diff --git a/handler/default.go b/handler/default.go
index 34a1cf7..3810adc 100644
--- a/handler/default.go
+++ b/handler/default.go
@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"me-fit/db"
 	"me-fit/middleware"
 	"me-fit/service"
 
@@ -8,17 +9,17 @@ import (
 	"net/http"
 )
 
-func GetHandler(db *sql.DB) http.Handler {
+func GetHandler(d *sql.DB) http.Handler {
 	var router = http.NewServeMux()
 
-	router.HandleFunc("/", service.HandleIndexAnd404(db))
+	router.HandleFunc("/", service.HandleIndexAnd404(d))
 
-	handlerAuth := NewHandlerAuth(db, service.NewServiceAuthImpl(db))
+	handlerAuth := NewHandlerAuth(d, service.NewServiceAuthImpl(db.NewDbAuthSqlite(d)))
 
 	// Serve static files (CSS, JS and images)
 	router.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.Dir("./static/"))))
 
-	handleWorkout(db, router)
+	handleWorkout(d, router)
 
 	handlerAuth.handle(router)
 
diff --git a/service/auth.go b/service/auth.go
index 4340c1f..e8b2eaa 100644
--- a/service/auth.go
+++ b/service/auth.go
@@ -51,9 +51,9 @@ type ServiceAuthImpl struct {
 	dbAuth db.DbAuth
 }
 
-func NewServiceAuthImpl(d *sql.DB) *ServiceAuthImpl {
+func NewServiceAuthImpl(dbAuth db.DbAuth) *ServiceAuthImpl {
 	return &ServiceAuthImpl{
-		dbAuth: db.NewDbAuthSqlite(d),
+		dbAuth: dbAuth,
 	}
 }
 
@@ -64,7 +64,7 @@ func (service ServiceAuthImpl) SignIn(email string, password string) (*User, err
 		if errors.Is(err, db.ErrUserNotFound) {
 			return nil, ErrInvaidCredentials
 		} else {
-			return nil, err
+			return nil, types.ErrInternal
 		}
 	}
 
diff --git a/service/auth_test.go b/service/auth_test.go
new file mode 100644
index 0000000..295ada2
--- /dev/null
+++ b/service/auth_test.go
@@ -0,0 +1,107 @@
+package service
+
+import (
+	"me-fit/db"
+	"me-fit/types"
+
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type DbAuthStub struct {
+	user *db.User
+	err  error
+}
+
+func (d DbAuthStub) GetUser(email string) (*db.User, error) {
+	return d.user, d.err
+}
+
+func TestSignIn(t *testing.T) {
+	t.Parallel()
+	t.Run("should return user if password is correct", func(t *testing.T) {
+		t.Parallel()
+		salt := []byte("salt")
+		stub := DbAuthStub{
+			user: db.NewUser(
+				uuid.New(),
+				"test@test.de",
+				true,
+				time.Date(2020, 1, 2, 0, 0, 0, 0, time.UTC),
+				false,
+				getHashPassword("password", salt),
+				salt,
+				time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC),
+			),
+			err: nil,
+		}
+		underTest := NewServiceAuthImpl(stub)
+
+		actualUser, err := underTest.SignIn("test@test.de", "password")
+		if err != nil {
+			t.Errorf("Expected nil, got %v", err)
+		}
+
+		expectedUser := User{
+			Id:            stub.user.Id,
+			Email:         stub.user.Email,
+			EmailVerified: stub.user.EmailVerified,
+		}
+		if *actualUser != expectedUser {
+			t.Errorf("Expected %v, got %v", expectedUser, actualUser)
+		}
+	})
+
+	t.Run("should return ErrInvalidCretentials if password is not correct", func(t *testing.T) {
+		t.Parallel()
+		salt := []byte("salt")
+		stub := DbAuthStub{
+			user: db.NewUser(
+				uuid.New(),
+				"test@test.de",
+				true,
+				time.Date(2020, 1, 2, 0, 0, 0, 0, time.UTC),
+				false,
+				getHashPassword("password", salt),
+				salt,
+				time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC),
+			),
+			err: nil,
+		}
+		underTest := NewServiceAuthImpl(stub)
+
+		_, err := underTest.SignIn("test@test.de", "wrong password")
+		if err != ErrInvaidCredentials {
+			t.Errorf("Expected %v, got %v", ErrInvaidCredentials, err)
+		}
+	})
+	t.Run("should return ErrInvalidCretentials if user has not been found", func(t *testing.T) {
+		t.Parallel()
+		stub := DbAuthStub{
+			user: nil,
+			err:  db.ErrUserNotFound,
+		}
+		underTest := NewServiceAuthImpl(stub)
+
+		_, err := underTest.SignIn("test", "test")
+		if err != ErrInvaidCredentials {
+			t.Errorf("Expected %v, got %v", ErrInvaidCredentials, err)
+		}
+	})
+	t.Run("should forward ErrInternal on any other error", func(t *testing.T) {
+		t.Parallel()
+		stub := DbAuthStub{
+			user: nil,
+			err:  errors.New("Some error"),
+		}
+		underTest := NewServiceAuthImpl(stub)
+
+		_, err := underTest.SignIn("test", "test")
+		if err != types.ErrInternal {
+			t.Errorf("Expected %v, got %v", types.ErrInternal, err)
+		}
+	})
+}