Handling login functionality for native app connec

After extensive research, I have not been able to find a clear answer to my question. Firstly, can anyone tell me the basic logic of handling "login functionality" for a native iphone app connecting to a web service? For instance, the facebook app ask for a username and password immediately after launch, and from there you have full access to your account in all successive views of the app. Each time you post something etc, you do not have to re-login... Can someone please explain this process to me? Is it done through cookies or sessions? is Keychain involved?

I have a semi-working app right now but I'm almost positive I could be doing it better and more securely. Here is what I'm doing:

1) Set up a local server with a database of users (username and password columns and other tables etc.) using mysql. Wrote a simple web-service that takes in POST data and queries the database to check that the username exists... and if it does, that the passwords are equal. Using sha1 hashing. Echo true or false accordingly.

2) My app has an initial login screen with a 2 textfields (1 for username and 1 for password) and a button that calls the login method. My login method does the following:

init an *NSURL with a string (the url of my web service: @"http://webservice.com/login.php")
init an *ASIFormDataRequst with that url
set the post value with the password and email text in the text fields
set the delegate to itself
call startAsycronous on the request
implemented the requestFininshed method to retrieve the "true" or "false" echo-ed from the webservice
depending on the response, move forward to the next view, else, make an alert telling the user to retry

So, my questions are:

1) Is this secure for sending passwords? (via ASIHTTPRequest and the POST method?) 2) In the succeeding views, the user should be able to interact with their account (like posting messages and status's and pictures on the Facebook) How do I persist the user's logged in status so that every time the user interacts with the database, I can ensure that the user is still logged in and that it's the same user? For instance, the only way I can think of doing this is if I store a cookie on the users device with the username and password, and then every successive interaction with the web service / database, it does an authentication with the cookie values (username and password).

There has got to be a better way of doing this? Maybe sessions or cookies? or by using keychain??

Thanks for the help guys, and sorry for the long question!

Here are my thoughts based on what I know:

1) Is this secure for sending passwords? (via ASIHTTPRequest and the POST method?)

You need to make sure you are sending this information via https (SSL) and not a plain Http. The reason is, you don't have control over where the user wireless access point is. For all you know, the user could connect to open access point that is belong to a particular hacker. Having it transmitted will enable him to sniff the packet and get the required information to access the WebService even though the password is hashed. Having it send via https would ensure that the packet is encrypted with strong key. Even if a hacker manage to sniff the packet out, it will take him a long time before he/she is able to decrypt the message.

2) In the succeeding views, the user should be able to interact with their account (like posting > messages and status's and pictures on the Facebook) How do I persist the user's logged in status > so that every time the user interacts with the database, I can ensure that the user is still logged in and that it's the same user?

One commonly employed method to do this is to get the session token after the user logged in. That is, you create a random generated ID that you return upon successful login. You would then map this token with the user id in the backend and it is associated with a session time out. You refresh this time out every time the user connects to a webservice and time it out after certain period to avoid breach of security. You would then persist the session token in your device and then used that for subsequent call. As long the session is alive then the user is logged in. As the token is associated with a specific user, you also ensure the identity of the caller.

To prevent someone else using other people token is the reason why you need SSL to secure the channel and prevent sniffing. Assuming that you have secured your connection channels, the only way to get the token is

to verify the identity via login
The phone is stolen by hackers who could take the token by inspecting the local storage.

The mapping is necessary so you could validate the token is a real token that has been associated with the user via login activity. Furthermore,for number 2, you can offer remote wipe out feature that basically kills the mapping and making that token invalid.

Another very important part of this token is the token cannot be guessable and have to be random cryptographically (see Randomness Recommendations for Security). If the token is only based on pseudo randomness, a clever hacker might be able to guess its algorithm and can guess its next/previous token and has a possibility of obtaining any valid token in the table.

There are many algorithm to generate this token. For example, Java Programming Language provides a SecureRandom class in order to provide cryptographically randomness and .NET has similar secure RandomGenerator class.

If you want to look at the algorithm OATH has proposed Time-Based One-Time Password Algorithm (TOTP) which is an extension of HOTP. Most of the languages/platforms would have the cryptographically strong random generator that you could leverage immediately though without you having to write it yourself.

Depending on your service implementation/platform, you might want to ask SO for a suitable class/module for cryptographically random generator such as the one asked here "How do you generate cryptographically secure random numbers with php"