Lock file on iOS Device programmatically

I want to lock a file (located in the Documents directory of my Application Bundle) programmatically using a password. Is there an API for that, or do开发者_StackOverflow i have to write my own encryption and decription mechanism?

I've allready searched on the internet, but i couldn't find the proper solution for my problem.

To be clearly, i want to store a NSDictionary to a file located in the Documents directory. There i'd simply use NSDictionary's method writeToFile:atomically: but there is no possibility to set a password?

Thanks for your help, Br Nic

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You can do that in two steps:

1. convert your NSDictionary to NSData by using NSKeyedArchiver archivedDataWithRootObject

2. encrypt/decrypt your NSData like explained in this S.O. topic