[Rails] Web Services for iphone App

Hi All
I am working an Rails 2.3.14.I am providing web services for
an iphone app thru xml.
My Rails app uses an authlogic gem for authentication.How to
authenticate a API call from an iphone app.
Initially i can able to sign up and login thru XML post
request.After login how my rails app identify me as a logged
user.Whether i need to provide any token after login and using token
for further request for an API call.I am novice in this.please suggest
me the best way to implement authentication for an iphone app.

Thanks in advance....

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