paperclip.rb

# Building authenticated and expiring URL for S3
# Based on
#   http://github.com/logandk/paperclip_hacks/blob/master/lib/paperclip.rb
# and
#   http://thewebfellas.com/blog/2009/8/29/protecting-your-paperclip-downloads
module Paperclip
  class Attachment
    def authenticated_url(style = nil, expires_in = 5.minutes)
      if @storage.to_s == 's3' && file?
        AWS::S3::S3Object.url_for(path(style || default_style), bucket_name, :expires_in => expires_in, :use_ssl => s3_protocol == 'https')
      else
        url
      end
    end
  end
end

Nice ! thx for this gist