Specing

# User Authentication System

## Abstract
Secure user authentication system with email/password login, session management, 
and password reset functionality. Supports 10,000+ concurrent users with 
enterprise security standards.

## Motivation

### Problem
Our application currently has no user authentication, blocking us from:
- Launching multi-user features
- Personalizing user experiences  
- Protecting sensitive user data
- Meeting enterprise security requirements

### Business Impact
- **Revenue**: Blocking $50K MRR from enterprise customers requiring auth
- **Growth**: Cannot launch social features that drive 40% user retention
- **Risk**: Regulatory compliance issues without proper user data protection

### Success Criteria
- Support 10,000 concurrent authenticated users
- 99.9% authentication service uptime
- Enterprise-grade security compliance
- <500ms login response time (p95)

## Scope

### In Scope
- Email/password registration and login
- Session management with secure tokens
- Password reset via email verification
- Account email verification
- Basic user profile management

### Out of Scope
- OAuth/SSO integration (future iteration)
- Multi-factor authentication (future iteration)  
- User roles and permissions (separate spec)
- Account deletion/GDPR compliance (separate spec)

### Dependencies
- Email service integration (SendGrid)
- Database schema updates (User and Session tables)
- Frontend authentication UI components

### ARCH-AUTH-1: Authentication Service Layer
**Pattern**: Clean Architecture with service separation
**Components**:
- AuthController (API layer)
- AuthService (business logic)
- UserRepository (data layer)
- SessionManager (state management)

**Dependencies**:
- Database connection pool
- Email service integration
- Redis for session storage

**Constraints**:
- Controllers MUST NOT contain business logic
- Services MUST be stateless
- Repositories MUST use prepared statements

### BEH-AUTH-1: User Registration Flow
**Trigger**: User submits registration form
**Normal Flow**:
1. Validate email format and uniqueness
2. Hash password with bcrypt
3. Create user record in database
4. Send verification email
5. Return success response

**Error Flows**:
- Invalid email → Show format error
- Duplicate email → Suggest login instead
- Email service down → Queue for retry, show success

**Success Criteria**:
- Registration completes within 2 seconds
- Verification email delivered within 30 seconds

### DATA-USER-1: User Account Model
```typescript
interface User {
  id: string;           // UUID v4 primary key
  email: string;        // RFC 5322 format, unique, lowercase
  password_hash: string; // bcrypt cost=12, never exposed
  email_verified: boolean; // default false
  created_at: Date;
  last_login: Date | null;
}

### Test Requirements (TEST-*)

Specify how to verify correctness and handle edge cases.

```markdown
### TEST-AUTH-1: Registration Endpoint Tests
**Unit Tests**:
- Valid registration → Returns 201 with user_id
- Duplicate email → Returns 409 with error message
- Invalid email format → Returns 400 with validation error
- Weak password → Returns 422 with strength requirements

**Integration Tests**:
- End-to-end registration flow with email verification
- Rate limiting enforcement (6th attempt returns 429)
- Database rollback on email service failure

**Performance Tests**:
- 1000 concurrent registrations complete within SLA
- Password hashing latency under 100ms (p95)

### BEH-AUTH-1: User Registration
Users can register with email and password

### BEH-AUTH-1: User Registration Flow
**Input Validation**:
- BEH-AUTH-1.1: Email MUST match RFC 5322 format
- BEH-AUTH-1.2: Email MUST be unique in system
- BEH-AUTH-1.3: Password MUST be 8+ characters with uppercase, lowercase, number, special char

**Processing Steps**:
- BEH-AUTH-1.4: Convert email to lowercase before storage
- BEH-AUTH-1.5: Hash password with bcrypt cost=12
- BEH-AUTH-1.6: Generate UUID v4 for user ID
- BEH-AUTH-1.7: Create user record with email_verified=false
- BEH-AUTH-1.8: Generate email verification token (UUID v4)
- BEH-AUTH-1.9: Queue verification email for delivery
- BEH-AUTH-1.10: Return 201 with user_id (do not wait for email)

**Error Handling**:
- BEH-AUTH-1.11: Invalid email format → 400 with specific field error
- BEH-AUTH-1.12: Duplicate email → 409 with suggestion to login
- BEH-AUTH-1.13: Weak password → 422 with strength requirements
- BEH-AUTH-1.14: Database error → 500 with request ID for debugging

// @spec BEH-AUTH-1.4: Convert email to lowercase before storage
const normalizedEmail = email.toLowerCase();

// @spec BEH-AUTH-1.5: Hash password with bcrypt cost=12  
const passwordHash = await bcrypt.hash(password, 12);

// @spec BEH-AUTH-1.6: Generate UUID v4 for user ID
const userId = uuidv4();

// @spec BEH-AUTH-1.7: Create user record with email_verified=false
const user = await User.create({
  id: userId,
  email: normalizedEmail,
  password_hash: passwordHash,
  email_verified: false
});