NString comparsions operators -- CONTAINS/LIKE -- it doesn't work on NSData

001Background

Problem: Once encrypted values are stored as NSData in persistent store, if fetch takes place with NSPredicate which uses string (not basic) comparisons operators -- CONTAINS/LIKE -- it doesn't work for NSData.

Note: I have to stop having random values for 'iv' & 'salt' generationed in DataGenerator.m
Please find my code in files listed below. I have provided comments against working & non-working code.

Question: In order to fetch data, how to form sql query (predicate) with string operators 'like, contains or match' for encrypted data.

002AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSManagedObjectContext *context = [self managedObjectContext];
    
    // Create new core data object
    NSManagedObject *coreDataObject = [NSEntityDescription insertNewObjectForEntityForName:@"CoreDataObject" inManagedObjectContext:context];
    
    NSString * value = @"Hello world";
    // Set our encrypted attribute
    [coreDataObject setValue:value forKey:@"encrypted_field"];
    
    // Save our object
    NSError *error = nil;
    if (![context save:&error]) {
        NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
    }
    
    // Fetch the Object back from core data
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"CoreDataObject" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    
    // Works as expected: In NSPredicate, basic Comparisons operators works. See http://nshipster.com/nspredicate/ from details for valid basic operators
    [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"%K == %@", @"encrypted_field", value]];
    
    // Problem: Once encrypted values are stored as NSData in persistent store, if fetch takes place with NSPredicate which
    // uses string (not basic) comparisons operators -- CONTAINS/LIKE -- it doesn't work on NSData.
    // [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"%K LIKE %@",  @"encrypted_field", value]];

    // Execute request
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
    
    // Remove items from core CoreDataObject
    for (NSManagedObject *info in fetchedObjects) {
        [context deleteObject:info];
    }
    
    error = nil;
    if (![context save:&error]) {
        NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
    }
    
    
    return YES;
}

003DataGenerator.m

@implementation DataGenerator
+ (NSData *)createRandomDataOfLength:(size_t)length {
    NSMutableData *data = [NSMutableData dataWithLength:length];

// Stop having random 'Salt' & 'iv' value. As it changes the encryption value (i.e different transformerValue than reverseTransformedValue).
//    int result = SecRandomCopyBytes(kSecRandomDefault,
//                                    length,
//                                    data.mutableBytes);
//    NSAssert(result == 0, @"Unable to generate random bytes: %d",
//             errno);
    
    return data;
}

Did you get this working? I am also stuck with similar issue.