IOS Upload Images To AWS 3 Generate Random Unique Key Names

I am currently using Amazon S3 to upload images from an IOS app with Cognito Identities and a custom back-end server.

When uploading the image, a key needs to be set for the image URL

_uploadRequest.key = @"filename.jpg";

The final image URL will be:

http://ift.tt/1K9vnjk

Is there a good way to generate unique random strings in IOS in order to have unique filenames or would it be to generate them server side (Rails back-end) ?