Look to the parent class (NSDictionary)
- (NSArray *)allKeysForObject:(id)anObject
Which will return a NSArray of all the keys for a given Object Value. BUT it does this by sending an isEqual message to each object of the Dictionary so for your large dataset this may not be best performance.
Maybe you need to hold some form of additional indexing structure structure(s) to allow you to locate the objects on some critical values within them, linked to the key without direct object comparison