BIP352: clarify input-sum accumulation — tolerate intermediate point at infinity

[SatsAndSports]

BIP352 defines A = A₁ + … + Aₙ on the receiving side and a = a₁ + … + aₙ on the sending side, and it says what to do when the total is the identity (receiver: skip; sender: fail). It does not say what happens when an intermediate partial sum equals the point at infinity during accumulation. This pins the intended behavior normatively:

• accumulation MUST tolerate intermediate infinity (it is the group identity — it does not affect the total);
• accumulation MUST be order-invariant;
• EC backends that error on point-at-infinity addition MUST catch the condition and continue accumulating.

This is a clarification, not a behavior change: the reference implementation (secp256k1lab GE.sum) already tolerates intermediate infinity, and the official send_and_receive_test_vectors.json already contains the intermediate-cancellation (zero-sum) case. It also matches the text's existing claim — the sum is a group operation, whose identity element is the point at infinity.

Why reachable, not theoretical

For any private key a with P = a·G, the negated public key −P = (n − a)·G is also spendable by the same holder. Consequently a transaction whose input-key list contains an intermediate cancellation (e.g. [P, −P, Q] summing to Q, non-infinite total) is ordinary, consensus-valid, and cheap — reachable by construction, with ordering as the knob. An implementation that errors on intermediate cancellation becomes input-order-dependent, dropping or mis-deriving payments that conformant implementations accept, and receiver software scans arbitrary on-chain transactions containing any of the eligible input types (P2TR, P2WPKH, P2SH-P2WPKH, P2PKH).

Evidence (backend contracts differ at today’s public pins)

• The BIP352 reference implementation tolerates intermediate cancellation (secp256k1lab GE.sum); the official vectors already contain that case.
• go-libsecp256k1.PubKeyAdd errors with invalid public key: [0,0] not on secp256k1 curve on an intermediate infinity — the index then silently drops the transaction depending on input ordering.
• rust-secp256k1 reports InvalidPublicKeySum / InvalidTweak on the same case in the Rust silent-payments crates (rust-silentpayments / spdk) — send aborts, receive errors, both diverging from the reference verdict.
• libsecp256k1 (C core) handles infinity correctly at the group-element level (secp256k1_ge/gej ops), while serialized-pubkey helpers (secp256k1_ec_pubkey_combine) reject it — so wrappers inherit either behavior depending on which API layer they call.

Cross-implementation divergence on this clause was found during a public differential of open implementations against the BIP's own official vectors; the fix is to pin the reference semantics in prose, and no vector changes are needed.

[jonatack]

cc BIP authors @josibake @RubenSomsen @theStack for feedback here