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I have stored an important document on the SDHC card on my Samsung Galaxy 2 phone (so that I can edit it on the go). I don't want others to access this file or document without my permission.

I know I can lock the phone but this option means that nothing could be done on the phone unless it is unlocked.

I want others to still make calls from my phone but not be able to access the memory card. (Letting others use my phone is an internal arrangement i.e. so that there is one point of contact when we are travelling together)

Is there a way to lock the memory card (with a password) but leave the phone unlocked?

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I would highly doubt it unless you're rooted, and if you're rooted the user can easily get around it. What's wrong with encrypting the file? That actually prevents against theft and so on as well. –  Matthew Read Jan 3 '12 at 23:32
    
Locking the memory card will prevent access to anything on the card. This approach would be more suited because then I will not have to worry about locking individual files (e.g. documents, photos etc.). –  Adhesh Josh Jan 5 '12 at 10:50
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5 Answers

up vote 1 down vote accepted

Simply, use BoxCryptor.. It'll encrypt your files & folders with AES-256 standard (AES-256 is classified by the U.S. Government to protect “TOP SECRET” information). The best thing: You can use it on any platform: Android, iOS, Windows, Mac, Linux. So, you can sync your encrypted files with Dropbox, too.
Its free Android app: https://market.android.com/details?id=com.boxcryptor.android (Free version has limitation of Read-only access). You don't even need a rooted device whats not the case of fully free LUKS Manager. If your device is rooted & you also need write access without spending money, go for LUKS Manager, but you'll miss cross-platform access. So, my recommendation is paid BoxCryptor if you need write access.

With encryption, your files & folders will be safe even if your friends are hacker. With even fastest super computers, breaking AES-256 standard needs decades or even hundreds/thousands years (based on password strength & encryption passphrase).

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Encryption is most probably the most appropriate solution for your case. Something along the lines of LUKSManager could be the right tool. Though you would need a rooted phone in any case and perhaps a modified kernel, too.

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Encryption is the only appropriate solution in this case. Everything else can simply be bypassed by just stealing the sd-card. –  Robert Jan 27 '12 at 10:58
    
Root permission is not necessary for encryption... Some application you must try to check it: LastPass (Free), BoxCryptor with license (Paid).. –  Sachin Shekhar Jan 29 '12 at 23:37
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You cant lock, in your definition, the sdcard or one file. But you have some ways to protect your file.

  1. Store it in a password protected Zip file.
  2. Encrypt your sdcard
  3. Use a AppLocker to lock the apps they are needed to view / edit the file.
  4. Use BoxCryptor.

The only safe way is, to encrypt it. If you lock your sdcard with a software, the thief can easily connect it to a pc and steal the file or even steal the whole sdcard.

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If your phone is rooted(which it may not), you might be able to unmount the card, altough I dont know if the system tries to remout it on itself after some period.

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Have you considered just removing the SD card from the phone and keeping it in a safe spot until you need to access your important files?

AFAIK there isn't a way to encrypt/protect a single file on an SD card. That leaves you looking at fully encrypting the card which Android doesn't support out of the box. Instead of that hassle, why not simply remove it? You don't need an SD card to use basic phone functions (calls, SMS) and many apps can either do without SD storage or be moved to internal storage. Your device requires you to power down and pull the battery to access the micro SD card slot; insertion or removal of the card and then powering the phone back on, though, shouldn't take you much longer than a minute.

Keep it simple, I say.

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