# File lib/aws/dynamo_db/request.rb, line 29
      def add_authorization!(signer)

        self.access_key_id = signer.access_key_id

        headers["x-amz-date"] ||= (headers["date"] ||= Time.now.rfc822)
        headers["host"] ||= host

        raise ArgumentError, "a security token is required" unless
          signer.session_token

        headers["x-amz-security-token"] = signer.session_token

        # compute the authorization
        request_hash = OpenSSL::Digest::SHA256.digest(string_to_sign)
        signature = signer.sign(request_hash)
        headers["x-amzn-authorization"] =
          "AWS3 "+
          "AWSAccessKeyId=#{signer.access_key_id},"+
          "Algorithm=HmacSHA256,"+
          "SignedHeaders=#{headers_to_sign.join(';')},"+
          "Signature=#{signature}"
      end