Object Encoding and Decoding with NSSecureCoding Protocol

NSCoding is a fantastically simple and convenient way to store data on iOS or Mac OS by turning your model objects directly into a file and then loading it back into memory again, without needing to write any file parsing and serialization logic. To save any object to a data file (assuming that it implements the NSCoding protocol), you can just do this:

And to load it again later:

That’s fine for resources that are compiled into your app (such as nib files, which use NSCoding under the hood), but the problem with using NSCoding for reading and writing user data files is that, because you are encoding whole classes in a file, you are implicitly giving that file (with potentially unknown origin) permission to instantiate classes inside your app.

Although you cannot store executable code in an NSCoded file (on iOS at least), a hacker could use a specially crafted file to trick your app into instantiating classes that you never intended it to, or do so in a different context to what you intended. Whilst it would be difficult to do any real harm in this way, it could certainly lead to app crashes and/or data loss for users.

In iOS6, Apple introduced a new protocol built on top of NSCoding called NSSecureCoding. NSSecureCoding is identical to NSCoding, except that when decoding objects you specify both the key and class of the object you are decoding, and if the expected class doesn’t match the class of the object decoded from the file, the NSCoder will throw an exception to tell you that the data has been tampered with.

Most of the system objects that support NSCoding have been upgraded to support NSSecureCoding, so by enabling secure coding on your NSKeyedUnarchiver, you can make sure that the data file you are loading is safe. You would do that like this:

Note that if you enable secure coding on your NSKeyedUnarchiver, every object stored in the file must conform to NSSecureCoding, otherwise you will get an exception. To tell the framework that your own classes support NSSecureCoding, you have to implement the new decoding logic in your initWithCoder: method, and return YES from the supportsSecureCoding method. There are no changes to the encodeWithCoder: method needed, as the security concerns are around loading, not saving.

A few weeks ago, I wrote about how to implement NSCoding automatically by using introspection to determine the properties of your class at runtime.

This is a great way to add NSCoding support to all of your model objects without having to write repetitive and error-prone initWithCoder:/encodeWithCoder: methods. But the approach we used would not support NSSecureCoding because we do not attempt to validate the types of the objects being loaded.

So how can we enhance our automatic NSCoding system to support NSSecureCoding right out of the box?

If you recall, the original implementation worked by using the class_copyPropertyList() and property_getName() runtime functions to generate a list of property names, which we stored in an array:

Using KVC (Key-Value Coding), we were then able to set and get all the properties of an object by name and encode/decode them in an NSCoder object.

To implement NSSecureCoding, we’ll follow the same principle, but instead of just getting the property names, we’ll also need to get their types. Fortunately, the objective C runtime stores detailed information about the types of class properties, so we can easily extract this data along with the names.

Properties of a class can be either primitive types (such as integers, BOOLs and structs) or objects (such as NSString, NSArray, etc). The KVC valueForKey: and setValue:forKey: methods implement automatic “boxing” of primitive types, meaning that they will turn integers, BOOLs and structs into NSNumber and NSValue objects, respectively. That makes things a bit easier for us because we only have to deal with boxed types (objects), so we can represent all of our property types as classes, instead of having to call different decoding methods for different property types.

The runtime methods don’t give us the boxed class name for each property though, instead they give us the type encoding – a specially formatted C-string containing the type information (this is the same format returned by the @encode(var); syntax). There’s no automatic way to get the equivalent boxed class for a primitive type, so we’ll need to parse this string and then specify the appropriate class ourselves.

The format of a type encoding string is documented here.

The first character represents the primitive type. Objective C uses a unique character for each supported primitive, for example ‘i’ represents an integer, ‘f’ a float, ‘d’ a double, and so on. Objects are represented by ‘@’ (followed by the class name) and then there are other more obscure types such as ‘:’ for selectors, or ‘#’ for classes.

Struct and union types are represented by expressions wrapped in {…} brackets. Only some of these types are supported by the KVC mechanism, but the ones that are supported are always boxed as NSValue objects, so we can treat any value starting with ‘{‘ the same way.

If we switch based on the first character of the string, we can handle all the known types:

To handle ‘@’ types, we need to extract the class name. The class name may include protocols (which we don’t really need to worry about), so we’ll split the string to extract just the class name, and then use NSClassFromString to get the class:

Finally, we can combine this parsing logic with the propertyNames method logic from our previous implementation to create a method that returns a dictionary of property classes, keyed by property name. Here is the complete implementation:

That’s the hard part done. Now, to implement NSSecureCoding, we’ll just modify the initWithCoder: method from our previous automatic coding implementation to take the property class into account when parsing. We’ll also need to return YES from the supportsSecureCoding method:

And there you have it: A simple base class for your models that supports NSSecureCoding out of the box. Alternatively, you can use my AutoCoding category that uses this approach to add automatic NSCoding and NSSecureCoding support to any object that doesn’t already implement it.

Nick Lockwood is the author of iOS Core Animation: Advanced Techniques. Nick also wrote iCarousel, iRate and other Mac and iOS open source projects.
  1. Foo *someFoo = [unarchiver decodeObjectForKey:NSKeyedArchiveRootObjectKey];
    Will not work since “unarchiver” requires secure coding.

    should use
    [unarchiver decodeObjectOfClass: [Foo class] forKey: NSKeyedArchiveRootObjectKey];

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