PostQuantum-KeyExchange.md

sequenceDiagram
    participant Alice as Alice (Sender)
    participant AliceLib as LibEmiddle (Alice)
    participant Server as Transport Server
    participant BobLib as LibEmiddle (Bob)
    participant Bob as Bob (Receiver)

    Note over Alice, Bob: Post-Quantum + Classical Hybrid Key Exchange

    Alice->>AliceLib: Initialize with Post-Quantum crypto
    AliceLib->>AliceLib: PostQuantumCrypto.GenerateKeyPair()
    AliceLib->>AliceLib: Generate hybrid key bundle (Classic + PQ)

    Bob->>BobLib: Initialize with Post-Quantum crypto
    BobLib->>BobLib: PostQuantumCrypto.GenerateKeyPair()
    BobLib->>BobLib: Generate hybrid key bundle (Classic + PQ)
    BobLib->>Server: Upload hybrid key bundle

    Alice->>AliceLib: Request session with Bob
    AliceLib->>Server: Fetch Bob's hybrid key bundle
    Server->>AliceLib: Return hybrid key bundle

    AliceLib->>AliceLib: Perform hybrid key exchange
    AliceLib->>AliceLib: X3DHProtocol + PostQuantum exchange
    AliceLib->>AliceLib: Derive combined shared secret

    AliceLib->>Server: Send hybrid encrypted message
    Server->>BobLib: Deliver to Bob

    BobLib->>BobLib: Process hybrid key exchange
    BobLib->>BobLib: X3DHProtocol + PostQuantum exchange  
    BobLib->>BobLib: Derive combined shared secret
    BobLib->>BobLib: Initialize DoubleRatchet with hybrid keys
    BobLib->>Bob: Deliver decrypted message

    AliceLib->>AliceLib: All future messages use hybrid encryption
    
    Note over Alice, Bob: Quantum-resistant security achieved through classical + post-quantum key combination

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