diff --git a/astronaut b/astronaut
index 757702a..ea4c5ab 100755
--- a/astronaut
+++ b/astronaut
@@ -54,9 +54,13 @@ msg() {
 download() {
     local name=$(basename "$1")
     local path="$dir_source/$name"
-    if [ ! -f "$path" ]; then
+    local checksum=""
+    if [ "$2" -a -f "$path" ]; then
+        checksum="$(md5sum ""$path"" 2> /dev/null | cut -d' ' -f1)"
+    fi
+    if [ ! -f "$path" -o "$checksum" != "$2" ]; then
         msg "Downloading $name"
-        if [ "$2" ]; then
+        if [ "$3" ]; then
             local cmd="$(echo "$2" | sed -e 's@{dst}@'"$path"'@g' -e 's@{source}@'"$1"'@g')"
             $cmd
         else
@@ -76,7 +80,7 @@ extract() {
 }
 
 dlextract() {
-    download "$1"
+    download "$1" "$2"
     extract "$(basename ""$1"")"
 }
 
diff --git a/hello.sat b/hello.sat
index 19e006f..10a522e 100644
--- a/hello.sat
+++ b/hello.sat
@@ -3,7 +3,8 @@
 name=hello
 version=2.9
 
-dlextract "http://ftp.gnu.org/pub/gnu/$name/$name-$version.tar.gz"
+dlextract "http://ftp.gnu.org/pub/gnu/$name/$name-$version.tar.gz" \
+          "67607d2616a0faaf5bc94c59dca7c3cb"
 
 cd "$name-$version"
 ./configure