Amazon S3 POST upload (from iPhone)

A bit of background: I am building an iPhone app with a complementary server backend (written in Rails or possibly Sinatra, but probably not relevant for this discussion). Part of the functionalit开发者_如何转开发y involves uploading pictures from the iPhone to the server. These ultimately get stored on S3, so in order to simplify the app and conserve bandwidth, I would like to upload the pictures directly from the iPhone to S3, skipping my backend server.

Using the S3 REST API (in which case I would likely use ASIHTTPRequest) would mean storing the AWS key and secret in the iPhone app, which I don't want to do for security reasons.

For similar reasons I don't want to make my S3 bucket publicly writable.

Now it looks like S3 also has support for browser-based uploads using POST. If I understand it correctly, this works by generating a signed policy document on the server, which then allows the client app to directly POST the file to S3. It seems like in principle this should work not only for browsers, but also for iPhone apps.

However, I have a hard time figuring out the exact way of getting this working (not the iPhone specific part, just S3 POST uploads in general). What information needs to be sent to the server in order to calculate the signature (e.g. does it need the file size or any other file information)? I'll dig through the official docs some more and start experimenting with this, but if anybody could point me to some tutorials or sample code, that would be much appreciated.

When you generate the policy you can restrict what is uploaded in various ways (key name, mime-type, file size etc) by constructing a JSON string. These restrictions (including an expirydate) are then signed using your AWS secret key. You then post the signed policy and you access key as form parameters to AWS along with the key for the new resource, it's content and whatever other meta-data you like.

The official doco is the only reference I know of (but I haven't googled for it either...)

http://docs.amazonwebservices.com/AmazonS3/2006-03-01/dev/HTTPPOSTForms.html#HTTPPOSTConstructPolicy

is the page you're probably most interested in.