iOS Core Data encryption using NSValueTransformer

Question

I'm experimenting with encrypting data with Core Data and CommonCrypto. I am trying to use a NSValueTransformer to lazily encrypt and decrypt.

However when I'm now trying save the encrypted data to the persistent store coordinator, it fails. Every time im trying to save my data to the database, it gives me:

-[__NSCFString bytes]: unrecognized selector sent to instance

I'm sure it's some sort of database and NSManagedObject mismatch, but I can't figure it out. I feel it's probably rather simple, but I can't find the solution. My code:

NSValueTransformer

class TryHardEncryption: NSValueTransformer {

override class func transformedValueClass() -> AnyClass {
    return NSString.self
}

override class func allowsReverseTransformation() -> Bool {
    return true
}


override func reverseTransformedValue(value: AnyObject?) -> AnyObject? {
    if let message = value as? NSString {
        let keyString        = "12345678901234567890123456789012"
        let keyData: NSData! = (keyString as NSString).dataUsingEncoding(NSUTF8StringEncoding) as NSData!
        let keyBytes         = UnsafeMutablePointer<Void>(keyData.bytes)
        print("keyLength   = \(keyData.length), keyData   = \(keyData)")

        let data: NSData! = (message as NSString).dataUsingEncoding(NSUTF8StringEncoding) as NSData!
        let dataLength    = size_t(data.length)
        let dataBytes     = UnsafeMutablePointer<Void>(data.bytes)
        print("dataLength  = \(dataLength), data      = \(data)")

        let cryptData    = NSMutableData(length: Int(dataLength) + kCCBlockSizeAES128)
        let cryptPointer = UnsafeMutablePointer<Void>(cryptData!.mutableBytes)
        let cryptLength  = size_t(cryptData!.length)

        let keyLength              = size_t(kCCKeySizeAES256)
        let operation: CCOperation = UInt32(kCCDecrypt)
        let algoritm:  CCAlgorithm = UInt32(kCCAlgorithmAES128)
        let options:   CCOptions   = UInt32(kCCOptionPKCS7Padding + kCCOptionECBMode)

        var numBytesEncrypted :size_t = 0

        let cryptStatus = CCCrypt(operation,
            algoritm,
            options,
            keyBytes, keyLength,
            nil,
            dataBytes, dataLength,
            cryptPointer, cryptLength,
            &numBytesEncrypted)

        if UInt32(cryptStatus) == UInt32(kCCSuccess) {
            //  let x: UInt = numBytesEncrypted
            cryptData!.length = Int(numBytesEncrypted)
            print("DecryptcryptLength = \(numBytesEncrypted), Decrypt = \(cryptData)")

            // Not all data is a UTF-8 string so Base64 is used
            let base64cryptString = cryptData!.base64EncodedStringWithOptions(.Encoding64CharacterLineLength)
            print("base64DecryptString = \(base64cryptString)")
            print( "utf8 actual string = \(NSString(data: cryptData!, encoding: NSUTF8StringEncoding))");
            return base64cryptString
        } else {
            print("Error: \(cryptStatus)")
        }
    }
    return nil
}

override func transformedValue(value: AnyObject?) -> AnyObject? {
    if let message = value as? NSString {
        let keyString        = "12345678901234567890123456789012"
        let keyData: NSData! = (keyString as NSString).dataUsingEncoding(NSUTF8StringEncoding) as NSData!
        let keyBytes         = UnsafePointer<UInt8>(keyData.bytes)
        print("keyLength   = \(keyData.length), keyData   = \(keyData)")

        let data: NSData! = message.dataUsingEncoding(NSUTF8StringEncoding) as NSData!
        let dataLength    = Int(data.length)
        let dataBytes     = UnsafePointer<UInt8>(data.bytes)
        print("dataLength  = \(dataLength), data      = \(data)")

        let cryptData    = NSMutableData(length: Int(dataLength) + kCCBlockSizeAES128)!
        let cryptPointer = UnsafeMutablePointer<UInt8>(cryptData.mutableBytes)
        let cryptLength  = size_t(cryptData.length)

        let keyLength              = size_t(kCCKeySizeAES256)
        let operation: CCOperation = UInt32(kCCEncrypt)
        let algoritm:  CCAlgorithm = UInt32(kCCAlgorithmAES128)
        let options:   CCOptions   = UInt32(kCCOptionECBMode + kCCOptionPKCS7Padding)

        var numBytesEncrypted :size_t = 0

        let cryptStatus = CCCrypt(operation,
            algoritm,
            options,
            keyBytes, keyLength,
            nil,
            dataBytes, dataLength,
            cryptPointer, cryptLength,
            &numBytesEncrypted)

        if UInt32(cryptStatus) == UInt32(kCCSuccess) {
            cryptData.length = Int(numBytesEncrypted)
            print("cryptLength = \(numBytesEncrypted), cryptData = \(cryptData)")

            // Not all data is a UTF-8 string so Base64 is used
            let base64cryptString = cryptData.base64EncodedStringWithOptions(.Encoding64CharacterLineLength)
            print("base64cryptString = \(base64cryptString)")
            return NSString(string: base64cryptString) as NSObject

        } else {
            print("Error: \(cryptStatus)")
        }
    }
    return nil
}

}

What I had to do to get the NSValueTransformer to work is:

let transformer: TryHardEncryption = TryHardEncryption()
    NSValueTransformer.setValueTransformer(transformer, forName: "TryHardEncryption")

Without the above code, the NSValueTransformer was never called.

I've marked the database field as the Transformable type and named it: TryHardEncryption. Do you guys know what is wrong here?

EDIT The property it's regarding is:

@NSManaged var establishmentDescription: String?

Both the encrypt and decrypt functions return a String when I debug them.

Answer 1

I finally figured it out... I was confused as to what type very object had to be and what my nsvaluetransformer had to return. I wasnt casting the value to the proper type in the nsvaluetransformer and thus I was returning nil in the code. However it wasnt crashing in a way that I would easily figure this out. Secondly I changed the entity column back to an NSObject. Returned an NSData object in the transformedValue method and a NSString in the reverseTransformedValue method. This was actually all i needed to make it work. Thanks a lot for your help. It indeed was a type error.

Answer 2

To help anyone who would like to see the corrected code sample, replace the return line of transformedValue() in the question with this: return base64cryptString.dataUsingEncoding(NSUTF8StringEncoding)