specs/001-auth-security/spec.md

# Feature Specification: Authentication & API Security

**Feature Branch**: `001-auth-security`
**Created**: 2026-01-09
**Status**: Draft
**Input**: User description: "Authentication & API Security – Phase II Todo Web App"

## User Scenarios & Testing *(mandatory)*

### User Story 1 - User Sign Up (Priority: P1)

A new user visits the application and creates an account to start managing their tasks. The system securely registers the user and establishes their identity for future sessions.

**Why this priority**: Without user registration, no one can use the application. This is the entry point for all users and must work reliably.

**Independent Test**: Can be fully tested by submitting registration form with valid credentials and verifying account creation. Delivers immediate value by allowing users to establish their identity in the system.

**Acceptance Scenarios**:

1. **Given** a new user on the sign-up page, **When** they provide valid email and password, **Then** their account is created and they receive confirmation
2. **Given** a user attempting to sign up, **When** they provide an email that already exists, **Then** they receive a clear error message indicating the email is already registered
3. **Given** a user on the sign-up page, **When** they provide invalid credentials (weak password, malformed email), **Then** they receive specific validation feedback before submission

---

### User Story 2 - User Sign In (Priority: P2)

A registered user returns to the application and signs in with their credentials. The system authenticates them and provides a secure token for accessing their personal data.

**Why this priority**: After registration, users need to authenticate to access their tasks. This enables returning users to access the application.

**Independent Test**: Can be fully tested by submitting valid credentials and verifying successful authentication with token issuance. Delivers value by allowing registered users to access their accounts.

**Acceptance Scenarios**:

1. **Given** a registered user on the sign-in page, **When** they provide correct email and password, **Then** they are authenticated and receive a secure token
2. **Given** a user attempting to sign in, **When** they provide incorrect credentials, **Then** they receive a generic error message without revealing which field was incorrect
3. **Given** an authenticated user, **When** their session token is issued, **Then** the token contains their user identity and has a defined expiration time

---

### User Story 3 - Protected API Access (Priority: P3)

An authenticated user makes requests to the API to manage their tasks. The system verifies their identity on every request and ensures they can only access their own data.

**Why this priority**: This enforces the security boundary that prevents users from accessing each other's data. Critical for data privacy and security.

**Independent Test**: Can be fully tested by making API requests with valid tokens and verifying that only the authenticated user's data is returned. Delivers value by ensuring data isolation between users.

**Acceptance Scenarios**:

1. **Given** an authenticated user with a valid token, **When** they request their tasks via the API, **Then** they receive only their own tasks
2. **Given** an authenticated user, **When** they attempt to access another user's task by ID, **Then** the request is denied with appropriate error
3. **Given** a user making an API request, **When** the token is included in the Authorization header, **Then** the backend extracts and verifies the token signature

---

### User Story 4 - Invalid Token Handling (Priority: P4)

A user attempts to access protected resources without a valid token (expired, malformed, or missing). The system rejects the request and returns a clear unauthorized response.

**Why this priority**: This prevents unauthorized access and provides clear feedback when authentication fails. Essential for security but lower priority than the happy path flows.

**Independent Test**: Can be fully tested by making API requests with invalid/missing tokens and verifying 401 responses. Delivers value by enforcing authentication requirements.

**Acceptance Scenarios**:

1. **Given** a user making an API request, **When** no token is provided, **Then** the system returns 401 Unauthorized
2. **Given** a user with an expired token, **When** they make an API request, **Then** the system returns 401 Unauthorized with indication that token is expired
3. **Given** a user with a malformed token, **When** they make an API request, **Then** the system returns 401 Unauthorized without exposing internal error details

---

### Edge Cases

- What happens when a user tries to sign up with an email that's already registered?
- How does the system handle concurrent sign-in attempts from the same user?
- What happens when the shared secret (BETTER_AUTH_SECRET) is missing or misconfigured?
- How does the system handle tokens that are syntactically valid but signed with the wrong secret?
- What happens when a user's token expires mid-session while they're actively using the application?
- How does the system handle extremely long passwords or email addresses?
- What happens when the backend receives a token with valid signature but for a user that no longer exists?

## Requirements *(mandatory)*

### Functional Requirements

- **FR-001**: System MUST allow new users to create accounts with email and password
- **FR-002**: System MUST validate email format and password strength during registration
- **FR-003**: System MUST prevent duplicate account creation with the same email address
- **FR-004**: System MUST authenticate users by verifying their email and password credentials
- **FR-005**: System MUST issue JWT tokens upon successful authentication
- **FR-006**: System MUST include user identity (user ID, email) in the JWT token payload
- **FR-007**: System MUST sign JWT tokens using the shared secret (BETTER_AUTH_SECRET)
- **FR-008**: System MUST set token expiration time to prevent indefinite access
- **FR-009**: System MUST require JWT token in Authorization header for all protected API endpoints
- **FR-010**: System MUST verify JWT signature on every protected API request
- **FR-011**: System MUST extract user identity from verified JWT tokens
- **FR-012**: System MUST filter all task queries by the authenticated user's ID
- **FR-013**: System MUST return 401 Unauthorized for requests without valid tokens
- **FR-014**: System MUST return 401 Unauthorized for expired tokens
- **FR-015**: System MUST return 401 Unauthorized for tokens with invalid signatures
- **FR-016**: System MUST prevent users from accessing or modifying other users' tasks
- **FR-017**: System MUST use the same BETTER_AUTH_SECRET value in both frontend and backend
- **FR-018**: System MUST store passwords securely using industry-standard hashing
- **FR-019**: System MUST not expose sensitive error details in authentication failure responses
- **FR-020**: System MUST maintain stateless authentication (no server-side session storage)

### Key Entities

- **User**: Represents a registered user account with email, hashed password, and unique identifier. Each user owns a collection of tasks and can only access their own data.
- **JWT Token**: A cryptographically signed token containing user identity claims (user ID, email, expiration time). Used to authenticate API requests without server-side session state.
- **Authentication Session**: The period during which a user's JWT token is valid, allowing them to make authenticated requests to the API.

## Success Criteria *(mandatory)*

### Measurable Outcomes

- **SC-001**: Users can complete account registration in under 1 minute with clear validation feedback
- **SC-002**: Users can sign in and receive authentication token in under 5 seconds
- **SC-003**: 100% of API requests without valid tokens receive 401 Unauthorized responses
- **SC-004**: 100% of authenticated users can only retrieve and modify their own tasks
- **SC-005**: System successfully verifies token signatures for 100% of valid tokens
- **SC-006**: Zero instances of users accessing other users' data in testing
- **SC-007**: Authentication flow handles 100 concurrent sign-in requests without errors
- **SC-008**: Token verification adds less than 50ms latency to API requests

## Assumptions *(optional)*

- Better Auth library is already configured in the frontend application
- Database schema includes a users table with email and password fields
- Frontend and backend share the same BETTER_AUTH_SECRET environment variable
- JWT tokens will use HS256 (HMAC with SHA-256) signing algorithm
- Access tokens will expire after 1 hour (industry standard for web applications)
- Password requirements: minimum 8 characters, at least one uppercase, one lowercase, one number
- Email validation follows RFC 5322 standard format
- The application uses HTTPS in production to protect tokens in transit
- Rate limiting for authentication endpoints will be handled separately (not in this spec)

## Dependencies *(optional)*

- Better Auth library must be installed and configured in the Next.js frontend
- Backend must have JWT library for token verification (e.g., PyJWT for Python)
- Database must have users table with appropriate schema
- Environment configuration must support BETTER_AUTH_SECRET in both frontend and backend
- Task CRUD API endpoints must be implemented (from Spec 001-task-crud)

## Out of Scope *(optional)*

- OAuth providers (Google, GitHub, etc.) and social login
- Password reset and forgot password functionality
- Email verification and account activation
- Multi-factor authentication (MFA)
- Token refresh mechanism and refresh tokens
- Remember me functionality
- Session management across multiple devices
- Account deletion and data export
- Role-based access control (RBAC) beyond basic user isolation
- Rate limiting and brute force protection
- Chatbot or AI-powered features
- UI/UX polish and advanced form interactions
- Password strength meter or complexity requirements beyond basic validation