使用Java加密/解密pdf文件

Question

I want to encrypt/decrypt a pdf file on Android (but it's a java common issue)

I have this code to generate my key :

 public static byte[] getRawKey(byte[] seed) throws Exception {
     KeyGenerator kgen = KeyGenerator.getInstance("AES");
     SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
     sr.setSeed(seed);
     kgen.init(128, sr);
     SecretKey skey = kgen.generateKey();
     byte[] raw = skey.getEncoded();
     return raw;
 }

my code to write encrypted file :

 inStream = new BufferedInputStream(conn.getInputStream());
 outFile = new File(path + fileName);
 outStream = new BufferedOutputStream(new FileOutputStream(outFile), 4096);
 byte[] data = new byte[4096];
 String seed = "password";
 byte[] rawKey = Utils.getRawKey(seed.getBytes());
 SecretKeySpec skeySpec = new SecretKeySpec(rawKey, "AES");
 Cipher cipher = Cipher.getInstance("AES");
 cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
 int bytesRead = 0;
 while((bytesRead = inStream.read(data, 0, data.length)) >= 0)
 {
      outStream.write(cipher.doFinal(data),0, bytesRead);
 }
 outStream.flush();
 outStream.close();  
 inStream.close();

And my code to decrypt it (and save it, to a new decrypted file) :

  FileInputStream fis = new FileInputStream(file);
  FileOutputStream fos = new FileOutputStream(tmp_file);
  String seed = "password";
  byte[] rawKey = Utils.getRawKey(seed.getBytes());
  SecretKeySpec skeySpec = new SecretKeySpec(rawKey, "AES");
  Cipher cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.DECRYPT_MODE, skeySpec);
  int b;
  byte[] data = new byte[4096];
  while((b = fis.read(data)) != -1) {
       fos.write(cipher.doFinal(data), 0, b);
  }
 fos.flush();
 fos.close();
 fis.close();

I read a lot on stackoverflow, and try to follow instructions, but this not working and I got this error :

 javax.crypto.BadPaddingException: pad block corrupted at com.android.org.bouncycastle.jcajce.provider.symmetric.util.BaseBlockCipher.engineDoFinal(BaseBlockCi

What am I doing wrong ? Is there any specificity related to pdf file ?

Answer 1

Try changing the decryption to use this:

int encryptedCount;
final byte[] decryptedData = new byte[4096];
final byte[] encryptedData = new byte[4096];
while ((encryptedCount = fis.read(encryptedData)) != -1) {
    final int decryptedCount = cipher.update(encryptedData, 0, encryptedCount, decryptedData);
    fos.write(decryptedData, 0, decryptedCount);
}
fos.write(cipher.doFinal());

Likewise, calling doFinal in the encryption block is problematic. You would need to alter that as well. Note that you could use CipherOutputStream and CipherInputStream to hide the details of encrypting and writing/reading the bytes. I would actually recommend that for these requirements.

Also, I don't think that seeding the SecureRandom with "password" will have the intended effect you are looking for. I think that could also be a source of your problems, as you need to ensure that encrypting and decrypting use the same key.

Update: Here is code that makes use of the same key for encryption and decryption and utilizes CipherInputStream and CipherOutputStream :

// get the key
final KeyGenerator generator = KeyGenerator.getInstance("AES");
generator.init(128);
final SecretKey secretKey = generator.generateKey();

// perform encryption
final Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, secretKey);
FileInputStream fis = new FileInputStream(System.getProperty("user.home") + java.io.File.separatorChar + "plain.pdf");
FileOutputStream fos = new FileOutputStream(System.getProperty("user.home") + java.io.File.separatorChar + "test.enc");
final CipherOutputStream output = new CipherOutputStream(fos, cipher);

int bytesRead = 0;
final byte[] plainText = new byte[4096];
while ((bytesRead = fis.read(plainText)) >= 0) {
    output.write(plainText, 0, bytesRead);
}
output.flush();
output.close();
fos.close();
fis.close();
final byte[] iv = cipher.getIV();

// decrypt the file
cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));
fis = new FileInputStream(System.getProperty("user.home") + java.io.File.separatorChar + "test.enc");
fos = new FileOutputStream(System.getProperty("user.home") + java.io.File.separatorChar + "test.pdf");
final CipherInputStream input = new CipherInputStream(fis, cipher);

final byte[] decryptedData = new byte[4096];
int decryptedRead;
while ((decryptedRead = input.read(decryptedData)) >= 0) {
    fos.write(decryptedData, 0, decryptedRead);
}
fos.flush();
fos.close();
input.close();
fis.close();

Answer 2

You shouldn't be calling cipher.doFinal inside your loop. Instead call cipher.update(data, ...) and then call cipher.doFinal() after the loop completes. Otherwise you're only processing the last block.

Answer 3

you must use a password (in keyspec) with only 16, 24,32 character length . good luck