session management and one-time user login - iphone

I'm creating an iphone app where the user logins once (when they open the app for the first time), then will never have to login again (like how instagram does it). The app will automatically log them in the next time they open it up. However, the app makes a bunch of requests to a web server.

What is the best way for the server to issue session tokens? How long should the session tokens be valid for? How can I ensure the user never has to log in again, while still providing secure session tokens.

One approach is for the server to issue a token to the user when the user logs in for开发者_如何学C the first time, and make that token permanent. That, however, does not seem secure.

Thanks for the help!

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Well, generally the session is already handled through session cookies. Unless you're planning to have third parties connect to your service, I think it's a bit overkill to do anything besides basic http authentication. I would definitely send all of your connection requests over an https connection though.

As far as persisting the session on the iPhone side, you can save the user and password in the Keychain, and then automatically retrieve and send it to the server when it requires you to log in again, without having to prompt the user to log in again. How often you want the sessions to last on the server end is really up to you.

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What is the best way for the server to issue session tokens?

One way to do it is using OAuth. It is more complex than cookies but it has more features.
A token is granted to each application and can be revoked by the user from a page in the server. This token can be permanent or temporary. You can store it as plain text or inside the iPhone keychain, depending on the level of security you need. There is open free code for server and client implementations. Another benefit is that clients can log in your service using their Twitter/Facebook/... account so they don't need to register on your site.