hm := hmac.New(sha1.New, []byte(secretAccessKey))
	hm.Write([]byte(stringToSign))

	// Calculate signature.
	signature := base64.StdEncoding.EncodeToString(hm.Sum(nil))

	query := req.URL.Query()
	// Handle specially for Google Cloud Storage.
	query.Set("AWSAccessKeyId", accessKeyID)
	// Fill in Expires for presigned query.
	query.Set("Expires", strconv.FormatInt(epochExpires, 10))

	// Encode query and save.