I found an android app named Super Erase that deletes files and folder permanently from android device so that the file deleted cant be recovered anymore..here is the application i am talking about ...but i was wondering how to that and i know it is made with android studio ..i tried the regular way to delete file.delete() but still the file can be recovered.can i have any help .
For starters, secure file deletion on flash media is a complex problem, with no quick and easy answers. The paper Reliably Erasing Data From Flash-Based Solid State Drives gives a good overview of the problems, the potential solutions, and their limitations. They conclude that
For sanitizing entire disks, ... software techniques work most, but not
all, of the time. We found that none of the available software
techniques for sanitizing individual files were effective. [emphasis added]
NIST 800-88 also has a good overview of the technology trends contributing to the problem, along with some minimum recommendations (appendix A) for Android devices. However they tend to be either whole-disk erasure (factory reset), or rely on cryptographic erasure (CE), rather than being general file erasure methods.
But all is not lost. Even if you can't sanitize individual files, you could hope to wipe all the unallocated space after deleting files. The article Secure Deletion on Log-structured File Systems (Reardon, et al.) describes a fairly promising way to do that in user-mode software. Android's internal memory uses (always?) a log-structured file system.
This paper's "purging" method does not require kernel-level access, and doesn't seem to require any native code on Android. (Note that the term "purging" is used a little differently in documents like NIST 800-88.) The basic idea is to delete all the sensitive data, then fill in the remaining space on the drive with a junk data file, and finally delete the junk data file.
While that takes more time and effort than just overwriting the deleted files themselves (several times in different patterns), it seems to be very robust even when you have to deal with the possibility of wear-leveling and log-structure FS.
Caveat and Further Measures
The main caveat for me is about the conditions mentioned by Reardon et al. in the above paper:
Purging will work for any log-structured file system provided both the
user’s disk quota is unlimited and the file system always performs
garbage collection to reclaim even a single chunk of memory before
declaring that the drive is unwritable. [emphasis mine]
The second condition seems pretty likely to be fulfilled, but I don't know about the first one. Does Android (or some manufacturers' versions of it) enforce quotas on disk space used by apps? I have not found any info about user quotas, but there are quotas for other niches like browser persistent storage. Does Android reserve some space for system use, or for each app's caching, for example, that can't be used for other things? If so, it should help (albeit with no guarantees) if we begin purging immediately after the sensitive files are deleted, so there is little time for other filesystem activity to stake a claim to the recently freed space.
Maybe we could mitigate these risks by cyclical purging:
Determine the remaining space available (call it S) on the relevant partition, e.g. using File.getUsableSpace()
Write a series of files to the partition; each one is, say, 20% of the initial S (subject to file size limits).
When we run out of space, delete the first couple of files that we created, then write another file or two as space allows.
Repeat that last step a few times, until you've reached a threshold you're satisfied with. Maybe up to the point where you've written 2*S worth of filler files; tweak that number to balance speed against thoroughness. How much you actually need to do this would be an area for more research.
Delete the remaining filler files.
The idea with cyclical purging is that if we run out of quota to overwrite all free space, deleting the filler files just written will free up more quota; and then the way log-structured filesystems allocate new blocks should allow us to continue overwriting the remaining blocks of free space in sequence, rather than rewriting the same space again.
I'm implementing this method in a test app, and will post it when it's working.
What about FAT-formatted microSD cards?
Would the same methods work on external storage or microSD cards? FAT is block-structured, so would the purge method apply to FAT-formatted SD cards?
On most contemporary flash memory devices, such as CompactFlash and
Secure Digital cards, [wear leveling] techniques are implemented in
hardware by a built-in microcontroller. On such devices, wear leveling
is transparent and most conventional file systems can be used on them
as-is. (https://en.wikipedia.org/wiki/Wear_leveling)
...which suggests to me that even on a FAT-formatted SD card, wear leveling means that the traditional Gutmann methods would not work (see his "Even Further Epilogue") and that a method like "purging" would be necessary.
Whether purging is sufficient, depends on your security parameters. Wear leveling seems to imply that a block could potentially be "retired" at any time, in which case there is no way to erase it without bypassing the microcontroller's wear leveling. AFAIK this can't be done in software, even if you had kernel privileges; you'd have to design special hardware.
However, "retiring" a bad block should be a fairly rare event relative to the life of the media, so for many scenarios, a purging method would be secure enough.
Erasing the traces
Note that Gutmann's method has an important strength, namely, to erase possible traces of old data on the storage media that could remain even after a block is overwritten with new data. These traces could theoretically be read by a determined attacker with lots of resources. A truly thorough approach to secure deletion would augment a method like Gutmann's with purging, rather than replacing it.
However, on log-structured and wear-leveled filesystems, the much bigger problem is trying to ensure that the sensitive blocks get overwritten at all.
Do existing apps use these methods?
I don't have any inside information about apps in the app store, but looking at reviews for apps like iShredder would suggest that at best, they use methods like Reardon's "purging." For example, they can take several hours to do a single-pass wipe of 32GB of free space.
Also note limitations: The reviews on some of the secure deletion apps say that in some cases, the "deleted" files were still accessible after running the "secure delete" operation. Of course we take these reviews with a grain of salt -- there is a possibility of user error. Nevertheless, I wouldn't assume these apps are effective, without good testing.
iShredder 4 Enterprise helpfully names some of the algorithms they use, in their app description:
Depending on the edition, the iShredder™ package comes with deletion
algorithms such as DoD 5220.22-M E, US Air Force (AFSSI-5020), US Army
AR380-19, DoD 5220.22-M ECE, BSI/VS-ITR TL-03423 Standard,
BSI-VS-2011, NATO Standard, Gutmann, HMG InfoSec No.5, DoD 5220.22 SSD
and others.
This impressive-sounding list gives us some pointers for further research. It's not clear how these methods are used -- singly or in combination -- and in particular whether any of them are represented as being effective on their own. We know that Gutmann's method would not be. Similarly, DoD 5220.22-M, AFSSI-5020, AR380-19, and Infosec No. 5 specify Gutmann-like procedures for overwriting sectors on hard drives, which would not be effective for flash-based media. In fact, "The U.S. Department of Defense no longer references DoD 5220.22-M as a method for secure HDD erasure", let alone for flash-based media, so this reference is misleading to the uninformed. (The DoD is said to reference NIST 800-88 instead.) "DoD 5220.22 SSD" sounds promising, but I can't find any informative references for it. I haven't chased down the other algorithms listed, but the results so far are not encouraging.
When you delete file with standard methods like file.delete() or runtime.exec("rm -f my_file") the only job that kernel does is removing info about file from auxiliary filesystem structures. But storage sectors that contain actual data remain untouched. And because of this recovering is possible.
This gives an idea about how we can try to remove file entirely - we should erase all sectors somehow. Easiest approach is to rewrite all file content with random data few times. After each pass we must flush file buffers to ensure that new content is written to storage. All existing methods of secure file removal spin around above principle. For example this one. Note that there is no universal method that works well across all storage types and filesystems. I guess you should experiment by yourself and try to implement some of the existing approaches or design your own. E.g. you can start from next:
Overwrite and flush file 10 times with random data (use FileOutputStream methods). Note!!! Don't use zeros or another low entropy data. Some filesystems may optimize such sparse files and leave some sectors with original content. You can use /dev/urandom file as source of random data (this is a virtual file and it is endless). It gives better results and works faster then well-known Random class.
Rename and move file 10 times. Choose new file names randomly.
Then truncate file with FileChannel.truncate().
And finally remove file with File.delete().
Of course you can write all logic in native code, it may be even somewhat easier than in Java. Described algorithm is just an example. Try doing in that way.
The standard filesystem API won't give you a simple function call for that.
You will have to use the underlaying native API for FileIO. Although I have never used it, theres a library for that:
https://github.com/johanneslumpe/react-native-fs
There are two answers to this question.
First, to answer the direct question of how some of these apps might be doing secure single file delete: what you do is actually open the file and replace the contents with zeros many times. The method sounds stupid, but I have worked with filesystem-level encryption on Android in the past and I found that the above holds true for many secure file delete solutions out there. For a seemingly compliant security, you can repeat writing zeros 7 times (or whatever the NIST standards specify for your hardware type).
Charset charset = StandardCharsets.UTF_8;
String content = new String(Files.readAllBytes(path), charset);
content = content.replaceAll("*", "0");
Files.write(path, content.getBytes(charset));
The right answer to this question is however different. On modern SSD drives and operating systems, it is insecure to delete single files. Therefore, these apps don't really offer a compelling product. Modern operating systems store fragments of the file in different places, and it is possible that even after you have zeroed out the most recent version of the file block-by-block and also overwrote all metadata, that a fragment from an older version of the file might be left over in another part of the drive.
The only secure way to delete sensitive content from a disk is to zero out the entire disk multiple times before discarding the disk.
#LarsH's answer about wiping all unallocated space after deleting files is compelling, but perhaps impractical. If you simply want to secure delete files so no one can scan the disk to recover it, then a better solution is the full-disk encryption. This was in-fact the entire appeal of full-disk encryption. This is why Apple stopped supporting secure file delete in their Mac OSX and iOS, and switched to full-disk encryption as default on all iPhones. Android phones have full-disk encryption as well now.
EDIT:
If you are looking for a true solution for a customer, your best bet is to use single file encryption. Once you destroy your key which only your app would know, there is no way to decrypt the file even if someone found it on the disk.
There exists no real solution for deleting files securely on SSDs. You can only give a false sense of security to non-technical people who still remember the old HDD days.
i believe this question is already asked but i am not satisfied with their answers and posting it again here.
can someone please tell me how to safeguard my android app assets from copy cats who want to build similar app?
As always there is a trade-off between convenience and security. The more secure you want your app the less convenient it will be for you to develop.
The source code is inherently insecure due to ease of decompiling especially with rooted phone. To protect your source code you can obfuscate and/or encrypt your code which will prevent decompiling. Not exactly sure what tools are available for Android, but I am sure it will complicate your build process. If you just obfuscate, decompiling may still be possible, but will be much more difficult and will likely require the person attempting to decompile your code to know and understand Bytecode if a strong level of obfuscation is used.
To protect your assets, I believe your only option is to use encryption. Again this will complicate the app and/or build process depending on where you implement.
Even if you use encryption to protect your assets, you must protect the encryption key within your source code. Obviously, it does not matter what encryption scheme you use if your encryption key is in plaintext in the source code then anybody can grab the key and your asset and decrypt. All this does is add one more small fence to jump over.
However, if you correctly protect the encryption key and use a good encryption algorithm you should have less to worry about. This is a fairly complicated process though, it is difficult to use a key for encryption within your code and not keep it in plaintext. Even if you don't keep it in plaintext within the code, at some point it must be in memory to perform decryption. So if somebody can attach a debugger or dump memory at the right time, it will compromise the key. Of course, this requires a much more skilled adversary.
Overall, you need to decide exactly who you are worried about stealing your assets. If you are worried about the average Joe copying them, then you should be ok. If you are worried about a professional hacker, script kiddie, etc. gaining access to them then you are probably out of luck.
can someone please tell me how to safeguard my android app assets from copy cats who want to build similar app?
Generally, you can't. If it's in the app, anyone who wants to can get to them.
You are welcome to roll your own encryption scheme, or use tools like DexGuard. However, since the decryption engine and key must be in the app itself, all these do is increase the level of effort required to get to your assets. Making it more difficult will reduce the odds that somebody grabs the assets out of your APK, but it does not prevent the possibility. And, of course, there are other ways to get at much of this stuff (e.g., screenshots and image editors, recording audio played back by the app).
You can secure assets folder contents by encrypt it using strong encryption algorithms and decrypt them at runtime. Copycats cannot easily decrypt and get assets folder contents by simply extract apk using zip tools.
I today released an Android application which includes some language-dependent MP3 files. My concern is that the more languages, the bigger the application size.
What would be your recommendation:
Release one application per language (not really easy to deal with, would have to release n apps each time I add new features to the software)
Provide a mechanism to download the MP3 files at first launch (would require an internet connection whereas the user may think he doesn't need one. It could bring frustration.)
Optimize the MP3 files (not that scalable if I have to support 10 languages)
[ ] fill the blank !
Hervé.
I'm a big fan of user-choice. As such, I might make the MP3 files optional, but on first run explain to the user that you have some language-dependant features and allow them to download them if they desire.
The reasons for this are many:
It keeps the file size low.
It is easily supportable with one distribution.
It allows you to extend/correct the language files independent of the software.
It is upfront, and gives the user the choice of language (even potentially multiple).
It allows the user to change the aural language without having to uninstall/reinstall or have duplicate installs.
There are many users (such as myself) who never speak to their phone, and never want to hear it speak to them. :) (Yes, I'm crazy)
Just a suggestion!
FuzzicalLogic
I would like to write application (as background service) which will encrypt whole file system totally. The questions are:
Is it possible, such that all Android services will work smoothly? Like, say Microsoft's BitLocker?
If so - can someone point me to some sources/docs?
No this is not possible thought the API.
You'd have to get the source code of Android and try to implement that yourself baking your own custom system image.
However I don't think it is possible at all.
Encrypted file system would be possible only via kernel-mode driver, which means a custom ROM for a device.
Its not clear if you are doing this to be secure, or only in order for a trojan to claim payment for restoring the files ;)
Encrypting files after they have been written in plaintext will leave the plaintext spread around your Flash (or disk) until that space is later reclaimed for new files. Its basically not secure. You have to encrypt before bytes get written to disk.
Android runs on Linux, and device drivers for storage, whilst modular, are compiled into the kernel. So unless you are distributing a custom Android image, you cannot post-install install a driver on someone's device.
There has been discussion like this on the mailing list here.
How can we make Android assets secure so that no one can read them after app deployment?
There is nothing you can do that will stop a determined attacker from reading them.
Using your own application level encryption would at least make the problem unique to your application, but someone could still do code analysis of your app to figure out how to decrypt them.
The platform's limited copy protection mechanisms are weaker, because they only have to be defeated once for all applications (such as by rooting the phone).
Assets utilized by platform functionality would also vulnerable to a modified platform configured to dump out copies of them.
Do what is easy to stop casual copying by unsophisticated users if you like, but then save your time and energy for battles you can actually win, such as the quality of your application.