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Alice → Bob:
1. Fetch Bob's PreKey Bundle (Identity + Signed PreKey + One-Time PreKey + PQ PreKey)
2. 4× X25519 DH computations
3. ML-KEM-1024.Encaps(Bob's PQ Public Key) → (pq_ciphertext, pq_shared_secret)
4. Root Key = HKDF-SHA256(DH1‖DH2‖DH3‖DH4‖pq_shared_secret)
5. Send: Alice's keys + PQ ciphertext (1,088 bytes) + encrypted message

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DH Ratchet (direction change):
  New Root Key = HKDF-SHA256(current_root_key, X25519_DH_output)

Symmetric Ratchet (per-message):
  Message Key  = HMAC-SHA256(chain_key, 0x01)
  Next Chain   = HMAC-SHA256(chain_key, 0x02)

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Ciphertext = AES-256-GCM.Encrypt(
  key:   message_key (32 bytes),
  nonce: random (12 bytes, never reused),
  aad:   ratchet_public_key ‖ prev_chain_length ‖ message_number,
  plaintext: message content
)

Per-message overhead: nonce (12B) + auth tag (16B) = 28 bytes

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XEdDSA Signing (via libsignal):
  payload   = senderId ‖ "|" ‖ timestamp ‖ "|" ‖ content
  signature = XEdDSA.Sign(payload, X25519_private_key)
  → 64 bytes attached to message
  (Converts X25519 key to Ed25519 internally — no separate Ed25519 key needed)

Verification:
  XEdDSA.Verify(payload, signature, sender_X25519_public_key)
  → verified | failed | noSignature

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Sender:
1. Generate ephemeral X25519 keypair
2. DH = X25519(ephemeral_private, recipient_identity_public)
3. AES_Key = HKDF-SHA256(DH, info="Arc Sealed Sender v1")
4. Envelope = AES-256-GCM(senderId + "\0" + payload, AES_Key)

→ Sealed Message = ephemeral_pub(32B) ‖ nonce(12B) ‖ ciphertext ‖ mac(16B)

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Encryption (libsignal group_encrypt):
  Chain Key → Message Key → AES-256-GCM  (libsignal internal)
  HMAC signature                          (libsignal internal)
  → Output: SenderKeyMessage bytes

Decryption (libsignal group_decrypt):
  Signature verify → Replay detect → Decrypt (libsignal internal)

All crypto operations delegated to libsignal — no custom primitives.

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