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3dbc95c3f6
`ecdhSharedSecret`, the input to every NIP-44 conversation key
(preferences, read state, reminders, themes, mutes, stars, sections,
sort, pairing, agent observer), used pointycastle's BigInt point
multiplication. That costs about 5.9 ms per call on an iPhone SE (2nd
gen, Profile) and is variable time in the secret scalar. It now calls
libsecp256k1's constant-time `secp256k1_ecdh` through `ecdhX` from #8654
(39.8 µs on the same device), returning the same raw x-coordinate NIP-44
requires. The decoded secret key bytes are zeroed after the call.
Invalid keys now throw `ArgumentError` instead of pointycastle's errors;
callers already treat any throw as a failed derivation.
### Validation
- Added tests: `ecdh_test.dart` compares the native result with the
previous pointycastle algorithm for 32 random key pairs, checks the
NIP-44 v2 `get_conversation_key` vector (secret 1, peer pubkey of secret
2), and checks rejection of a zero secret, an off-curve public key and
wrong lengths.
- Full mobile analyze and test suite pass after rebasing onto main at
da1190e2c, including the desktop nostr-rs NIP-44 interop vector.
- On a device: an iPhone 12 mini Profile build of the full native stack,
connected to the internal relay.
buzz-review-completed
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Signed-off-by: Tom Brow <tomb@block.xyz>
Co-authored-by: george <973be878edd6d4f79692a3827f20a6f7fe0ed0362f269ff6b6ba8867033d7d8e@sprout-oss.stage.blox.sqprod.co>