After the new update to the Spotify iOS SDK and Web API I have been working with the new libraries (they're great!)
However I am wondering how does one authenticate a user once, say on iOS, and then share this authentication token to the other services to keep a single user session.
Specifically since libSpotify uses a password as opposed to an access token, how would one fix this issue?
You can't. The iOS SDK is intended to replace libspotify/CocoaLibSpotify entirely, and the authentication stacks aren't compatible. If there's something in CocoaLibSpotify that you rely on that's missing from the iOS SDK, file an issue on the iOS SDK's issue tracker to let us know about it and stick with CocoaLibSpotify for now.