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Add double ratchet transport with opt-in flag - #11
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| def receive(self, header: Header) -> bytes: | ||
| key = (header.dh_pub, header.n) | ||
| if key in self.state.skipped: | ||
| mk = self.state.skipped.pop(key) | ||
| return fernet_key_from_mk(mk) | ||
| if self.state.dh_remote != header.dh_pub: | ||
| self.ratchet_step(header.dh_pub) | ||
| self._skip_message_keys(header.n, header.dh_pub) | ||
| self.state.CKr, mk = kdf_chain(self.state.CKr) |
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The receive method never uses the header’s pn and does not guard against header.n values that are less than the current Nr. When a peer sends multiple messages on a chain and then performs a DH ratchet, any out‑of‑order delivery will call ratchet_step before deriving the missing message keys, so those messages become undecryptable. Similarly, if the network repeats a frame with an already processed n, the code will derive and discard the next chain key, permanently desynchronising the ratchet and breaking all later frames. The double‑ratchet spec derives header.pn keys before ratcheting and rejects duplicates; this implementation advances state on every call, so retransmissions or reorderings will wedge the session.
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Testing
https://chatgpt.com/codex/tasks/task_e_68ec96ebda888331ac0e675f76dccf10