Authentication Flow:
1. User enters username and password
2. System validates credentials
3. If MFA enabled, send verification code
4. User enters code from authenticator app or SMS
5. System validates code and issues JWT token

MFA Methods:
- Authenticator apps (Google Authenticator, Authy)
- SMS verification codes
- Email verification codes
- Hardware security keys (YubiKey)
- Biometric authentication (Touch ID, Face ID)

Implementation:
- Store MFA secret encrypted in database
- Use TOTP (Time-based One-Time Password) algorithm
- 30-second time window for code validity
- Rate limit MFA attempts (5 attempts per 15 minutes)

Password Requirements:
- Minimum 8 characters
- At least one uppercase letter
- At least one lowercase letter
- At least one number
- At least one special character

Password Storage:
- Hash with bcrypt (cost factor: 12)
- Salt per password (random 16-byte salt)
- Never store plaintext passwords
- Rotate hashing algorithm periodically

Password Reset:
- Send reset link to verified email
- Token expires in 1 hour
- One-time use token
- Invalidate all sessions on password change

JWT Token Structure:
{
  "user_id": 123456789,
  "username": "johndoe",
  "verified": true,
  "iat": 1704672000,
  "exp": 1704675600,
  "jti": "unique_token_id"
}

Token Security:
- Sign with RS256 (asymmetric encryption)
- Short expiration (1 hour for access token)
- Refresh token (30 days, stored in database)
- Rotate refresh tokens on use
- Revoke tokens on logout or suspicious activity

Session Storage:
- Store active sessions in Redis
- Track device, IP, location, user agent
- Allow users to view and revoke sessions
- Automatic session expiration

User Roles:
- Regular User: Post tweets, follow users, like/retweet
- Verified User: Blue checkmark, higher rate limits
- Moderator: Content moderation, user suspension
- Admin: Full system access, user management

Permissions:
- tweet:create, tweet:delete, tweet:edit
- user:follow, user:block, user:mute
- admin:suspend_user, admin:delete_tweet
- moderator:review_content, moderator:flag_spam

Implementation:
- Store roles and permissions in database
- Check permissions on every API request
- Cache permissions in Redis for performance
- Audit log for all permission checks

Privacy Levels:
1. Public: Anyone can see and interact
2. Protected: Only approved followers can see
3. Private: Only mentioned users can see (DMs)

Implementation:
- Check privacy settings before serving tweet
- Filter timelines based on follow relationships
- Respect block and mute lists
- Handle protected accounts in search results

Database Encryption:
- AES-256 encryption for all data at rest
- Transparent Data Encryption (TDE) for databases
- Encrypted backups and snapshots
- Key rotation every 90 days

File Storage Encryption:
- S3 server-side encryption (SSE-S3)
- Encrypt media files before upload
- Encrypted object storage
- Separate encryption keys per region

Key Management:
- AWS KMS for key management
- Hardware Security Modules (HSM) for key storage
- Key hierarchy (master key → data keys)
- Audit logging for key usage

TLS Configuration:
- TLS 1.3 for all connections
- Strong cipher suites only
- Perfect Forward Secrecy (PFS)
- HSTS (HTTP Strict Transport Security)
- Certificate pinning for mobile apps

API Security:
- HTTPS only (no HTTP)
- TLS certificate from trusted CA
- Automatic certificate renewal
- Monitor certificate expiration

E2E Encryption Protocol:
- Signal Protocol for messaging
- Double Ratchet algorithm
- X3DH key agreement protocol
- Forward secrecy and post-compromise security

Key Management:
- Identity keys stored on device
- Pre-keys uploaded to server
- Session keys derived per conversation
- Key verification via safety numbers

Implementation:
- Encrypt message on sender device
- Server routes encrypted envelope
- Decrypt on recipient device
- Server cannot read message content

Data Subject Rights:
1. Right to Access:
   - Provide data export in JSON format
   - Include all tweets, likes, follows, DMs
   - Deliver within 30 days

2. Right to Deletion:
   - Delete account and all associated data
   - 30-day grace period before permanent deletion
   - Propagate deletion across all systems
   - Remove from backups within 90 days

3. Right to Rectification:
   - Allow users to update profile information
   - Correct inaccurate data
   - Update privacy settings

4. Right to Portability:
   - Export data in machine-readable format
   - Allow transfer to other platforms
   - Include all user-generated content

5. Right to Object:
   - Opt-out of data processing for marketing
   - Opt-out of personalized ads
   - Opt-out of data sharing with third parties

Implementation:
- Data export API endpoint
- Async deletion pipeline
- Audit logging for all data access
- Consent management system
- Privacy policy updates with user notification

California Consumer Privacy Act:
1. Right to Know:
   - Disclose data collection practices
   - Provide categories of data collected
   - Explain data usage purposes

2. Right to Delete:
   - Delete personal information on request
   - Exceptions for legal obligations

3. Right to Opt-Out:
   - Opt-out of data sale to third parties
   - "Do Not Sell My Personal Information" link

4. Non-Discrimination:
   - Cannot discriminate for exercising rights
   - Same service quality for all users

Implementation:
- Privacy dashboard for users
- Opt-out mechanism for data sale
- Transparent data collection disclosure
- Regular privacy audits

Retention Periods:
- Active tweets: Indefinite (until user deletes)
- Deleted tweets: 30-day soft delete, then permanent
- User accounts: 30-day grace period after deletion
- Access logs: 90 days
- Audit logs: 7 years (compliance)
- Backup data: 90 days

Automated Cleanup:
- Cron jobs for data deletion
- Cascade deletes across all systems
- Verify deletion completion
- Audit trail for deletions

ML-Based Detection:
1. Spam Detection:
   - Train on labeled spam data
   - Features: content similarity, posting frequency
   - Real-time scoring (0-1 scale)
   - Auto-hide if score >0.9

2. Hate Speech Detection:
   - NLP models for toxic content
   - Multi-language support
   - Context-aware detection
   - Human review for borderline cases

3. NSFW Content Detection:
   - Image classification models
   - Video frame analysis
   - Audio content analysis
   - Blur sensitive media by default

4. Misinformation Detection:
   - Fact-checking partnerships
   - Source credibility scoring
   - Add context labels to disputed content
   - Reduce distribution of false information

Implementation:
- Real-time content scanning on upload
- Async processing for heavy ML models
- Queue flagged content for human review
- Feedback loop to improve models

Moderation Workflow:
1. User reports content
2. Automated triage (ML scoring)
3. Queue for human review
4. Moderator reviews and decides
5. Action taken (remove, warn, suspend)
6. User notified of decision
7. Appeal process available

Moderator Tools:
- Content review dashboard
- Context information (user history, reports)
- Action buttons (remove, warn, suspend)
- Escalation to senior moderators
- Audit trail for all actions

Moderation Policies:
- Violence and threats
- Hate speech and harassment
- Sexual content and nudity
- Spam and manipulation
- Misinformation and fake news
- Copyright and trademark violations

Blocking and Muting:
- Block: Prevent user from seeing your content
- Mute: Hide user's content from your timeline
- Mute keywords: Filter tweets with specific words
- Mute conversations: Stop notifications for thread

Reporting:
- Report tweets, users, DMs
- Categories: spam, abuse, harassment, etc.
- Attach screenshots and context
- Track report status
- Feedback on action taken

Safety Mode:
- Auto-block accounts sending harmful content
- Temporary blocks (7 days)
- ML-based detection of harmful behavior
- User can review and unblock

Rate Limit Tiers:
Free Tier:
- 300 tweets per 3 hours
- 180 timeline requests per 15 minutes
- 45 search requests per 15 minutes

Premium Tier:
- 1000 tweets per 3 hours
- 900 timeline requests per 15 minutes
- 300 search requests per 15 minutes

Implementation:
- Token bucket algorithm
- Redis for distributed rate limiting
- Per-user and per-IP limits
- Graceful degradation (return 429)

OAuth 2.0 Flow:
1. App requests authorization
2. User grants permission
3. App receives authorization code
4. App exchanges code for access token
5. App uses token for API requests

Token Types:
- Access Token: Short-lived (1 hour)
- Refresh Token: Long-lived (30 days)
- API Key: For server-to-server

Scopes:
- read: Read tweets and user data
- write: Post tweets, follow users
- dm: Access direct messages
- admin: Full account access

Validation Rules:
- Tweet text: Max 280 characters, UTF-8 encoded
- Username: 4-15 characters, alphanumeric + underscore
- Email: Valid email format, max 255 characters
- URL: Valid URL format, HTTPS only
- Media: Max 5MB images, 512MB videos

Sanitization:
- Escape HTML special characters
- Remove SQL injection attempts
- Strip malicious scripts
- Validate file types and content
- Check for malware in uploads

Implementation:
- Server-side validation (never trust client)
- Whitelist allowed characters
- Reject invalid input with clear error
- Log validation failures for monitoring

Monitoring Metrics:
- Failed login attempts
- Unusual API usage patterns
- Suspicious account activity
- Data access patterns
- Permission changes
- Configuration changes

Alerting:
- Real-time alerts for critical events
- Anomaly detection with ML
- Threshold-based alerts
- Integration with PagerDuty
- Escalation policies

Audit Logging:
- Log all authentication attempts
- Log all data access
- Log all permission changes
- Log all admin actions
- Retain logs for 7 years

Incident Response Plan:
1. Detection: Automated monitoring and alerts
2. Triage: Assess severity and impact
3. Containment: Isolate affected systems
4. Investigation: Root cause analysis
5. Remediation: Fix vulnerability
6. Recovery: Restore normal operations
7. Post-Mortem: Document lessons learned

Security Incident Types:
- Data breach
- Account compromise
- DDoS attack
- Malware infection
- Insider threat
- API abuse

Response Team:
- Security engineers
- DevOps engineers
- Legal team
- PR/Communications
- Executive leadership

Security Practices:
- Regular security audits
- Penetration testing (quarterly)
- Bug bounty program
- Dependency scanning
- Code security reviews
- Security training for engineers

Vulnerability Disclosure:
- Responsible disclosure policy
- Bug bounty rewards ($100-$10,000)
- 90-day disclosure timeline
- Coordinated disclosure with researchers
- Public security advisories