EncryptionTransformer class

EncryptionTransformer.h

@interface EncryptionTransformer : NSValueTransformer
{}

/**
 * Returns the key used for encrypting / decrypting values during transformation.
 */
- (NSString*)key;

@end

EncryptionTransformer.m

@implementation EncryptionTransformer

+ (Class)transformedValueClass
{
   return [NSData class];
}

+ (BOOL)allowsReverseTransformation
{
   return YES;
}

- (NSString*)key
{
   // Your version of this class might get this key from the app delegate or elsewhere.
   return @"secret key";
}

- (id)transformedValue:(NSData*)data
{
   // If there's no key (e.g. during a data migration), don't try to transform the data
   if (nil == [self key])
   {
      return data;
   }

   if (nil == data)
   {
      return nil;
   }

   return [data dataAES256EncryptedWithKey:[self key]];
}

- (id)reverseTransformedValue:(NSData*)data
{
   // If there's no key (e.g. during a data migration), don't try to transform the data
   if (nil == [self key])
   {
      return data;
   }

   if (nil == data)
   {
      return nil;
   }
   
   return [data dataAES256DecryptedWithKey:[self key]];
}

@end