# 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](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:

```swift
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:

```swift
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:

```swift
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:

```swift
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

* [https://mas.owasp.org/MASTG/iOS/0x06h-Testing-Platform-Interaction/#object-persistence](https://mas.owasp.org/MASTG/iOS/0x06h-Testing-Platform-Interaction/#object-persistence)

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