diff --git a/backend/internal/errors/auth.go b/backend/internal/errors/auth.go
index 5d2a4f1..36f89fe 100644
--- a/backend/internal/errors/auth.go
+++ b/backend/internal/errors/auth.go
@@ -24,6 +24,7 @@ import (
 var (
 	ErrUnauthorized = errors.New("User is not authorized")
 	ErrUsernameTaken = errors.New("Provided username is already in use")
+	ErrUserNotFound = errors.New("User was not found")
 	ErrInvalidCredentials = errors.New("Invalid username, password or TOTP code")
 	ErrInvalidToken = errors.New("Token is invalid or expired")
 	ErrServerError = errors.New("Internal server error")
diff --git a/backend/internal/errors/general.go b/backend/internal/errors/general.go
index e3bdc79..641a4f6 100644
--- a/backend/internal/errors/general.go
+++ b/backend/internal/errors/general.go
@@ -24,4 +24,5 @@ import (
 var (
 	ErrNotImplemented = errors.New("Feature is not implemented")
 	ErrBadRequest = errors.New("Bad request")
+	ErrForbidden = errors.New("Access is denied")
 )
diff --git a/backend/internal/models/auth.go b/backend/internal/models/auth.go
index 1d63a09..5532009 100644
--- a/backend/internal/models/auth.go
+++ b/backend/internal/models/auth.go
@@ -28,9 +28,8 @@ type RegistrationBeginRequest struct {
 	Password string  `json:"password" binding:"required"` // TODO: password checking
 }
 
-// TODO: length check
 type RegistrationCompleteRequest struct {
-	Username 					string  `json:"username" binding:"required"`
+	Username 					string  `json:"username" binding:"required,min=3,max=20"`
 	VerificationCode	string  `json:"verification_code" binding:"required"`
 	Name							string  `json:"name" binding:"required,max=75"`
 	Birthday					*string `json:"birthday"`
@@ -41,10 +40,9 @@ type RegistrationCompleteResponse struct {
 	Tokens
 }
 
-// TODO: length check
 type LoginRequest struct {
-	Username string `json:"username" binding:"required"`
-	Password string `json:"password" binding:"required"`
+	Username string `json:"username" binding:"required,min=3,max=20"`
+	Password string `json:"password" binding:"required,max=100"`
 	TOTP 		 *string `json:"totp"`
 }
 
diff --git a/backend/internal/utils/pointer.go b/backend/internal/utils/pointer.go
new file mode 100644
index 0000000..c6cd178
--- /dev/null
+++ b/backend/internal/utils/pointer.go
@@ -0,0 +1,5 @@
+package utils
+
+func NewPointer[T any](d T) *T {
+    return &d
+}