iOS Serialisation and Encoding

NSCoding and NSSecureCoding

iOS comes with two protocols for object serialisation for Objective-C or NSObjects: NSCoding and NSSecureCoding. When a class conforms to either of the protocols, the data is serialized to NSData: a wrapper for byte buffers. Note that Data in Swift is the same as NSData or its mutable counterpart: NSMutableData. The NSCoding protocol declares the two methods that must be implemented in order to encode/decode its instance-variables. A class using NSCoding needs to implement NSObject or be annotated as an @objc class. The NSCoding protocol requires to implement encode and init as shown below.

class CustomPoint: NSObject, NSCoding {

    //required by NSCoding:
    func encode(with aCoder: NSCoder) {
        aCoder.encode(x, forKey: "x")
        aCoder.encode(name, forKey: "name")
    }

    var x: Double = 0.0
    var name: String = ""

    init(x: Double, name: String) {
            self.x = x
            self.name = name
    }

    // required by NSCoding: initialize members using a decoder.
    required convenience init?(coder aDecoder: NSCoder) {
            guard let name = aDecoder.decodeObject(forKey: "name") as? String
                    else {return nil}
            self.init(x:aDecoder.decodeDouble(forKey:"x"),
                                name:name)
    }

    //getters/setters/etc.
}

The issue with NSCoding is that the object is often already constructed and inserted before you can evaluate the class-type. This allows an attacker to easily inject all sorts of data. Therefore, the NSSecureCoding protocol has been introduced. When conforming to NSSecureCoding you need to include:

static var supportsSecureCoding: Bool {
        return true
}

when init(coder:) is part of the class. Next, when decoding the object, a check should be made, e.g.:

let obj = decoder.decodeObject(of:MyClass.self, forKey: "myKey")

The conformance to NSSecureCoding ensures that objects being instantiated are indeed the ones that were expected. However, there are no additional integrity checks done over the data and the data is not encrypted. Therefore, any secret data needs additional encryption and data of which the integrity must be protected, should get an additional HMAC.

Object Archiving with NSKeyedArchiver

NSKeyedArchiver is a concrete subclass of NSCoder and provides a way to encode objects and store them in a file. The NSKeyedUnarchiver decodes the data and recreates the original data. Let's take the example of the NSCoding section and now archive and unarchive them:

// archiving:
NSKeyedArchiver.archiveRootObject(customPoint, toFile: "/path/to/archive")

// unarchiving:
guard let customPoint = NSKeyedUnarchiver.unarchiveObjectWithFile("/path/to/archive") as?
    CustomPoint else { return nil }

You can also save the info in primary plist NSUserDefaults:

// archiving:
let data = NSKeyedArchiver.archivedDataWithRootObject(customPoint)
NSUserDefaults.standardUserDefaults().setObject(data, forKey: "customPoint")

// unarchiving:
if let data = NSUserDefaults.standardUserDefaults().objectForKey("customPoint") as? NSData {
    let customPoint = NSKeyedUnarchiver.unarchiveObjectWithData(data)
}

Codable

It is a combination of the Decodable and Encodable protocols. A String, Int, Double, Date, Data and URL are Codable by nature: meaning they can easily be encoded and decoded without any additional work. Let's take the following example:

struct CustomPointStruct:Codable {
    var x: Double
    var name: String
}

By adding Codable to the inheritance list for the CustomPointStruct in the example, the methods init(from:) and encode(to:) are automatically supported. Fore more details about the workings of Codable check the Apple Developer Documentation.

You can also use codable to save the data in the primary property list NSUserDefaults:

struct CustomPointStruct: Codable {
        var point: Double
        var name: String
    }

    var points: [CustomPointStruct] = [
        CustomPointStruct(point: 1, name: "test"),
        CustomPointStruct(point: 2, name: "test"),
        CustomPointStruct(point: 3, name: "test"),
    ]

    UserDefaults.standard.set(try? PropertyListEncoder().encode(points), forKey: "points")
    if let data = UserDefaults.standard.value(forKey: "points") as? Data {
        let points2 = try? PropertyListDecoder().decode([CustomPointStruct].self, from: data)
    }

JSON Encoding

There are a lot of thrid party libraries to encode data in JSON (like exposed here). However, Apple provides support for JSON encoding/decoding directly by combining Codable together with a JSONEncoder and a JSONDecoder:

struct CustomPointStruct: Codable {
    var point: Double
    var name: String
}

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

let test = CustomPointStruct(point: 10, name: "test")
let data = try encoder.encode(test)
let stringData = String(data: data, encoding: .utf8)

// stringData = Optional ({
// "point" : 10,
// "name" : "test"
// })

XML

There are multiple ways to do XML encoding. Similar to JSON parsing, there are various third party libraries, such as: Fuzi, Ono, AEXML, RaptureXML, SwiftyXMLParser, SWXMLHash

They vary in terms of speed, memory usage, object persistence and more important: differ in how they handle XML external entities. See XXE in the Apple iOS Office viewer as an example. Therefore, it is key to disable external entity parsing if possible. See the OWASP XXE prevention cheatsheet for more details. Next to the libraries, you can make use of Apple's XMLParser class

When not using third party libraries, but Apple's XMLParser, be sure to let shouldResolveExternalEntities return false.

All these ways of serialising/encoding data can be used to store data in the file system. In those scenarios, check if the stored data contains any kind of sensitive information. Moreover, in some cases you may be able to abuse some serialised data (capturing it via MitM or modifying it inside the filesystem) deserializing arbitrary data and making the application perform unexpected actions (see Deserialization page). In these cases, it's recommended to send/save the serialised data encrypted and signed.

References

iOS Platform APIsMobile Security Testing Guide