diff --git a/internal/domain/user.go b/internal/domain/user.go
index 1ff4967..74a431e 100644
--- a/internal/domain/user.go
+++ b/internal/domain/user.go
@@ -46,12 +46,18 @@ type User struct {
 	DeletedAt   *time.Time
 }
 
+// TODO: move to another file?
 type Challenge struct {
 	Message   uuid.UUID
 	TimeStamp time.Time
 	ExpiresAt time.Time
 }
 
+// TODO: check EIP712 in here for challenge validation
+func (c *Challenge) IsExpired() bool {
+	return time.Now().After(c.ExpiresAt)
+}
+
 func NewUser(pubKey, name, lastName, phoneNumber, email, nationalID string) (*User, error) {
 
 	_, err := phone.IsValid(phoneNumber)
diff --git a/internal/usecase/auth_service.go b/internal/usecase/auth_service.go
index 4f7edd2..53b362a 100644
--- a/internal/usecase/auth_service.go
+++ b/internal/usecase/auth_service.go
@@ -11,6 +11,7 @@ import (
 
 type AuthService interface {
 	GenerateChallenge(ctx context.Context, pubKey string) (*domain.Challenge, error)
+	Authenticate(ctx context.Context, pubKey string, signature string, challenge *domain.Challenge, chainID uint) (*UserToken, error)
 }
 
 type authService struct {
@@ -21,6 +22,12 @@ type authService struct {
 	refreshTokenExp uint
 }
 
+type UserToken struct {
+	AuthorizationToken string
+	RefreshToken       string
+	ExpiresAt          int64
+}
+
 func NewAuthService(
 	userRepo *domain.UserRepo,
 	sessionRepo *domain.SessionRepo,
@@ -47,3 +54,7 @@ func (s *authService) GenerateChallenge(ctx context.Context, pubKey string) (*do
 	}
 	return challenge, nil
 }
+
+func (s *authService) Authenticate(ctx context.Context, pubKey string, signature string, challenge *domain.Challenge, chainID uint) (*UserToken, error) {
+	return nil, nil
+}