diff --git a/sync/user_management.go b/sync/user_management.go
index 7a226bf..ecd3c3f 100644
--- a/sync/user_management.go
+++ b/sync/user_management.go
@@ -54,7 +54,7 @@ func GetUsedUserIDs() map[int64]bool {
 // GetFreeUserID returns the smallest valid unused user id
 func GetFreeUserID() int64 {
 	used := GetUsedUserIDs()
-	for i := int64(0); i > 0; i++ {
+	for i := int64(0); i >= 0; i++ {
 		if !used[i] {
 			return i
 		}
@@ -79,7 +79,7 @@ func AddKey(userID int64, key string) {
 	}
 
 	destFileName := fmt.Sprintf("%d_keys", userID)
-	destFile, err := os.OpenFile(filepath.Join(PublicKeyLocation, destFileName), os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0644)
+	destFile, err := os.OpenFile(filepath.Join(PublicKeyLocation, destFileName), os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0o644)
 	if err != nil {
 		log.Fatalf("Error adding key. Unable to create new key file or write to existing key file with user id %v", userID)
 	}
diff --git a/sync/user_management_test.go b/sync/user_management_test.go
new file mode 100644
index 0000000..1700ce5
--- /dev/null
+++ b/sync/user_management_test.go
@@ -0,0 +1,110 @@
+/*
+Copyright 2026 - timew-sync-server contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the Software without restriction, including without limitation the
+rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the
+Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
+WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+*/
+
+package sync
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestGetUsedUserIDs_Empty(t *testing.T) {
+	tmpDir := t.TempDir()
+	oldLocation := PublicKeyLocation
+	PublicKeyLocation = tmpDir
+	defer func() { PublicKeyLocation = oldLocation }()
+
+	ids := GetUsedUserIDs()
+	if len(ids) != 0 {
+		t.Errorf("expected empty map, got %v", ids)
+	}
+}
+
+func TestGetUsedUserIDs_WithFiles(t *testing.T) {
+	tmpDir := t.TempDir()
+	oldLocation := PublicKeyLocation
+	PublicKeyLocation = tmpDir
+	defer func() { PublicKeyLocation = oldLocation }()
+
+	for _, name := range []string{"0_keys", "2_keys", "5_keys"} {
+		if err := os.WriteFile(filepath.Join(tmpDir, name), []byte("key"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(tmpDir, "notakey"), []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	ids := GetUsedUserIDs()
+	if len(ids) != 3 {
+		t.Errorf("expected 3 ids, got %d", len(ids))
+	}
+	for _, id := range []int64{0, 2, 5} {
+		if !ids[id] {
+			t.Errorf("expected id %d to be present", id)
+		}
+	}
+}
+
+func TestGetFreeUserID_Empty(t *testing.T) {
+	tmpDir := t.TempDir()
+	oldLocation := PublicKeyLocation
+	PublicKeyLocation = tmpDir
+	defer func() { PublicKeyLocation = oldLocation }()
+
+	id := GetFreeUserID()
+	if id != 0 {
+		t.Errorf("expected 0, got %d", id)
+	}
+}
+
+func TestGetFreeUserID_WithGap(t *testing.T) {
+	tmpDir := t.TempDir()
+	oldLocation := PublicKeyLocation
+	PublicKeyLocation = tmpDir
+	defer func() { PublicKeyLocation = oldLocation }()
+
+	for _, name := range []string{"0_keys", "1_keys", "3_keys"} {
+		if err := os.WriteFile(filepath.Join(tmpDir, name), []byte("key"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	id := GetFreeUserID()
+	if id != 2 {
+		t.Errorf("expected 2, got %d", id)
+	}
+}
+
+func TestGetFreeUserID_StartingFromZero(t *testing.T) {
+	tmpDir := t.TempDir()
+	oldLocation := PublicKeyLocation
+	PublicKeyLocation = tmpDir
+	defer func() { PublicKeyLocation = oldLocation }()
+
+	for _, name := range []string{"1_keys", "2_keys"} {
+		if err := os.WriteFile(filepath.Join(tmpDir, name), []byte("key"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	id := GetFreeUserID()
+	if id != 0 {
+		t.Errorf("expected 0, got %d", id)
+	}
+}