What should I do to convert a encrypted NSData to NSDictionary?

NSData *jsonDataEncrypted = [jsonStringEncrypted dataUsingEncoding:NSUTF8StringEncoding];
NSDictionary *jsonDicEncrypted = [NSJSONSerialization JSONObjectWithData:jsonDataEncrypted options:kNilOptions error:nil];

The code above is'n work. The

jsonDicEncrypted

is null.