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## Summary Two pure-Dart crypto paths that run on the UI isolate during normal use: - **NIP-98 body hashes.** Every relay HTTP query (`RelaySessionNotifier._postForEvents`) hashes its request body for the NIP-98 auth header with pointycastle's `SHA256Digest`. This switches that hash, the media upload hash and the NIP-OA preimage hash to `package:crypto`'s `sha256`, which is already a dependency. The output is identical. - **Public key derivation.** `AccountIdentityStore.publicKeyForNsec` runs on every relay auth assertion and derived the key with the `nostr` package's pure-Dart curve math. It now uses the native `derivePublicKeyHex` (libsecp256k1) with the same validation. The default-agent writer now signs with the native `signNostrEvent` instead of `nostr.Event.from`. Left as they were: `nostr.Keys` construction for signing keys (nostr 2.0.0 always derives in Dart, with no way to pass a known public key), and `nostr.Keys.generate()` on the rare key-creation paths. ## Measurements - **Device** (iPhone 12 mini, Profile build, #4things captures): NIP-98 hashing took 17 to 24 ms of UI CPU per 10 to 20 s capture, mostly from channel notices queries. `publicKeyForNsec` took about 7 ms per relay auth assertion. - **Synthetic** (Mac, AOT, same output): SHA-256 of a 1 KB body takes 39.9 µs with pointycastle and 6.6 µs with `package:crypto`. 50 KB: 1,518 µs vs 293 µs. 1 MB: 29.6 ms vs 6.1 ms. - **Device A/B after the change** (iPhone 12 mini, Profile/AOT, Tom's Block community, two app resumes per build from the home screen, 15 s windows; A is `origin/main`3d26ed7, B is A plus this PR and [#8966](https://github.com/block/buzz/pull/8966)): NIP-98 signing per resume went from 42 / 50 ms to 9 / 13 ms of UI CPU, and all SHA-256 work per resume from 74 / 73 ms to 33 / 33 ms. Dart public-key derivation through `nostr.Keys` (left as is, see above) still takes 15 to 23 ms per resume. - **Same A/B on current `main`** (14037e3): pointycastle SHA-256 per resume went from 39 / 45 ms to 7 / 0 ms, and NIP-98 header building from 4 / 5 ms to 2 / 0 ms. `nostr.Keys` derivation: 12 to 17 ms per resume on both builds. ## Testing - New known-vector tests for the NIP-98 header hash: an empty body, `abc`, and the one-million-`a` FIPS vector. They also check the event's kind, method, signature and pubkey. - New `publicKeyForNsec` tests: the generator-point vector, matching the pure-Dart derivation, and rejecting invalid keys (scalar 0 and n, garbage, an npub). - These are correctness tests only. A timing test would be flaky, so none would fail if the old pure-Dart code came back. - `just mobile-check mobile-test` passes (pre-push hook). - Automated device run in place of manual testing: the B build above on the 12 mini, launched and resumed twice into a signed-in Block community, loading the channel list, channel notices and read state over authenticated relay queries (NIP-98 headers and relay auth assertions on every request). Not exercised on the device: media upload and the default-agent writer. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Signed-off-by: Tom Brow <tomb@squareup.com> Co-authored-by: Tom Brow <tomb@squareup.com> Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz> Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>