documentation

docs/adr/0001-noise-transport.md

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+# Transport security uses Noise, not TLS
+
+We secure the session with the Noise protocol (an `XX` handshake) using each
+device's long-term identity key, rather than mutual TLS with per-device certs.
+Noise avoids a CA/cert machinery on a peer-to-peer LAN, maps cleanly onto the
+SAS pairing flow, and is light enough for the Odin daemon and Android
+(Conscrypt). The pinned trust record and the session key are the *same* key,
+simplifying the model.