从NSManagedObject内部引用NSValueTransformer实体

Question

我使用NSValueTranformer加密某些核心数据属性。这一切都很好，但我需要能够使用不同的加密密钥取决于NSManagedObject。无论如何，我可以从我的变压器类中访问这个实体吗？

用例是，我有多个具有不同密码的用户，可以访问不同的NSManagedObject实体。如果我对所有对象使用相同的加密密钥，就可以重新分配谁在SQL db中拥有它们，它们仍然会解密。

对这件事的最佳方式有什么想法吗？

编辑:我应该说我是用iOS来做这个的。

Answer 1

第三次魅力？让我看看我能不能解决你的唯一转换时磁盘的要求。把这看作是另外两种方法的混合体。

@interface UserSession : NSObject

+ (UserSession*)currentSession;
+ (void)setCurrentSession: (UserSession*)session;
- (id)initWithUserName: (NSString*)username andEncryptionKey: (NSData*)key;

@property (nonatomic, readonly) NSString* userName;
@property (nonatomic, readonly) NSData* encryptionKey;

@end

@implementation UserSession

static UserSession* gCurrentSession = nil;

+ (UserSession*)currentSession
{
    @synchronized(self)
    {
        return gCurrentSession;
    }
}

+ (void)setCurrentSession: (UserSession*)userSession
{
    @synchronized(self)
    {
        gCurrentSession = userSession;
    }
}

- (id)initWithUserName: (NSString*)username andEncryptionKey: (NSData*)key
{
    if (self = [super init])
    {
        _userName = [username copy];
        _encryptionKey = [key copy];
    }
    return self;
}

- (void)dealloc
{
    _userName = nil;
    _encryptionKey = nil;
}

@end

@interface EncryptingValueTransformer : NSValueTransformer
@end

@implementation EncryptingValueTransformer

- (id)transformedValue:(id)value
{    
    UserSession* session = [UserSession currentSession];
    NSAssert(session, @"No user session! Can't decrypt!");

    NSData* key = session.encryptionKey;
    NSData* decryptedData = Decrypt(value, key);
    return decryptedData;
}

- (id)reverseTransformedValue:(id)value
{
    UserSession* session = [UserSession currentSession];
    NSAssert(session, @"No user session! Can't encrypt!");

    NSData* key = session.encryptionKey;
    NSData* encryptedData = Encrypt(value, key);
    return encryptedData;
}

@end

这里唯一棘手的部分是，您必须确保在创建托管对象上下文之前设置了当前的UserSession，并且直到上下文保存和释放之后才进行更改。

希望这能有所帮助。

Answer 2

您可以创建具有状态(即加密密钥)的NSValueTransformer子类的自定义实例，并使用-bind:toObject:withKeyPath:options:密钥在options字典中将它们传递给NSValueTransformerBindingOption。

您将无法在IB中直接设置它，因为IB按类名引用值转换器，但您可以在代码中这样做。如果您感到非常雄心勃勃，您可以在IB中设置绑定，然后在代码中用不同的选项替换它们。

它看起来可能是这样的：

@interface EncryptingValueTransformer : NSValueTransformer

@property (nonatomic,readwrite,copy) NSData* encryptionKey;

@end

@implementation EncryptingValueTransformer

- (void)dealloc
{
    _encryptionKey = nil;
}

- (id)transformedValue:(id)value
{
    if (!self.encryptionKey)
        return nil;

    // do the transformation

    return value;
}

- (id)reverseTransformedValue:(id)value
{
    if (!self.encryptionKey)
        return nil;

    // Do the reverse transformation

    return value;
}

@end


@interface MyViewController : NSViewController

@property (nonatomic, readwrite, assign) IBOutlet NSControl* controlBoundToEncryptedValue;

@end

@implementation MyViewController

// Other stuff...

- (void)loadView
{
    [super loadView];

    // Replace IB's value tansformer binding settings (which will be by class and not instance) with specific,
    // stateful instances.
    for (NSString* binding in [self.controlBoundToEncryptedValue exposedBindings])
    {
        NSDictionary* bindingInfo = [self.controlBoundToEncryptedValue infoForBinding: binding];
        NSDictionary* options = bindingInfo[NSOptionsKey];
        if ([options[NSValueTransformerNameBindingOption] isEqual: NSStringFromClass([EncryptingValueTransformer class])])
        {
            // Out with the old
            [self.controlBoundToEncryptedValue unbind: binding];

            // In with the new
            NSMutableDictionary* mutableOptions = [options mutableCopy];
            mutableOptions[NSValueTransformerNameBindingOption] = nil;
            mutableOptions[NSValueTransformerBindingOption] = [[EncryptingValueTransformer alloc] init];
            [self.controlBoundToEncryptedValue bind: binding
                                           toObject: bindingInfo[NSObservedObjectKey]
                                        withKeyPath: bindingInfo[NSObservedKeyPathKey]
                                            options: mutableOptions];
        }
    }
}

// Assuming you're using the standard representedObject pattern, this will get set every time you want
// your view to expose new model data. This is a good place to update the encryption key in the transformers'
// state...

- (void)setRepresentedObject:(id)representedObject
{
    for (NSString* binding in [self.controlBoundToEncryptedValue exposedBindings])
    {
        id transformer = [self.controlBoundToEncryptedValue infoForBinding: NSValueBinding][NSOptionsKey][NSValueTransformerBindingOption];
        EncryptingValueTransformer* encryptingTransformer = [transformer isKindOfClass: [EncryptingValueTransformer class]] ? (EncryptingValueTransformer*)transformer : nil;
        encryptingTransformer.encryptionKey = nil;
    }

    [super setRepresentedObject:representedObject];

    // Get key from model however...
    NSData* encryptionKeySpecificToThisUser = /* Whatever it is... */ nil;

    for (NSString* binding in [self.controlBoundToEncryptedValue exposedBindings])
    {
        id transformer = [self.controlBoundToEncryptedValue infoForBinding: NSValueBinding][NSOptionsKey][NSValueTransformerBindingOption];
        EncryptingValueTransformer* encryptingTransformer = [transformer isKindOfClass: [EncryptingValueTransformer class]] ? (EncryptingValueTransformer*)transformer : nil;
        encryptingTransformer.encryptionKey = encryptionKeySpecificToThisUser;
    }
}

// ...Other stuff

@end

Answer 3

好的。这让我心烦意乱所以我再想一想.我认为最简单的方法是有某种类型的“会话”对象，然后在托管对象上有一个“派生属性”。假设您有一个名为UserData的实体，该实体具有一个名为encryptedData的属性，我编写了一些代码，这些代码可能有助于说明：

@interface UserData : NSManagedObject
@property (nonatomic, retain) NSData * unencryptedData;
@end

@interface UserData () // Private
@property (nonatomic, retain) NSData * encryptedData;
@end

// These functions defined elsewhere
NSData* Encrypt(NSData* clearData, NSData* key);
NSData* Decrypt(NSData* cipherData, NSData* key);

@interface UserSession : NSObject

+ (UserSession*)currentSession;

- (id)initWithUserName: (NSString*)username andEncryptionKey: (NSData*)key;

@property (nonatomic, readonly) NSString* userName;
@property (nonatomic, readonly) NSData* encryptionKey;

@end

@implementation UserData

@dynamic encryptedData;
@dynamic unencryptedData;

+ (NSSet*)keyPathsForValuesAffectingUnencryptedData
{
    return [NSSet setWithObject: NSStringFromSelector(@selector(encryptedData))];
}

- (NSData*)unencryptedData
{
    UserSession* session = [UserSession currentSession];
    if (nil == session)
        return nil;

    NSData* key = session.encryptionKey;
    NSData* encryptedData = self.encryptedData;
    NSData* decryptedData = Decrypt(encryptedData, key);
    return decryptedData;
}

- (void)setUnencryptedData:(NSData *)unencryptedData
{
    UserSession* session = [UserSession currentSession];
    NSAssert(session, @"No user session! Can't encrypt!");

    NSData* key = session.encryptionKey;
    NSData* encryptedData = Encrypt(unencryptedData, key);

    self.encryptedData = encryptedData;
}

@end

@implementation UserSession

static UserSession* gCurrentSession = nil;

+ (UserSession*)currentSession
{
    @synchronized(self)
    {
        return gCurrentSession;
    }
}

+ (void)setCurrentSession: (UserSession*)userSession
{
    @synchronized(self)
    {
        gCurrentSession = userSession;
    }
}

- (id)initWithUserName: (NSString*)username andEncryptionKey: (NSData*)key
{
    if (self = [super init])
    {
        _userName = [username copy];
        _encryptionKey = [key copy];
    }
    return self;
}

-(void)dealloc
{
    _userName = nil;
    _encryptionKey = nil;
}

@end

这里的想法是，当给定的用户登录时，您将创建一个新的UserSession对象并调用+[UserSession setCurrentSession: [[UserSession alloc] initWithUserName: @"foo" andEncryptionKey: <whatever>]]。派生属性(unencryptedData)访问器和变量获取当前会话，并使用键将值来回转换为“实”属性。(另外，不要跳过+keyPathsForValuesAffectingUnencryptedData方法。这将告诉运行时这两个属性之间的关系，并将有助于更无缝地工作。)