diff --git a/utils/download.go b/utils/download.go
index 12b6cf8e..8e9894e7 100644
--- a/utils/download.go
+++ b/utils/download.go
@@ -10,7 +10,19 @@ import (
 )
 
 // Downloader is parallel HTTP fetcher
-type Downloader struct {
+type Downloader interface {
+	Download(url string, destination string) <-chan error
+	DownloadTemp(url string) (*os.File, error)
+	Shutdown()
+}
+
+// Check interface
+var (
+	_ Downloader = &downloaderImpl{}
+)
+
+// downloaderImpl is implementation of Downloader interface
+type downloaderImpl struct {
 	queue   chan *downloadTask
 	stop    chan bool
 	stopped chan bool
@@ -26,8 +38,8 @@ type downloadTask struct {
 
 // NewDownloader creates new instance of Downloader which specified number
 // of threads
-func NewDownloader(threads int) (downloader *Downloader) {
-	downloader = &Downloader{
+func NewDownloader(threads int) Downloader {
+	downloader := &downloaderImpl{
 		queue:   make(chan *downloadTask, 1000),
 		stop:    make(chan bool),
 		stopped: make(chan bool),
@@ -38,12 +50,12 @@ func NewDownloader(threads int) (downloader *Downloader) {
 		go downloader.process()
 	}
 
-	return
+	return downloader
 }
 
 // Shutdown stops downloader after current tasks are finished,
 // but doesn't process rest of queue
-func (downloader *Downloader) Shutdown() {
+func (downloader *downloaderImpl) Shutdown() {
 	for i := 0; i < downloader.threads; i++ {
 		downloader.stop <- true
 	}
@@ -54,7 +66,7 @@ func (downloader *Downloader) Shutdown() {
 }
 
 // Download starts new download task
-func (downloader *Downloader) Download(url string, destination string) <-chan error {
+func (downloader *downloaderImpl) Download(url string, destination string) <-chan error {
 	ch := make(chan error, 1)
 
 	downloader.queue <- &downloadTask{url: url, destination: destination, result: ch}
@@ -65,7 +77,7 @@ func (downloader *Downloader) Download(url string, destination string) <-chan er
 // DownloadTemp starts new download to temporary file and returns File
 //
 // Temporary file would be already removed, so no need to cleanup
-func (downloader *Downloader) DownloadTemp(url string) (*os.File, error) {
+func (downloader *downloaderImpl) DownloadTemp(url string) (*os.File, error) {
 	ch := make(chan error, 1)
 
 	tempfile, err := ioutil.TempFile(os.TempDir(), "aptly")
@@ -87,7 +99,7 @@ func (downloader *Downloader) DownloadTemp(url string) (*os.File, error) {
 }
 
 // handleTask processes single download task
-func (downloader *Downloader) handleTask(task *downloadTask) {
+func (downloader *downloaderImpl) handleTask(task *downloadTask) {
 	log.Printf("Downloading %s...\n", task.url)
 
 	resp, err := http.Get(task.url)
@@ -115,7 +127,7 @@ func (downloader *Downloader) handleTask(task *downloadTask) {
 }
 
 // process implements download thread in goroutine
-func (downloader *Downloader) process() {
+func (downloader *downloaderImpl) process() {
 	for {
 		select {
 		case <-downloader.stop: