Set up other parameters to be used by the password-based
       // encryption.
       PBEParameterSpec pbeParamSpec = new PBEParameterSpec(salt, 20);
       SecretKeyFactory keyFac = SecretKeyFactory
               .getInstance("PBEWithMD5AndDES");
       SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);
       // Make a PBE Cyper object and initialize it to decrypt using
       // the given password.
       Cipher pbeCipher = Cipher.getInstance("PBEWithMD5AndDES");
       pbeCipher.init(Cipher.DECRYPT_MODE, pbeKey, pbeParamSpec);
       // Decrypt the ciphertext and then print it out.
       /*byte[] cleartext = pbeCipher.doFinal(ciphertext);
       System.out.println(new String(cleartext));*/
       FileOutputStream output = new FileOutputStream("sheepTest.png");
       CipherOutputStream cos = new CipherOutputStream(
              output, pbeCipher);
        ImageIO.write(input,"PNG",  cos);
        cos.close();

   }
   }
}

推荐答案

我猜这行是返回null：

I'd guess that this line is returning null:

BufferedImage input = ImageIO.read(inputFile);

ImageIO.read 的文档说：

如果没有注册的ImageReader声称能够读取结果流，则返回null。

"If no registered ImageReader claims to be able to read the resulting stream, null is returned."

该null被传递给此调用，这导致NPE：

That null is passed on to this call, which results in the NPE:

ImageIO.write(input,"PNG",  cos);

我不熟悉这个API，但是从文档和我在这里看到的，我认为我可以推断出 ImageIO.read 返回null的原因是因为它试图解释文件中的图像数据，但它不能因为它是加密的。您需要首先解密数据，然后才能将其解释为图像。

I'm not familiar with this API, but from the documentation and what I see here, I think I can infer that the reason ImageIO.read is returning null is because it's trying to interpret the image data in the file, but it can't because it's encrypted. You need to decrypt the data first before you can interpret it as an image.

您正在进行解密，但您的格式转换是使用原始的加密文件作为其输入，而不是解密的图像数据。

You are doing the decryption, but your format conversion is using the original, encrypted file as its input, not the decrypted image data.

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