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iOS Serialisation and Encoding

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Code and more information in https://mas.owasp.org/MASTG/iOS/0x06h-Testing-Platform-Interaction/#object-persistence.

Object Serialization in iOS Development

In iOS, object serialization involves converting objects into a format that can be easily stored or transmitted, and then reconstructing them from this format when needed. Two main protocols, NSCoding and NSSecureCoding, facilitate this process for Objective-C or NSObject subclasses, allowing objects to be serialized into NSData, a format that wraps byte buffers.

NSCoding Implementation

To implement NSCoding, a class must inherit from NSObject or be marked as @objc. This protocol mandates the implementation of two methods for encoding and decoding instance variables:

class CustomPoint: NSObject, NSCoding {
    var x: Double = 0.0
    var name: String = ""

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

    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)
    }
}

Enhancing Security with NSSecureCoding

To mitigate vulnerabilities where attackers inject data into already constructed objects, NSSecureCoding offers an enhanced protocol. Classes conforming to NSSecureCoding must verify the type of objects during decoding, ensuring that only the expected object types are instantiated. However, it's crucial to note that while NSSecureCoding enhances type safety, it doesn't encrypt data or ensure its integrity, necessitating additional measures for protecting sensitive information:

static var supportsSecureCoding: Bool {
    return true
}

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

Data Archiving with NSKeyedArchiver

NSKeyedArchiver and its counterpart, NSKeyedUnarchiver, enable encoding objects into a file and later retrieving them. This mechanism is useful for persisting objects:

NSKeyedArchiver.archiveRootObject(customPoint, toFile: "/path/to/archive")
let customPoint = NSKeyedUnarchiver.unarchiveObjectWithFile("/path/to/archive") as? CustomPoint

Using Codable for Simplified Serialization

Swift's Codable protocol combines Decodable and Encodable, facilitating the encoding and decoding of objects like String, Int, Double, etc., without extra effort:

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

This approach supports straightforward serialization to and from property lists and JSON, enhancing data handling in Swift applications.

JSON and XML Encoding Alternatives

Beyond native support, several third-party libraries offer JSON and XML encoding/decoding capabilities, each with its own performance characteristics and security considerations. It's imperative to carefully select these libraries, especially to mitigate vulnerabilities like XXE (XML External Entities) attacks by configuring parsers to prevent external entity processing.

Security Considerations

When serializing data, especially to the file system, it's essential to be vigilant about the potential inclusion of sensitive information. Serialized data, if intercepted or improperly handled, can expose applications to risks such as unauthorized actions or data leakage. Encrypting and signing serialized data is recommended to enhance security.

References

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