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

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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

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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

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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

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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