Home Forums Peripherals & Multimedia iTunes, iPhone, iPod, iPad, Apple TV Opening an encrypted & compressed disk in Dropbox

I have an encrypted and compressed disk image on Dropbox. The DMG was originally a .doc (Word) file. When I access this file from my desktop, it opens as expected; Keychain does its thing and voilà!

But when I access this file from my iPhone 5, I am advised that the file type is unknown (and can’t be opened). Other .doc files can easily be created and read; it is only these DMGs that cause the angst.

Is this issue a result of not having DU on my iPhone?

Is there a solution for this or am I restricted to only accessing unencrypted and unstuffed vanilla files?

I don't have anything available that I can test my thesis with at the moment, so please take my comments with a grain of salt. IOS does not support a file system like that found in OS X. Therefore there is no DU nor is one needed. DMG files are based on and use a file system that mirrors the one in OS X. So an iOS device does not know how to handle the file system in a DMG. Similarly iOS would have no idea what to do with the directory or files in a DMG if it were to unpack one. Applications in iOS maintain their files as open objects or in the cloud. Dropbox itself provides a rudimentary file system to contain the files it manages and those are stored in a cloud provided by Dropbox..

You are stuck with unstuffed vanilla files.

it's possible that the necessary API's exist in iOS to mount a disk image, but there's no handler associated with .DMG to call the mount. diskutill itself probably doesn't exist as a part of the regular iOS installation. It's what normally mounts disk images, or any other volume that is trying to attach, by calling the appropriate APIs.

The necessary security framework to access keychains may not exist either. I don't think there's any implementation of keychains in iOS.

You can use Dropbox to access encrypted (but not compressed) files. I do that for some of my financial files.

Many thanks to you all. I suspected my problem was as you noted.

But, Ira, I made an encrypted file as suggested and omitted the compression step, but I am still unable to open the file on my iPhone 5.

Operator error (again)?

It may be a characteristic of the specific software. What type of file and trying to open it with which app?

For example, the app on my iPhone that opens the encrypted file is opening a file of its own type. I have not tried generic files with multiple apps, and even then, perhaps not every app can do it. Alas.

iOS devices can not open or use a .dmg, full stop. If they could, it would open the door to distributing apps outside of the Apple App Store.

Thanks, Tacit.

I had considered OS incompatibility, but it never dawned on me that the root of the issue relates to the distribution of App Store apps via .dmg.

Still, I wonder if there is a cheap encryption app that works on both my DT and iPhone. I just want to encrypt a file on my DT and decrypt it on my iPhone. Storing it on Dropbox is a plus, but not a requirement.

Work in which app on your iPhone?

That's not a rhetorical question. Every file that gets opened has to be opened by some application. Applications announce which formats they're able to open, and never get an opportunity to open any file they don't claim to be able to open.

Sandboxing on iOS is so tight that applications cannot even learn what other apps are installed, let alone what formats they can read. The closest they can come is to ask the OS to offer a file up for grabs. The OS (which does know) will construct a list of compatible apps and ask the user to pick one. That app will be told "some other app, I'm not saying which one, is offering up this file which I've given you temporary read-only access to. If you're interested, you can make your own copy of it, but you've gotta do it right now because your access is going to go away post haste." The program offering up the file never learns which app (if any) took the file, or even which apps were on the list.

That's how Mail passes attachments to other apps, and how Drop Box hands files off to other apps. Any app can pass files to Drop Box, because it claims to be able to open anything.

If your word processing application cannot already open encrypted files, compressed files, DMGs, etc., there's no way you can make it open one. The best you can do is write your own app that recognizes the encrypted file, decrypts it, and offers the decrypted file up for grabs. It will never know which, if any, application accepted the grab and made a copy. You will be forced to place the decrypted file on disk (well, Flash), but can delete it immediately afterwards.

Or maybe someone's already done that? Visit the iTunes App Store and search for "decryption". There are some apps there that may meet your needs.

Usage might be something like: you take a .doc file on your Mac and compress it to a .gz, which you then encrypt to a .pgp file. Put that in Drop Box. Drop Box would offer up the .pgp file, which the special purpose app would decrypt and expand back to a .doc file, which it would then throw out for grabs. The user would be confronted with two pop-up menus, first to choose the app to do the decryption/expansion, and then again to choose the app to consume the resulting .doc file. Two more user interactions to get the edited file compressed and encrypted and back to Drop Box.

There's a tradeoff between security and convenience. Apple puts the priority on security.

Indeed that is the rub. Well, that all that goes with Apple's Sandbox…

As my wants/requirements are small and the consequence of compromise are negligible, me thinks I'll just opt for a home made simple algorithm. Clearly, one could brute force the solution, but that is unlikely.

Now, re my numbered Credit Suisse account in the Caymans, ah, that is a different matter.

Always, my thanks kind sir, for the most enlightening (and educational) tutorial.