iOS File Protection

OS provides hardware-level encryption of files. Files marked for protection are encrypted using a per-device key, which is encrypted using the user’s password or PIN. Ten seconds after the device is locked, the unencrypted per-device key is removed from memory. When the user unlocks the device, the password or personal identification number (PIN) is used to decrypt the per-device key again, which is then used to decrypt the files.

You can configure the protection of files that you create with NSFileManager or NSData. The options, shown in the following list, have slightly different names. NSFileManager applies string attributes to the file, whereas NSData uses numeric options during creation, but the meanings are the same. The FileManager constants begin with NSFileProtection…, and the NSData constants begin with NSDataWritingFileProtection….

    • …None — The file is not protected and can be read or written at any time. This is the default value.
    • …Complete — Any file with this setting is protected ten seconds after the device is locked. This is the highest level of protection. Files with this setting may not be available when your program is running in the background. When the device is unlocked, these files are unprotected.
    • …CompleteUnlessOpen — Files with this setting are protected ten seconds after the device is locked unless they’re currently open. This allows your program to continue accessing the file while running in the background. When the file is closed, it will be protected if the device is locked.
    • …CompleteUntilFirstUserAuthentication — Files with this setting are protected only between the time the device boots and the first time the user unlocks the device. The files are unprotected from that point until the device is rebooted. This allows your application to open existing files while running in the background.

Sample usages with NSData –

[data writeToFile:path
             options:NSDataWritingFileProtectionComplete
               error:&error]

Sample NSFileManager –

[[NSFileManager defaultManager] createFileAtPath:[self filePath]
                                        contents:[@"super secret file contents" dataUsingEncoding:NSUTF8StringEncoding]
                                      attributes:[NSDictionary dictionaryWithObject:NSFileProtectionComplete forKey:NSFileProtectionKey]];

If your application needs to know whether protected data is available, you can use one of the following:
Implement the methods applicationProtectedDataWillBecomeUnavailable: and applicationProtectedDataDidBecomeAvailable: in your application delegate.
Observe the notifications UIApplicationProtectedDataWillBecomeUnavailable and UIApplicationProtectedDataDidBecomeAvailable (these constants lack the traditional Notification suffix). Check

[[UIApplication sharedApplication] protectedDataAvailable]

References
http://www.makebetterthings.com/iphone/ios-app-security-data-protection-part-1/

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