guggero / btcd

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Build # 29686067987

Build Type

push

github

Committed by guggero

Commit Message

silentpayments: add batched filter candidate derivation

Scanning derives candidate output keys for every served transaction
tweak, so per-tweak overhead multiplies by hundreds of thousands over
a spam-heavy block range. TransactionOutputKeysForFilterBatch derives
the candidates of many tweaks in one call and exploits two batch-level
shortcuts the per-tweak variant cannot:

  - The output tweak t_0 depends only on the shared secret, not on the
    spend key, so its point is computed once per transaction and the
    per-address candidates become cheap point additions instead of
    full base multiplications.

  - All per-point affine conversions (one modular inversion each, both
    for the ECDH shared secrets and for the candidate keys) collapse
    into two inversions for the entire batch via Montgomery's trick.

Benchmarks show 1.4x faster candidate derivation per tweak (102us to
72us native, 177us to 126us in wasm, for the base+change address
pair); the remaining cost is ~90% the one irreducible ECDH scalar
multiplication per tweak. Equivalence with the per-tweak variant is
tested against random keys, all official vectors and the
point-at-infinity corner case.

Coverage Stats

36581 of 69925 relevant lines covered (52.31%)

354391.51 hits per line