Token Cache Architecture

Overview

The Teams for Linux token cache system provides persistent authentication by implementing the missing _tokenCache interface that Microsoft Teams' authentication provider expects. This eliminates frequent re-authentication prompts and enables silent token refresh capabilities.

Problem Statement

Microsoft Teams web app uses an authentication provider that expects a token cache interface for silent token refresh. In browser environments, this works seamlessly, but Electron applications lack this interface by default. Without it, users experience:

• Authentication prompts every ~24 hours
• Re-authentication required after system sleep/wake cycles
• Authentication lost after application restarts
• Inability to use refresh tokens for silent authentication

Solution Architecture

Phase 1: Core Token Cache Bridge (Completed)

The initial implementation provides a localStorage-compatible interface that satisfies Teams' authentication provider requirements.

Phase 2: Secure Storage Integration (Completed)

Enhanced implementation adds OS-level encryption using Electron's safeStorage API while maintaining backward compatibility.

Implementation Details

Core Components

1. TeamsTokenCache Class

Located in app/browser/tools/tokenCache.js, this singleton class provides:

• localStorage Interface Compatibility: Full implementation of getItem, setItem, removeItem, clear, length, and key(index)
• Secure Storage Integration: Primary storage using Electron safeStorage with encryption
• Graceful Fallback: Automatic fallback to localStorage if secure storage unavailable
• Memory Fallback: Emergency fallback to in-memory storage for quota issues

2. Authentication Provider Injection

The token cache is injected into Teams' authentication provider via reactHandler.js:

// Inject token cache into Teams authentication provider
if (authProvider && !authProvider._tokenCache) {
    authProvider._tokenCache = tokenCache;
    console.log('[AUTH] Token cache injected successfully');
}

3. Cross-Platform Secure Storage

The implementation uses Electron's safeStorage API which automatically handles platform-specific encryption backends (macOS Keychain, Windows DPAPI, Linux Secret Service). The application only needs to check safeStorage.isEncryptionAvailable() - Electron abstracts all OS-specific details.

Security Considerations

1. Encryption at Rest

• Secure Storage: All tokens encrypted using OS-level cryptographic APIs
• Key Management: Encryption keys managed by the operating system
• Access Control: Only the application can decrypt stored tokens

2. Fallback Security

• localStorage Fallback: Plain text storage when secure storage unavailable
• Memory Fallback: Temporary storage for quota exceeded scenarios
• PII Protection: All logging sanitizes personally identifiable information

3. Natural Transition

• Gradual Security: New tokens automatically use secure storage
• Existing Compatibility: Old tokens continue working via fallback
• Token Refresh: Natural security improvement as tokens refresh

Token Storage Format

Secure Storage

Tokens are stored in localStorage with a secure prefix and encrypted content:

Key: secure_teams_<original_key>
Value: <base64_encoded_encrypted_data>

Token Types Supported

The system recognizes and handles multiple token formats:

• Teams Auth v1: tmp.auth.v1.* patterns
• Refresh Tokens: UUID-based refresh token patterns
• ID Tokens: UUID-based ID token patterns
• MSAL Tokens: msal.* patterns
• Custom Tokens: Any tokens matching authentication patterns

Configuration

Automatic Configuration

The token cache system requires no user configuration and works automatically:

1. Availability Detection: Automatically detects secure storage availability
2. Storage Selection: Chooses secure storage for new tokens when available
3. Fallback Selection: Selects appropriate storage backend based on availability

Runtime Information

Diagnostic information available via getStorageInfo():

{
  localStorage: true,
  memoryFallback: false,
  secureStorage: true,
  platform: "darwin",
  secureBackend: "electron-safeStorage"
}

Error Handling

Storage Errors

• Quota Exceeded: Automatic switch to memory fallback
• Security Errors: Graceful degradation to localStorage
• Encryption Errors: Fallback to unencrypted storage

Storage Fallback

• Graceful Degradation: Falls back to localStorage when secure storage unavailable
• Complete Failure: Leaves tokens in localStorage
• Recovery: No data loss in any failure scenario

Performance Considerations

Optimization Strategies

1. Lazy Initialization: Secure storage initialized only when needed
2. No Migration: Zero overhead natural transition
3. Efficient Fallback: Fast detection and switching between backends
4. Minimal Logging: Reduced logging overhead compared to complex systems

Benchmarks

Operation	Secure Storage	localStorage	Memory
getItem	~2ms	~1ms	~0.1ms
setItem	~3ms	~1ms	~0.1ms

Troubleshooting

Common Issues

1. Secure Storage Unavailable

Symptoms: Logs show "Secure storage not available" Cause: Secure storage unavailable (check safeStorage.isEncryptionAvailable()) Resolution: Automatic fallback to localStorage, no user action needed

2. Storage Availability

Symptoms: Some tokens not migrated to secure storage Cause: Individual token encryption failures Resolution: Failed tokens remain in localStorage, authentication still works

3. Memory Fallback Active

Symptoms: Logs show "Switching to memory fallback" Cause: localStorage quota exceeded Resolution: Tokens stored in memory for current session, consider clearing browser data

Debug Information

Token cache uses console logging with [TOKEN_CACHE] prefixes. Enable detailed logging to see debug output:

ELECTRON_DEBUG_LOGGING=true npm start

Log Patterns

[TOKEN_CACHE] TokenCache initialized { localStorage: true, secureStorage: true }
[TOKEN_CACHE] Secure storage available
[TOKEN_CACHE] Secure storage available and ready

Future Enhancements

Planned Improvements

• Simplified Architecture: No migration complexity to maintain
• Performance Optimization: Further reduce encryption/decryption overhead
• Advanced Diagnostics: Enhanced monitoring for enterprise environments

Not Planned

• Custom MSAL Integration: Would require developer app registration
• Token Value Modification: Only provides cache interface, doesn't alter tokens
• Real-time Token Monitoring: Focused on cache interface, not token lifecycle

API Reference

Public Methods

getItem(key: string): Promise<string | null>

Retrieve token from cache with secure storage priority.

setItem(key: string, value: string): Promise<void>

Store token in cache with encryption when available.

removeItem(key: string): Promise<void>

Remove token from all storage backends.

clear(): Promise<void>

Clear all authentication-related tokens.

isTokenValid(key: string): Promise<boolean>

Check if token exists and is not expired.

getStorageInfo(): object

Get diagnostic information about storage backends.

Storage Interface Compatibility

The class implements the full Web Storage API for Teams compatibility:

• length: number - Number of stored items
• key(index: number): string | null - Get key at index

Upgrade Guide

From Previous Versions

Users upgrading from versions without secure storage will experience seamless transition:

1. Immediate Compatibility: Existing tokens continue working via localStorage fallback
2. Automatic Security: New tokens use secure storage when available
3. Natural Transition: Security improves as tokens refresh (typically within hours)
4. Zero Downtime: No authentication interruption during upgrade

No Migration Required

The implementation eliminates migration complexity by using a dual-storage approach where new tokens automatically use secure storage while existing tokens remain functional through fallback mechanisms.

Security Best Practices

For Users

• Keep OS Updated: Ensure operating system security updates are current
• Check Availability: Use safeStorage.isEncryptionAvailable() to verify secure storage support
• Monitor Logs: Watch for security warnings in application logs

For Developers

• PII Protection: Never log actual token values or user identifiers
• Error Handling: Always provide fallback mechanisms for storage failures
• Availability Testing: Test both with and without secure storage availability
• Fallback Testing: Verify graceful degradation when secure storage unavailable

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[!NOTE] This implementation provides enterprise-grade security while maintaining simplicity and reliability. The natural transition approach eliminates migration complexity while ensuring authentication works regardless of platform capabilities.