# Authentication - v0.1.0

**Version**: 0.1.0
**Date**: 2025-12-22
**Status**: Initial Design

## Overview

This document defines the authentication and authorization flows for Sneaky Klaus. The system supports two types of users with different authentication mechanisms:

1. **Admin**: Single administrator account with password-based authentication
2. **Participants**: Passwordless authentication via magic links

Session management is handled by Flask-Session, which stores session data server-side in SQLite.

## Authentication Flows

### Initial Admin Setup Flow

The first-run experience for new Sneaky Klaus installations.

#### Sequence Diagram

```mermaid
sequenceDiagram
    participant User
    participant Browser
    participant App as Flask App
    participant DB as SQLite Database
    participant Session as Flask-Session

    User->>Browser: Navigate to application
    Browser->>App: GET /
    App->>DB: Check if admin exists
    DB-->>App: No admin found
    App->>Browser: Redirect to /setup
    Browser->>App: GET /setup
    App-->>Browser: Render setup form

    User->>Browser: Fill form (email, password)
    Browser->>App: POST /setup
    App->>App: Validate form (Flask-WTF)
    App->>App: Hash password (bcrypt)
    App->>DB: INSERT INTO admin
    DB-->>App: Admin created (id=1)
    App->>Session: Create admin session
    Session-->>App: Session ID
    App->>Browser: Set session cookie
    App->>Browser: Flash success message
    App->>Browser: Redirect to /admin/dashboard
    Browser->>App: GET /admin/dashboard
    App->>Session: Validate session
    Session-->>App: Admin authenticated
    App-->>Browser: Render dashboard

    Note over Browser,App: Success message auto-dismisses after 5 seconds
```

#### Implementation Details

**First-Run Detection**:
- Check `SELECT COUNT(*) FROM admin` on application startup
- If count == 0, set `app.config['REQUIRES_SETUP'] = True`
- Before request handler checks flag and redirects to `/setup` if True

**Route**: `/setup`

**Methods**: GET, POST

**Authorization**:
- Accessible only when no admin exists
- Returns 404 if admin already exists

**Form Fields** (Flask-WTF):
- `email`: EmailField, required, validated with email validator
- `password`: PasswordField, required, min length 12 characters
- `password_confirm`: PasswordField, required, must match password
- CSRF token (automatic via Flask-WTF)

**POST Workflow**:
1. Validate form with Flask-WTF
2. Double-check no admin exists (prevent race condition)
3. Hash password with bcrypt (cost factor 12)
4. Insert admin record
5. Create session (Flask-Session)
6. Set session cookie (HttpOnly, Secure, SameSite=Lax)
7. Flash success message: "Admin account created successfully!"
8. Redirect to `/admin/dashboard`

**Auto-login**:
- Session created immediately after admin creation
- No separate login step required
- User automatically authenticated

---

### Admin Login Flow

Standard password-based authentication for the admin user.

#### Sequence Diagram

```mermaid
sequenceDiagram
    participant User
    participant Browser
    participant App as Flask App
    participant RateLimit as Rate Limiter
    participant DB as SQLite Database
    participant Session as Flask-Session

    User->>Browser: Navigate to /admin/login
    Browser->>App: GET /admin/login
    App-->>Browser: Render login form

    User->>Browser: Fill form (email, password)
    Browser->>App: POST /admin/login
    App->>App: Validate form (Flask-WTF)

    App->>RateLimit: Check rate limit for email
    alt Rate limit exceeded
        RateLimit-->>App: Too many attempts
        App->>Browser: Flash error message
        App->>Browser: Render login form (429 status)
        Note over Browser: Error message persists until dismissed
    else Within rate limit
        RateLimit-->>App: Allowed
        App->>DB: SELECT * FROM admin WHERE email = ?
        DB-->>App: Admin record

        alt Invalid credentials
            App->>App: Verify password (bcrypt)
            App->>RateLimit: Increment failure count
            App->>Browser: Flash error message
            App->>Browser: Render login form
            Note over Browser: Error message persists until dismissed
        else Valid credentials
            App->>App: Verify password (bcrypt)
            App->>RateLimit: Reset rate limit counter
            App->>Session: Create admin session
            Session-->>App: Session ID
            App->>Browser: Set session cookie
            App->>Browser: Flash success message
            App->>Browser: Redirect to /admin/dashboard
            Browser->>App: GET /admin/dashboard
            App->>Session: Validate session
            Session-->>App: Admin authenticated
            App-->>Browser: Render dashboard
            Note over Browser: Success message auto-dismisses after 5 seconds
        end
    end
```

#### Implementation Details

**Route**: `/admin/login`

**Methods**: GET, POST

**Authorization**:
- Accessible to unauthenticated users only
- Redirects to dashboard if already authenticated

**Form Fields** (Flask-WTF):
- `email`: EmailField, required
- `password`: PasswordField, required
- `remember_me`: BooleanField, optional (extends session duration)
- CSRF token (automatic via Flask-WTF)

**Rate Limiting**:
- **Policy**: 5 attempts per 15 minutes per email
- **Key**: `login:admin:{email_lowercase}`
- **Implementation**: Check `rate_limit` table before authentication
- **Failure Handling**: Increment attempt counter on failed login
- **Success Handling**: Reset counter on successful login
- **Lockout Message**: "Too many login attempts. Please try again in {minutes} minutes."

**POST Workflow**:
1. Validate form with Flask-WTF
2. Normalize email to lowercase
3. Check rate limit for `login:admin:{email}`
4. If rate limited: flash error, return 429
5. Query admin by email
6. If admin not found or password invalid:
   - Increment rate limit counter
   - Flash error: "Invalid email or password"
   - Re-render form
7. If credentials valid:
   - Reset rate limit counter
   - Create session via Flask-Session
   - Set session expiration based on `remember_me`:
     - Checked: 30 days
     - Unchecked: 7 days (default)
   - Set session cookie
   - Flash success: "Welcome back!"
   - Redirect to `/admin/dashboard`

**Session Cookie Configuration**:
- `HttpOnly`: True (prevent JavaScript access)
- `Secure`: True (HTTPS only in production)
- `SameSite`: Lax (CSRF protection)
- `Max-Age`: Based on remember_me

---

### Admin Logout Flow

Terminates the admin session.

#### Sequence Diagram

```mermaid
sequenceDiagram
    participant User
    participant Browser
    participant App as Flask App
    participant Session as Flask-Session

    User->>Browser: Click logout
    Browser->>App: POST /admin/logout
    App->>Session: Get current session ID
    App->>Session: Delete session
    Session-->>App: Session deleted
    App->>Browser: Clear session cookie
    App->>Browser: Flash success message
    App->>Browser: Redirect to /
    Note over Browser: Success message auto-dismisses after 5 seconds
```

#### Implementation Details

**Route**: `/admin/logout`

**Methods**: POST (GET redirects to dashboard)

**Authorization**: Requires active admin session

**POST Workflow**:
1. Validate CSRF token
2. Delete session from Flask-Session store
3. Clear session cookie
4. Flash success: "You have been logged out"
5. Redirect to `/`

---

### Participant Magic Link Authentication Flow

Passwordless authentication for participants using time-limited magic links.

#### Sequence Diagram

```mermaid
sequenceDiagram
    participant Participant
    participant Browser
    participant App as Flask App
    participant RateLimit as Rate Limiter
    participant DB as SQLite Database
    participant Email as Resend
    participant Session as Flask-Session

    Note over Participant,Browser: Participant requests access
    Participant->>Browser: Navigate to /exchange/{slug}
    Browser->>App: GET /exchange/{slug}
    App->>DB: SELECT * FROM exchange WHERE slug = ?
    DB-->>App: Exchange record
    App-->>Browser: Render participant login form

    Participant->>Browser: Enter email
    Browser->>App: POST /exchange/{slug}/auth
    App->>App: Validate form (Flask-WTF)
    App->>RateLimit: Check rate limit for email

    alt Rate limit exceeded
        RateLimit-->>App: Too many requests
        App->>Browser: Flash error message
        App->>Browser: Render form (429 status)
        Note over Browser: Error message persists until dismissed
    else Within rate limit
        RateLimit-->>App: Allowed
        App->>DB: SELECT * FROM participant WHERE email = ? AND exchange_id = ?

        alt Participant not found
            App->>Browser: Flash error message
            App->>Browser: Render form
            Note over Browser: Generic error for security
        else Participant found
            DB-->>App: Participant record
            App->>App: Generate magic token (32 bytes, secrets module)
            App->>App: Hash token (SHA-256)
            App->>DB: INSERT INTO magic_token
            App->>Email: Send magic link email
            Email-->>Participant: Email with magic link
            App->>RateLimit: Increment request counter
            App->>Browser: Flash success message
            App->>Browser: Redirect to /exchange/{slug}/auth/sent
            Note over Browser: Success message auto-dismisses after 5 seconds
        end
    end

    Note over Participant,Email: Participant clicks magic link
    Participant->>Email: Click link
    Email->>Browser: Open /exchange/{slug}/auth/verify?token={token}
    Browser->>App: GET /exchange/{slug}/auth/verify?token={token}
    App->>App: Hash token
    App->>DB: SELECT * FROM magic_token WHERE token_hash = ?

    alt Token invalid/expired/used
        DB-->>App: No token found / Token expired
        App->>Browser: Flash error message
        App->>Browser: Redirect to /exchange/{slug}
        Note over Browser: Error message persists until dismissed
    else Token valid
        DB-->>App: Token record
        App->>DB: UPDATE magic_token SET used_at = NOW()
        App->>DB: SELECT * FROM participant WHERE id = ?
        DB-->>App: Participant record
        App->>Session: Create participant session
        Session-->>App: Session ID
        App->>Browser: Set session cookie
        App->>Browser: Flash success message
        App->>Browser: Redirect to /exchange/{slug}/dashboard
        Browser->>App: GET /exchange/{slug}/dashboard
        App->>Session: Validate session
        Session-->>App: Participant authenticated
        App-->>Browser: Render participant dashboard
        Note over Browser: Success message auto-dismisses after 5 seconds
    end
```

#### Implementation Details

**Route**: `/exchange/{slug}/auth`

**Methods**: GET, POST

**Authorization**: Public (unauthenticated)

**Form Fields** (Flask-WTF):
- `email`: EmailField, required
- CSRF token (automatic via Flask-WTF)

**Rate Limiting**:
- **Policy**: 3 requests per hour per email
- **Key**: `magic_link:{email_lowercase}`
- **Lockout Message**: "Too many magic link requests. Please try again in {minutes} minutes."

**POST Workflow** (Request Magic Link):
1. Validate form with Flask-WTF
2. Normalize email to lowercase
3. Check rate limit for `magic_link:{email}`
4. If rate limited: flash error, return 429
5. Query participant by email and exchange_id
6. If participant not found:
   - Flash generic error: "If this email is registered, you will receive a magic link."
   - Return success response (prevent email enumeration)
   - Do NOT send email
7. If participant found but withdrawn:
   - Same as not found (prevent information disclosure)
8. If participant found and active:
   - Generate token: 32 bytes from `secrets.token_urlsafe()`
   - Hash token: SHA-256
   - Store hash in `magic_token` table with:
     - `token_type`: 'magic_link'
     - `email`: participant email
     - `participant_id`: participant ID
     - `exchange_id`: exchange ID
     - `expires_at`: NOW() + 1 hour
   - Send email with magic link
   - Increment rate limit counter
   - Flash success: "Check your email for a magic link!"
   - Redirect to `/exchange/{slug}/auth/sent`

**Route**: `/exchange/{slug}/auth/verify`

**Methods**: GET

**Query Parameters**:
- `token`: Magic token (URL-safe base64)

**GET Workflow** (Verify Token):
1. Extract token from query string
2. Hash token with SHA-256
3. Query `magic_token` table by hash
4. Validate token:
   - Exists in database
   - `expires_at` > NOW()
   - `used_at` IS NULL
   - `token_type` = 'magic_link'
5. If invalid:
   - Flash error: "This magic link is invalid or has expired."
   - Redirect to `/exchange/{slug}`
6. If valid:
   - Mark token as used: `UPDATE magic_token SET used_at = NOW()`
   - Load participant record
   - Create session via Flask-Session with:
     - `user_id`: participant.id
     - `user_type`: 'participant'
     - `exchange_id`: exchange.id
   - Set session cookie (7-day expiration)
   - Flash success: "Welcome, {participant.name}!"
   - Redirect to `/exchange/{slug}/dashboard`

**Token Generation**:
```python
import secrets
import hashlib

# Generate token
token = secrets.token_urlsafe(32)  # 32 bytes = 43 URL-safe characters

# Hash for storage
token_hash = hashlib.sha256(token.encode()).hexdigest()

# Magic link URL
magic_link = f"{base_url}/exchange/{slug}/auth/verify?token={token}"
```

---

### Participant Logout Flow

Terminates the participant session.

#### Sequence Diagram

```mermaid
sequenceDiagram
    participant Participant
    participant Browser
    participant App as Flask App
    participant Session as Flask-Session

    Participant->>Browser: Click logout
    Browser->>App: POST /exchange/{slug}/logout
    App->>Session: Get current session ID
    App->>Session: Delete session
    Session-->>App: Session deleted
    App->>Browser: Clear session cookie
    App->>Browser: Flash success message
    App->>Browser: Redirect to /exchange/{slug}
    Note over Browser: Success message auto-dismisses after 5 seconds
```

#### Implementation Details

**Route**: `/exchange/{slug}/logout`

**Methods**: POST

**Authorization**: Requires active participant session for this exchange

**POST Workflow**:
1. Validate CSRF token
2. Verify session belongs to participant for this exchange
3. Delete session from Flask-Session store
4. Clear session cookie
5. Flash success: "You have been logged out"
6. Redirect to `/exchange/{slug}`

---

## Session Management

### Flask-Session Configuration

**Backend**: SQLAlchemy (SQLite)

**Table**: `sessions` (created and managed by Flask-Session)

**Configuration**:
```python
app.config['SESSION_TYPE'] = 'sqlalchemy'
app.config['SESSION_SQLALCHEMY'] = db  # SQLAlchemy instance
app.config['SESSION_PERMANENT'] = True
app.config['SESSION_USE_SIGNER'] = True  # Sign session cookies
app.config['SESSION_KEY_PREFIX'] = 'sk:'  # Prefix for session keys
app.config['PERMANENT_SESSION_LIFETIME'] = timedelta(days=7)  # Default
```

### Session Data Structure

**Admin Session**:
```python
{
    'user_id': 1,  # Admin ID
    'user_type': 'admin',
    '_fresh': True,  # For Flask-Login compatibility (future)
    '_permanent': True
}
```

**Participant Session**:
```python
{
    'user_id': 123,  # Participant ID
    'user_type': 'participant',
    'exchange_id': 456,  # Exchange participant is authenticated for
    '_fresh': True,
    '_permanent': True
}
```

### Session Validation

**Before Request Handler**:
```python
@app.before_request
def load_session_user():
    if 'user_id' in session and 'user_type' in session:
        if session['user_type'] == 'admin':
            g.admin = Admin.query.get(session['user_id'])
        elif session['user_type'] == 'participant':
            g.participant = Participant.query.get(session['user_id'])
            g.exchange_id = session.get('exchange_id')
```

### Session Expiration

**Default Expiration**: 7 days from last activity

**Remember Me** (Admin only): 30 days from last activity

**Sliding Window**: Flask-Session automatically updates `last_activity` on each request

**Cleanup**: Flask-Session handles expired session cleanup automatically

---

## Flash Messages

Flash messages provide user feedback for authentication actions.

### Flash Message Types

**Success Messages**:
- Category: `'success'`
- Auto-dismiss: 5 seconds
- Examples:
  - "Admin account created successfully!"
  - "Welcome back!"
  - "You have been logged out"
  - "Check your email for a magic link!"
  - "Welcome, {name}!"

**Error Messages**:
- Category: `'error'`
- Auto-dismiss: Manual (user must dismiss)
- Examples:
  - "Invalid email or password"
  - "Too many login attempts. Please try again in {minutes} minutes."
  - "This magic link is invalid or has expired."

### Implementation

**Backend** (Flask):
```python
from flask import flash

# Success message
flash("Admin account created successfully!", "success")

# Error message
flash("Invalid email or password", "error")
```

**Frontend** (Jinja2 + JavaScript):
```html
{% with messages = get_flashed_messages(with_categories=true) %}
  {% if messages %}
    {% for category, message in messages %}
      <div class="flash-message flash-{{ category }}"
           {% if category == 'success' %}data-auto-dismiss="5000"{% endif %}>
        {{ message }}
        <button class="flash-dismiss" aria-label="Dismiss">&times;</button>
      </div>
    {% endfor %}
  {% endif %}
{% endwith %}

<script>
  // Auto-dismiss success messages after 5 seconds
  document.querySelectorAll('[data-auto-dismiss]').forEach(el => {
    const delay = parseInt(el.dataset.autoDismiss);
    setTimeout(() => {
      el.style.opacity = '0';
      setTimeout(() => el.remove(), 300);
    }, delay);
  });

  // Manual dismiss for all messages
  document.querySelectorAll('.flash-dismiss').forEach(btn => {
    btn.addEventListener('click', () => {
      const message = btn.parentElement;
      message.style.opacity = '0';
      setTimeout(() => message.remove(), 300);
    });
  });
</script>
```

---

## Authorization Patterns

### Route Protection Decorators

**Admin-Only Routes**:
```python
from functools import wraps
from flask import session, redirect, url_for, flash, g

def admin_required(f):
    @wraps(f)
    def decorated_function(*args, **kwargs):
        if 'user_id' not in session or session.get('user_type') != 'admin':
            flash("You must be logged in as admin to access this page.", "error")
            return redirect(url_for('admin_login'))
        return f(*args, **kwargs)
    return decorated_function

@app.route('/admin/dashboard')
@admin_required
def admin_dashboard():
    return render_template('admin/dashboard.html')
```

**Participant-Only Routes**:
```python
def participant_required(f):
    @wraps(f)
    def decorated_function(slug, *args, **kwargs):
        if 'user_id' not in session or session.get('user_type') != 'participant':
            flash("You must be logged in to access this page.", "error")
            return redirect(url_for('exchange_auth', slug=slug))

        # Verify participant is authenticated for this exchange
        exchange = Exchange.query.filter_by(slug=slug).first_or_404()
        if session.get('exchange_id') != exchange.id:
            flash("You are not authorized to access this exchange.", "error")
            return redirect(url_for('exchange_auth', slug=slug))

        return f(slug, *args, **kwargs)
    return decorated_function

@app.route('/exchange/<slug>/dashboard')
@participant_required
def participant_dashboard(slug):
    return render_template('participant/dashboard.html')
```

### Setup Requirement Check

**Before Request Handler**:
```python
@app.before_request
def check_setup_required():
    # Skip check for setup route and static files
    if request.endpoint in ['setup', 'static']:
        return

    # Check if admin exists
    if app.config.get('REQUIRES_SETUP'):
        admin_count = Admin.query.count()
        if admin_count == 0:
            return redirect(url_for('setup'))
        else:
            app.config['REQUIRES_SETUP'] = False
```

---

## Security Considerations

### Password Security

**Hashing Algorithm**: bcrypt with cost factor 12

**Minimum Password Length**: 12 characters

**Password Validation**:
- Enforced at form level (Flask-WTF)
- No complexity requirements (length is primary security measure)

**Implementation**:
```python
from flask_bcrypt import Bcrypt

bcrypt = Bcrypt(app)

# Hash password
password_hash = bcrypt.generate_password_hash(password).decode('utf-8')

# Verify password
bcrypt.check_password_hash(admin.password_hash, password)
```

### CSRF Protection

**Implementation**: Flask-WTF automatic CSRF protection

**All Forms**: Include CSRF token automatically via `{{ form.csrf_token }}`

**All POST Routes**: Validate CSRF token automatically via Flask-WTF

**Exempt Routes**: None (all state-changing operations require CSRF token)

### Rate Limiting

**Storage**: SQLite `rate_limit` table

**Policies**:
- Admin login: 5 attempts / 15 minutes / email
- Magic link request: 3 requests / hour / email

**Bypass**: None (applies to all users including admin)

**Cleanup**: Expired entries purged daily via background job

### Session Security

**Cookie Settings**:
- `HttpOnly`: True (prevent XSS)
- `Secure`: True in production (HTTPS only)
- `SameSite`: Lax (CSRF protection)

**Session Signing**: Enabled via `SESSION_USE_SIGNER`

**Secret Key**: Loaded from environment variable `SECRET_KEY`

### Magic Link Security

**Token Generation**: `secrets.token_urlsafe(32)` (cryptographically secure)

**Token Storage**: SHA-256 hash only (original token never stored)

**Token Expiration**: 1 hour from creation

**Single Use**: Marked as used immediately upon verification

**Email Enumeration Prevention**: Generic success message for all email submissions

### Timing Attack Prevention

**Password Verification**: bcrypt naturally resistant to timing attacks

**Magic Link Lookup**: Use constant-time comparison for token hashes (handled by database query)

---

## Error Handling

### Authentication Errors

**Invalid Credentials**:
- HTTP Status: 200 (re-render form)
- Flash Message: "Invalid email or password"
- Rate Limit: Increment counter

**Rate Limit Exceeded**:
- HTTP Status: 429 Too Many Requests
- Flash Message: "Too many {action} attempts. Please try again in {minutes} minutes."
- Behavior: Re-render form with error

**Expired Magic Link**:
- HTTP Status: 200 (redirect to login)
- Flash Message: "This magic link is invalid or has expired."
- Behavior: Redirect to exchange login page

**CSRF Validation Failure**:
- HTTP Status: 400 Bad Request
- Flash Message: "Security validation failed. Please try again."
- Behavior: Redirect to form

### Session Errors

**Session Expired**:
- HTTP Status: 302 (redirect to login)
- Flash Message: "Your session has expired. Please log in again."
- Behavior: Redirect to appropriate login page

**Invalid Session**:
- HTTP Status: 302 (redirect to login)
- Flash Message: "You must be logged in to access this page."
- Behavior: Redirect to appropriate login page

---

## Testing Considerations

### Unit Tests

**Setup Flow**:
- Test admin creation with valid data
- Test duplicate admin prevention
- Test auto-login after setup
- Test setup route 404 when admin exists

**Admin Login**:
- Test successful login with valid credentials
- Test failed login with invalid credentials
- Test rate limiting after 5 failed attempts
- Test rate limit reset after successful login
- Test remember_me session duration

**Magic Link**:
- Test magic link generation for valid participant
- Test rate limiting after 3 requests
- Test token verification with valid token
- Test token expiration after 1 hour
- Test single-use token enforcement

### Integration Tests

**Full Authentication Flows**:
- Test setup → login → logout flow
- Test magic link request → email → verify → dashboard flow
- Test concurrent session handling
- Test session persistence across requests

### Security Tests

**CSRF Protection**:
- Test all POST routes require valid CSRF token
- Test CSRF token validation

**Rate Limiting**:
- Test rate limits are enforced correctly
- Test rate limit window expiration

**Session Security**:
- Test session cookie settings (HttpOnly, Secure, SameSite)
- Test session expiration

---

## Future Enhancements

Potential improvements for future versions:

1. **Two-Factor Authentication**: TOTP for admin login
2. **Password Reset**: Admin password reset via email
3. **Session Management UI**: Admin view of active sessions
4. **Account Lockout**: Temporary lockout after repeated failed logins
5. **Audit Logging**: Track all authentication events
6. **Multiple Admins**: Support for multiple admin accounts with roles
7. **OAuth Integration**: Social login options for participants

These enhancements are out of scope for v0.1.0.

---

## References

- [Flask-Session Documentation](https://flask-session.readthedocs.io/)
- [Flask-WTF Documentation](https://flask-wtf.readthedocs.io/)
- [bcrypt Documentation](https://github.com/pyca/bcrypt/)
- [OWASP Authentication Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Authentication_Cheat_Sheet.html)
- [ADR-0001: Core Technology Stack](../../decisions/0001-core-technology-stack.md)
- [ADR-0002: Authentication Strategy](../../decisions/0002-authentication-strategy.md)
- [Data Model v0.1.0](../data-model.md)