How to generate an S3 Access policy in Ruby on Rails

Server side is Rails.

Client side is Flash, users will upload directly to S3

I need a flexible way to generate S3 policy files, base64 encode them, and then distribute the resulting signed policy to the client.

Is there a good library/gem for this, or do I need to roll my own?

I'll be using paperclip to store the file, as per: http://www.railstoolkit.com/posts/fancyupload-amazon-s3-uploader-with-paperclip

I've had a look at:

• https://github.com/geemus/fog
• https://github.com/jnicklas/carrierwave
• https://github.com/marcel/aws-s3

These look 开发者_C百科like they'll help me get bits done, but I can't tell if they'll help me generate flexible policies.

EDIT: Going to give the "Generate an upload signature..." bit here a shot: http://www.kiakroas.com/blog/44/

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Here is a sample project for how to do this using Rails 3, Flash/Silverlight/GoogleGears/BrowserPlus and jQuery-based Plupload to upload directly to S3: https://github.com/iwasrobbed/Rails3-S3-Uploader-Plupload