Kychacha-crypto: Post-Quantum Secure Encryption Protocol

A post-quantum simple to use implementation for ML-KEM and ChaCha20

Hybrid cryptographic implementation using:

• ML-KEM (formerly Crystals-Kyber): Post-Quantum secure Key Encapsulation Mechanism (KEM) for key exchange, standardized by NIST.
• ChaCha20-Poly1305: Authenticated symmetric encryption.

Architecture

The following diagram describes the protocol flow between the "Sender" and the "Recipient":

sequenceDiagram
    participant Sender
    participant Recipient

    Recipient-->>Sender: Recipient public key (ML-KEM pub key 1184 bytes)
    
    Sender->>Sender: Encapsulate secret (ML-KEM)
    Note right of Sender: Generates ephemeral keypair and derives shared secret
    Sender->>Sender: Derive ChaCha key (HKDF-SHA256)
    Note right of Sender: Uses shared secret to derive symmetric key
    Sender->>Sender: Encrypt message (ChaCha20-Poly1305)
    
    Sender->>Recipient: Send {ciphertext, nonce, encrypted message}
    
    Recipient->>Recipient: Decapsulate secret (ML-KEM)
    Note right of Recipient: Recovers shared secret
    Recipient->>Recipient: Derive ChaCha key (HKDF-SHA256)
    Note right of Recipient: Derives the same symmetric key
    Recipient->>Recipient: Decrypt message

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Note: During the encapsulation process on the sender's side, an ephemeral keypair is generated.

Usage and documentation

https://docs.rs/kychacha_crypto

Safety Considerations

2. Randomness: Depends on the secure generator of the system.
3. HKDF context: Used for protocol binding.
4. Nonces: Generated with CSPRNG for each message.