diff --git a/Makefile b/Makefile
index 1c487c0..60aef9a 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ token-validator/token-validator: token-validator/*.go
 pkg/challenge: pkg/challenge/adlin pkg/challenge/issue pkg/challenge/init
 	$(LINUXKIT) pkg build --platforms linux/amd64 -org nemunaire pkg/challenge/
 
-challenge-initrd.img: challenge.yml subject/adlin.6.gz subject/adlin-TP1-topologie.png pkg/challenge pkg/shadow-up
+challenge-initrd.img: challenge.yml subject/adlin.6.gz subject/adlin-TP1-topologie.png pkg/challenge pkg/challenge/init pkg/shadow-up
 	$(LINUXKIT) build -docker $<
 
 
diff --git a/pkg/challenge/init b/pkg/challenge/init
index 294b330..c08a99c 100755
--- a/pkg/challenge/init
+++ b/pkg/challenge/init
@@ -87,8 +87,8 @@ losetup -P "${LOOPDEV}" /dev/blk > /dev/null 2> /dev/null
 mdev -s
 /sbin/mkfs.ext4 "${LOOPDEV}p1" > /dev/null 2> /dev/null
 mount "${LOOPDEV}p1" /mnt > /dev/null 2> /dev/null
-N1=$((72 - $RANDOM % 35))
-N2=$((25 + $RANDOM % 35))
+N1=$((29 - $RANDOM % 25))
+N2=$((3 + $RANDOM % 25))
 {
     echo -n $N1
     echo -n $N2
diff --git a/token-validator/challenge.go b/token-validator/challenge.go
index fe8fc7f..c0057ac 100644
--- a/token-validator/challenge.go
+++ b/token-validator/challenge.go
@@ -197,6 +197,9 @@ func challengeDisk(s *adlin.Student, t *givenToken, chid int) error {
 	if n1+n2 > len(pkey) {
 		n2 = len(pkey) - n1
 	}
+	if n1+n2 < n1 {
+		return errors.New("This is not the expected token.")
+	}
 
 	expectedToken := sha512.Sum512([]byte(pkey[n1 : n1+n2]))