diff --git a/tests/datamigration.nix b/tests/datamigration.nix
index d93117ed..56494b4f 100644
--- a/tests/datamigration.nix
+++ b/tests/datamigration.nix
@@ -20,7 +20,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
         
         $testtarget1->mustSucceed("mkdir -m 700 /root/.ssh");
         $testtarget1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/dbus.nix b/tests/dbus.nix
index adab2bb8..80df856a 100644
--- a/tests/dbus.nix
+++ b/tests/dbus.nix
@@ -28,12 +28,12 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         # which should return the path we have given.
         # This test should succeed.
 
-        my $result = $client->mustSucceed("disnix-client --print-invalid /nix/store/invalid");
+        my $result = $client->mustSucceed("disnix-client --print-invalid /nix/store/00000000000000000000000000000000-invalid");
 
-        if($result =~ /\/nix\/store\/invalid/) {
-            print "/nix/store/invalid is invalid\n";
+        if($result =~ /\/nix\/store\/00000000000000000000000000000000-invalid/) {
+            print "/nix/store/00000000000000000000000000000000-invalid is invalid\n";
         } else {
-            die "/nix/store/invalid should be invalid\n";
+            die "/nix/store/00000000000000000000000000000000-invalid should be invalid\n";
         }
 
         # Check invalid path. We query a valid path from the service
@@ -134,19 +134,19 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         # unprivileged user, which should fail. Then we try the same
         # command by a privileged user, which should succeed.
 
-        $client->mustFail("su - unprivileged -c 'disnix-client --print-invalid /nix/store/invalid'");
-        $client->mustSucceed("su - privileged -c 'disnix-client --print-invalid /nix/store/invalid'");
+        $client->mustFail("su - unprivileged -c 'disnix-client --print-invalid /nix/store/00000000000000000000000000000000-invalid'");
+        $client->mustSucceed("su - privileged -c 'disnix-client --print-invalid /nix/store/00000000000000000000000000000000-invalid'");
 
         # Logfiles test. We perform an operation and check the id of the
         # logfile. Then we stop the Disnix service and start it again and perform
         # another operation. It should create a logfile which id is one higher.
 
-        $client->mustSucceed("disnix-client --print-invalid /nix/store/invalid");
+        $client->mustSucceed("disnix-client --print-invalid /nix/store/00000000000000000000000000000000-invalid");
         $result = $client->mustSucceed("ls /var/log/disnix | sort -n | tail -1");
         $client->stopJob("disnix");
         $client->startJob("disnix");
         $client->waitForJob("disnix");
-        $client->mustSucceed("sleep 3; disnix-client --print-invalid /nix/store/invalid");
+        $client->mustSucceed("sleep 3; disnix-client --print-invalid /nix/store/00000000000000000000000000000000-invalid");
         my $result2 = $client->mustSucceed("ls /var/log/disnix | sort -n | tail -1");
 
         if((substr $result2, 0, -1) - (substr $result, 0, -1) == 1) {
diff --git a/tests/deployment.nix b/tests/deployment.nix
index a109718d..93b84f99 100644
--- a/tests/deployment.nix
+++ b/tests/deployment.nix
@@ -20,7 +20,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
         
         $testtarget1->mustSucceed("mkdir -m 700 /root/.ssh");
         $testtarget1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/distbuild.nix b/tests/distbuild.nix
index 2a9a1ba5..960e7b3f 100644
--- a/tests/distbuild.nix
+++ b/tests/distbuild.nix
@@ -20,7 +20,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
         
         $testtarget1->mustSucceed("mkdir -m 700 /root/.ssh");
         $testtarget1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/install.nix b/tests/install.nix
index 88842c07..d2380d48 100644
--- a/tests/install.nix
+++ b/tests/install.nix
@@ -19,7 +19,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
 
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
 
         $server->mustSucceed("mkdir -m 700 /root/.ssh");
         $server->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/locking.nix b/tests/locking.nix
index 8fd109d5..26eae9fa 100644
--- a/tests/locking.nix
+++ b/tests/locking.nix
@@ -20,7 +20,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
         
         $testtarget1->mustSucceed("mkdir -m 700 /root/.ssh");
         $testtarget1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/machine.nix b/tests/machine.nix
index 4fead669..770af227 100644
--- a/tests/machine.nix
+++ b/tests/machine.nix
@@ -30,6 +30,13 @@
     }
   ];
   
+  # We can't download any substitutes in a test environment. To make tests
+  # faster, we disable substitutes so that Nix does not waste any time by
+  # attempting to download them.
+  nix.extraOptions = ''
+    substitute = false
+  '';
+
   environment.systemPackages = [ dysnomia ];
 
   environment.etc."dysnomia/properties" = {
diff --git a/tests/pkgs.nix b/tests/pkgs.nix
index 6222e46a..8c250d01 100644
--- a/tests/pkgs.nix
+++ b/tests/pkgs.nix
@@ -20,7 +20,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         startAll;
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
         
         $testtarget1->mustSucceed("mkdir -m 700 /root/.ssh");
         $testtarget1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/runactivities.nix b/tests/runactivities.nix
index e27cf3d5..2a4295ba 100644
--- a/tests/runactivities.nix
+++ b/tests/runactivities.nix
@@ -27,12 +27,12 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         # which should return the path we have given.
         # This test should succeed.
 
-        my $result = $server->mustSucceed("disnix-run-activity --print-invalid /nix/store/invalid");
+        my $result = $server->mustSucceed("disnix-run-activity --print-invalid /nix/store/00000000000000000000000000000000-invalid");
 
-        if($result =~ /\/nix\/store\/invalid/) {
-            print "/nix/store/invalid is invalid\n";
+        if($result =~ /\/nix\/store\/00000000000000000000000000000000-invalid/) {
+            print "/nix/store/00000000000000000000000000000000-invalid is invalid\n";
         } else {
-            die "/nix/store/invalid should be invalid\n";
+            die "/nix/store/00000000000000000000000000000000-invalid should be invalid\n";
         }
 
         # Check invalid path. We query a valid path from the service
diff --git a/tests/snapshots.nix b/tests/snapshots.nix
index 08333f56..8a96e452 100644
--- a/tests/snapshots.nix
+++ b/tests/snapshots.nix
@@ -22,7 +22,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         #### Test disnix-ssh-client's snapshot operations
         
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
     
         $server->mustSucceed("mkdir -m 700 /root/.ssh");
         $server->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
diff --git a/tests/ssh.nix b/tests/ssh.nix
index 1fd8a1ad..c2a46b32 100644
--- a/tests/ssh.nix
+++ b/tests/ssh.nix
@@ -23,7 +23,7 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
 
         #### Test disnix-ssh-client
         # Initialise ssh stuff by creating a key pair for communication
-        my $key=`${pkgs.openssh}/bin/ssh-keygen -t dsa -f key -N ""`;
+        my $key=`${pkgs.openssh}/bin/ssh-keygen -t ecdsa -f key -N ""`;
 
         $server->mustSucceed("mkdir -m 700 /root/.ssh");
         $server->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
@@ -36,12 +36,12 @@ with import "${nixpkgs}/nixos/lib/testing.nix" { system = builtins.currentSystem
         # which should return the path we have given.
         # This test should succeed.
 
-        my $result = $client->mustSucceed("${env} disnix-ssh-client --target server --print-invalid /nix/store/invalid");
+        my $result = $client->mustSucceed("${env} disnix-ssh-client --target server --print-invalid /nix/store/00000000000000000000000000000000-invalid");
 
-        if($result =~ /\/nix\/store\/invalid/) {
-            print "/nix/store/invalid is invalid\n";
+        if($result =~ /\/nix\/store\/00000000000000000000000000000000-invalid/) {
+            print "/nix/store/00000000000000000000000000000000-invalid is invalid\n";
         } else {
-            die "/nix/store/invalid should be invalid\n";
+            die "/nix/store/00000000000000000000000000000000-invalid should be invalid\n";
         }
 
         # Check invalid path. We query a valid path from the service