NSSecureCoding

NSSecureCoding is a protocol in Apple's Foundation framework that extends NSCoding to require secure decoding of serialized objects. It aims to prevent object substitution attacks by ensuring that only objects of expected classes are unarchived, particularly when data originates from untrusted sources or is stored on disk.

To adopt NSSecureCoding, a class must conform to the protocol and implement the required methods. In Objective-C, this involves declaring + (BOOL)supportsSecureCoding and implementing -initWithCoder: and -encodeWithCoder:, with supportsSecureCoding returning YES. When decoding, developers should use decodeObjectOfClass:forKey: or decodeObjectOfClasses:forKey: to constrain the allowed classes. In Swift, a class conforms to NSSecureCoding, implements required init?(coder:), and encode(with:), and must provide a static var supportsSecureCoding: Bool { true }. Decoding should similarly use restricted decoding methods to validate object types, including containers such as arrays or dictionaries.

Security implications and usage guidelines: NSSecureCoding reduces the risk of instantiating unexpected or malicious classes during unarchiving. It is most effective when all objects participating in the archive support secure coding. If any involved class does not conform, the archive should not be surfaced to a secure unarchiver, or the data should be migrated accordingly.

History and scope: NSSecureCoding was introduced as part of Apple’s security enhancements for NSKeyedArchiver/NSKeyedUnarchiver, with support across macOS and iOS. It is recommended for custom classes that participate in keyed archiving to ensure safer deserialization. See also NSCoding, NSKeyedArchiver, NSKeyedUnarchiver.