gistfile1.rb

  def generate_secure_s3_url(s3_key)
    #
    # s3_key would be a path (including filename) to the file like:  "folder/subfolder/filename.jpg"
    # but it should NOT contain the bucket name or a leading forward-slash
    #
    # this was built using these instructions:
    # http://docs.amazonwebservices.com/AmazonS3/latest/dev/index.html?S3_QSAuth.html
    # http://aws.amazon.com/code/199?_encoding=UTF8&jiveRedirect=1
    
    s3_base_url       = MyApp::Application::S3_BASE_URL # i.e. https://mybucket.s3.amazonaws.com
    bucket            = MyApp::Application::S3_BUCKET # i.e. mybucket
    access_key_id     = MyApp::Application::S3_ACCESS_KEY_ID # your Amazon S3 access key ID
    secret_access_key = MyApp::Application::S3_SECRET_ACCESS_KEY # your Amazon S3 secret access key
    expiration_date   = 2.days.from_now.utc.to_i # 2 days from now in UTC epoch time (i.e. 1308172844)

    # this needs to be formatted exactly as shown below and UTF-8 encoded
    string_to_sign = "GET\n\n\n#{expiration_date}\n/#{bucket}/#{s3_key}".encode("UTF-8")

    # we have to CGI/URL escape the signature since it would fail if it included / or + characters
    signature = CGI.escape( Base64.encode64(
                              OpenSSL::HMAC.digest(
                                OpenSSL::Digest::Digest.new('sha1'),
                                  secret_access_key, string_to_sign)).gsub("\n","") )
  
    return "#{s3_base_url}/#{s3_key}?AWSAccessKeyId=#{access_key_id}
                                    &Expires=#{expiration_date}
                                    &Signature=#{signature}"
  end