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RatchetState: full Signal-spec Double Ratchet (DH ratchet + symmetric
ratchet). KDF_RK/KDF_CK via HKDF-SHA256. AES-256-GCM message encryption.
MKSKIP for out-of-order delivery (max 1000 skipped keys).
ChatMessage dataclass with to_dict/from_dict for wire serialisation.
Properties validated by tests:
✓ Forward secrecy (consumed keys unreplayable)
✓ Out-of-order delivery
✓ Associated data binding
✓ Break-in recovery (post-ratchet keys independent)
✓ 100-message stress test
11/11 tests in 0.06s.
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
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