Tuesday, April 10, 2012

Encryption of image files on Android — Cipher(Output|Input)Stream problems

I'm trying to encrypt image files on Android with password based encryption. To save the encrypted image I just do this:

FileOutputStream fos = new FileOutputStream(thumbnailFile);
CipherOutputStream cos = new CipherOutputStream(fos, encryptCipher);
Bitmap thumbnail = Bitmap.createScaledBitmap(bm2, 140, 140, true);
thumbnail.compress(Bitmap.CompressFormat.JPEG, 80, cos);

and to read it, this:

FileInputStream fis = new FileInputStream(f);
CipherInputStream cis = new CipherInputStream(fis, decryptCipher);
Bitmap b = BitmapFactory.decodeStream(cis);

but the Bitmap ends up as null. The code works when I bypass the encryption; that is when I use the File(Input|Output)Streams rather than the Cipher(Input|Output)streams.

My Ciphers are created as follows:

public void initCiphers(char password[]) {

PBEKeySpec pbeKeySpec;
PBEParameterSpec pbeParamSpec;
SecretKeyFactory keyFac;

byte[] salt = {
(byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
(byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99
int count = 20;
pbeParamSpec = new PBEParameterSpec(salt, count);
pbeKeySpec = new PBEKeySpec(password);
try {
keyFac = SecretKeyFactory.getInstance("PBEWithMD5AndDES");
SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);
encryptCipher = Cipher.getInstance("PBEWithMD5AndDES");
decryptCipher = Cipher.getInstance("PBEWithMD5AndDES");
encryptCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);
decryptCipher.init(Cipher.DECRYPT_MODE, pbeKey, pbeParamSpec);
} catch (Exception e) {
Log.v("tag", e.toString());

I don't get any exceptions.

There is obviously some problem with using Cipher(Output|Input)Streams with the android functions for encoding and/or decoding images, but since those functions are opaque and there are no exceptions, its hard to know what it is. I suspect it has to do with padding or flushing. Any assistance would be gratefully appreciated.

Source: Tips4all


  1. You could subclass CipherOutputStream or even just OutputStream, and just override the flush() method to do nothing.

  2. When writing to a CipherOutputStream, make sure you close() the stream after writing the data (and not closing the underlying stream before it). Closing makes sure the right padding is added. A flush() alone is not enough here.

    Also, I would advise to not use DES for new protocols - preferred nowadays is AES.