diff --git a/packages/auth/src/application/repositories/users-repository.interface.ts b/packages/auth/src/application/repositories/users-repository.interface.ts
new file mode 100644
index 0000000..9e03e0a
--- /dev/null
+++ b/packages/auth/src/application/repositories/users-repository.interface.ts
@@ -0,0 +1,7 @@
+import type { User } from "../../entities/user";
+
+export interface IUsersRepository {
+  getUser(id: string): Promise<User | undefined>;
+  getUserByUsername(username: string): Promise<User | undefined>;
+  createUser(input: User): Promise<User>;
+}
diff --git a/packages/auth/src/application/services/authentication-service.interface.ts b/packages/auth/src/application/services/authentication-service.interface.ts
new file mode 100644
index 0000000..0e5f873
--- /dev/null
+++ b/packages/auth/src/application/services/authentication-service.interface.ts
@@ -0,0 +1,14 @@
+import type { Cookie } from "../../entities/cookie";
+import type { Session } from "../../entities/session";
+import type { User } from "../../entities/user";
+
+export interface IAuthenticationService {
+  generateUserId(): string;
+  hashPassword(password: string): Promise<string>;
+  verifyPassword(hash: string, password: string): Promise<boolean>;
+  validateSession(
+    sessionId: string,
+  ): Promise<{ user: User; session: Session }>;
+  createSession(user: User): Promise<{ session: Session; cookie: Cookie }>;
+  invalidateSession(sessionId: string): Promise<{ blankCookie: Cookie }>;
+}