feat: add multi-device capabilities, tests, update README and add acknowledgement so we transition to whisper messages

Author: stdNullPtr

README.md

@@ -8,9 +8,11 @@ This repository serves as a technical demonstration of Signal Protocol implement
 
 - Complete Signal Protocol implementation
 - Key exchange mechanisms
-- Session establishment
+- Session establishment and proper acknowledgment
 - Message encryption/decryption
 - Multi-user communication scenarios
+- Multi-device support per user
+- Group messaging
 
 **Important**: This codebase is designed to be explored through its test suite. The tests serve as living documentation and executable examples of the Signal Protocol implementation.
 
@@ -43,8 +45,11 @@ The main Signal Protocol implementation featuring:
 - Identity key pair generation and management
 - PreKey bundle creation and distribution
 - Session establishment with other users
+- Proper session acknowledgment for Double Ratchet initialization
 - Message encryption using the Double Ratchet algorithm
 - Message decryption and session management
+- Multi-device support through device-specific sessions
+- Group messaging with sender key distribution
 
 #### `SignalClientState` (`lib/client/signal_client.freezed.dart`)
 Immutable state management using Freezed, containing:
@@ -59,12 +64,15 @@ Immutable state management using Freezed, containing:
 #### Data Models
 - `UserKeys` (`lib/server/models/user_keys.dart`): Represents a user's PreKey bundle for key exchange
 - `Message` (`lib/common/models/message.dart`): Encapsulates encrypted message data
+- `GroupMessage` (`lib/common/models/group_message.dart`): Represents encrypted group messages
 - `PublicPreKey` & `PublicSignedPreKey` (`lib/server/models/`): Key structures for exchange
 
 #### (Mock) Server Communication (`lib/server/server.dart`)
 - PreKey bundle upload and retrieval
 - Encrypted message routing
 - Message acknowledgment handling
+- Multi-device awareness
+- Group membership management
 
 #### Utilities
 - `Logger` (`lib/utils/logger.dart`): Logging utility for debugging
@@ -91,14 +99,16 @@ lib/
 
 test/
 ├── signal_protocol_test.dart      # Comprehensive test scenarios
-└── key_reuse_test.dart            # Tests for key reuse prevention
+├── key_reuse_test.dart            # Tests for key reuse prevention
+└── multi_device_test.dart         # Tests for multi-device support
 ```
 
 ## 🧪 Understanding the Implementation Through Tests
 
 The test suites serve as the primary documentation for this PoC. Each test file demonstrates specific aspects of the Signal Protocol:
 - `test/signal_protocol_test.dart`: Core protocol implementation scenarios
 - `test/key_reuse_test.dart`: Security tests for preventing key reuse attacks
+- `test/multi_device_test.dart`: Tests for multi-device support
 
 ### Running the Test Suite
 
@@ -129,18 +139,31 @@ flutter test --name "should establish session and exchange messages"
    - Illustrates initial session creation
    - Shows the X3DH key agreement protocol
    - Demonstrates session caching
+   - Properly acknowledges and transitions session states
 
 4. **Message Exchange**
    - Shows complete message encryption flow
    - Demonstrates the Double Ratchet algorithm
+   - Illustrates proper message type transitions (prekey → whisper)
    - Illustrates message decryption
 
 5. **Multi-User Scenarios**
    - Complex interactions between multiple users
    - Concurrent session management
    - Message ordering and delivery
 
-6. **Key Reuse Prevention** (in `key_reuse_test.dart`)
+6. **Multi-Device Support**
+   - Multiple devices for a single user
+   - Proper message routing to all user devices
+   - Independent session management per device
+
+7. **Group Messaging**
+   - Sender key distribution for groups
+   - Group membership management
+   - Encrypted group communication
+   - Handling members joining existing groups
+
+8. **Key Reuse Prevention** (in `key_reuse_test.dart`)
    - Ensures one-time PreKeys are not reused
    - Validates security against key reuse attacks
    - Tests proper key rotation mechanisms
@@ -158,6 +181,9 @@ The implementation follows the Signal Protocol specification:
 3. **One-Time PreKeys**: Ephemeral keys for perfect forward secrecy
 4. **Sessions**: Established using X3DH (Extended Triple Diffie-Hellman)
 5. **Double Ratchet**: Provides forward secrecy and break-in recovery
+6. **Session Acknowledgment**: Ensures proper state transitions for the ratchet
+7. **Multi-Device Support**: Manages separate cryptographic sessions for each device
+8. **Group Messaging**: Uses sender keys for efficient group communication
 
 ### State Management
 
@@ -170,7 +196,8 @@ Uses Freezed for immutable state management:
 
 The implementation requires a compatible server that:
 - Stores and serves PreKey bundles
-- Routes encrypted messages between users
+- Routes encrypted messages between users and devices
+- Manages group membership
 - Never has access to plaintext content
 
 ## 🔒 Security Considerations
@@ -181,14 +208,18 @@ This PoC demonstrates the following security properties:
 - **Perfect Forward Secrecy**: Compromised keys don't affect past communications
 - **Break-in Recovery**: Compromised keys don't affect future communications
 - **Deniable Authentication**: Messages can be authenticated by the recipient but anyone could have forged messages after the conversation - protecting users from being cryptographically proven to have sent a message
+- **Multi-Device Security**: Each device maintains its own cryptographic session, preventing compromise of all devices if one is breached
 
 ## 🚦 What This PoC Demonstrates
 
 ✅ Complete Signal Protocol implementation in Dart  
 ✅ Proper key management and rotation  
 ✅ Session establishment between users  
+✅ Session acknowledgment and state management  
 ✅ Message encryption and decryption  
 ✅ Multi-user communication patterns  
+✅ Multi-device support per user  
+✅ Group messaging with sender keys  
 ✅ Server integration for key exchange  
 
 ## 🚫 What This PoC Doesn't Include
@@ -197,8 +228,8 @@ This PoC demonstrates the following security properties:
 ❌ Persistent storage of keys and sessions  
 ❌ Production-ready error handling  
 ❌ Message delivery guarantees  
-❌ Group messaging  
 ❌ Media/file encryption  
+❌ Comprehensive cryptographic auditing  
 
 ## 📚 Learning from the Code
 
@@ -207,17 +238,45 @@ To understand the Signal Protocol implementation:
 1. Start with the main test file: `test/signal_protocol_test.dart`
 2. Follow the test scenarios in order
 3. Review security tests in `test/key_reuse_test.dart`
-4. Examine the `SignalClient` implementation in `lib/client/signal_client.dart`
-5. Review the state management in `SignalClientState`
-6. Understand the server interactions in `lib/server/server.dart`
-7. Explore the data models in `lib/common/models/` and `lib/server/models/`
+4. Explore multi-device capabilities in `test/multi_device_test.dart`
+5. Examine the `SignalClient` implementation in `lib/client/signal_client.dart`
+6. Review the state management in `SignalClientState`
+7. Understand the server interactions in `lib/server/server.dart`
+8. Explore the data models in `lib/common/models/` and `lib/server/models/`
+
+## Key Signal Protocol Features Explained
+
+### Session Acknowledgment
+
+The PoC implements proper session acknowledgment, which is crucial for the Double Ratchet algorithm:
+- Initial messages use PreKey messages (containing X3DH materials)
+- After session establishment, a cryptographic acknowledgment occurs
+- Subsequent messages use the more efficient "whisper" message type
+- This follows the Signal Protocol's security design for session establishment
+
+### Multi-Device Support
+
+The implementation demonstrates how Signal handles multiple devices for a single user:
+- Each device has its own identity and cryptographic material
+- Messages sent to a user are delivered to all their devices
+- Each device maintains independent sessions with other users' devices
+- This models Signal's approach to multi-device support
+
+### Group Messaging
+
+Group messaging is implemented using sender keys:
+- Each member distributes their sender key to the group
+- Messages are efficiently encrypted once and distributed to all members
+- New members can join existing groups
+- This matches Signal's efficient approach to group communication
 
 ## 🔗 References
 
 - [Signal Protocol Documentation](https://signal.org/docs/)
 - [libsignal_protocol_dart](https://pub.dev/packages/libsignal_protocol_dart)
 - [The X3DH Key Agreement Protocol](https://signal.org/docs/specifications/x3dh/)
 - [The Double Ratchet Algorithm](https://signal.org/docs/specifications/doubleratchet/)
+- [Sender Keys for Group Messaging](https://signal.org/docs/specifications/group-sessions/)
 
 ## 🤝 Contributing
 
@@ -240,4 +299,4 @@ The GPL-3.0 license ensures that:
 
 ---
 
-This proof-of-concept demonstrates how the Signal Protocol can be implemented in Flutter/Dart. Explore the test suite to understand the implementation details.
+This proof-of-concept demonstrates how the Signal Protocol can be implemented in Flutter/Dart. Explore the test suite to understand the implementation details.