Implementing End-to-End Encryption for Messaging

Overview

End-to-end encryption (E2EE) ensures that only the communicating parties can read messages, with no intermediary (including the server) able to decrypt them. This skill implements a simplified version of the Signal Protocol's Double Ratchet algorithm, using X25519 for key exchange, HKDF for key derivation, and AES-256-GCM for message encryption.

Objectives

• Implement X25519 Diffie-Hellman key exchange for session establishment
• Build the Double Ratchet key management algorithm
• Encrypt and decrypt messages with per-message keys
• Implement forward secrecy (compromise of current key does not reveal past messages)
• Handle out-of-order message delivery
• Implement key agreement using X3DH (Extended Triple Diffie-Hellman)

Key Concepts

Signal Protocol Components

Forward Secrecy

Each message uses a unique encryption key derived from a ratcheting chain. After a key is used, it is deleted, ensuring that compromise of the current state does not reveal previously sent/received messages.

Security Considerations

• Delete message keys immediately after decryption
• Implement message ordering and replay protection
• Use authenticated encryption (AES-GCM) for all messages
• Protect identity keys with device-level security
• Verify identity keys out-of-band (safety numbers)

Validation Criteria

• X25519 key exchange produces shared secret
• Messages encrypt and decrypt correctly between two parties
• Different messages produce different ciphertexts
• Forward secrecy: old keys cannot decrypt new messages
• Out-of-order messages can be decrypted
• Tampered messages are rejected by authentication