help with a useful implementation of keychain on an iphone app

Users have requested a "lock" on my iphone app, which makes good sense as it contains private information. I believe encryption of the data goes above and beyond what people expect on a phone, so my goal is simply to prevent unintended access to the app. My plan is to use keychain to store the password using m开发者_JS百科y app name as the ServiceName.

My concern is what happens for users if they lose/forget the password they typed in. Is there something I can do programatically to allow a user reset? Would deleting the app delete the keychain for the app?

I don't want to collect user emails. (Well I do, but I don't want this to be the justification.) And don't want the user to be permanently locked out of the app if they've lost the passsword.

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On the iPhone there is just a single keychain database, and there is no possibility to add a custom, application specific, keychain (as you can do on a Mac instead). By default, the keychain items you add to the keychain in your app are only available to your app. There is no documentation (to the best of my knowledge) stating the behavior occurring when the user deletes your app: may be iOS 4.1 deletes the app keychain items, may be it does leave them in the keychain.

Depending on how you stored the user's password, you may be able to retrieve it. For instance, if you stored a tag related to your application along with the user's password in a kSecAttrApplicationTag, then you may search for your exact tag in the keychain using the

OSStatus SecItemCopyMatching (
   CFDictionaryRef query,
   CFTypeRef *result
);

function. If the search is successful, then you may retrieve the password from the returned dictionary using the kSecValueData key or, if you prefer, you can even change it using

OSStatus SecItemUpdate (
   CFDictionaryRef query,
   CFDictionaryRef attributesToUpdate
);

For additional information, see the keychain reference and the Keychain Services Tasks for iOS documentation.

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The usual approach here is to collect a set of security questions and answers in addition to the (shorter, quicker) password or PIN. If the user forgets her password, she should hopefully remember the answers to her longer, more mnemonic security questions.

You can store both in the app's keychain.

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Looking back, I see I never updated this question with my actual implementation.

The core of my issue was that I do not have contact information for my user and I do not copy their data in any way. That is, I let their data remain private, in part, by enabling no internet or email features.

So, I used SecItemUpdate as suggested above, while also coding in a default password (ie, no password equivalent) for the app. Then if the user loses the password, they can tell the app that the password is lost at which point the app resets the password to the default password and DELETES all private data in the database. So, there is a cost to losing the password, but at no point can someone break in and see the private data.

I've used this method for a couple of years now and it seems to satisfy my users perfectly well.