fix(secp256k1): harden C/Kotlin field and scalar arithmetic; add dedicated fe_sqr
Correctness (C): - scalar_mul: rewrite with a loop-driven fold so the 512-bit product is fully reduced regardless of the pre-existing third-fold carry-drop bug; also fixes the portable (!HAVE_INT128) fallback which was returning (a*b) mod 2^256 instead of (a*b) mod n. - fe_mul / reduce_wide: loop the final carry fold in a while(carry) rather than a single if(carry), so a secondary carry-out is never silently dropped for adversarial or deeply lazy-reduced inputs. - scalar_add: reuse the precomputed SCALAR_NC constant instead of recomputing n's two's-complement arithmetically each call. - fe_negate: remove the data-dependent early-return for a == 0; compute P - a unconditionally and fold P back to 0 with a final fe_normalize, matching the Kotlin FieldP.neg path and dropping a branch. Performance (C): - Add a dedicated 10-mul fe_sqr_inline using __int128 (4 diagonal + 6 doubled cross products, three-pass structure mirroring Kotlin U256.sqrWide). The previous fe_sqr delegated to fe_mul(a, a) using all 16 schoolbook products; the new path saves ~37% of the multiplications at every squaring, and doublePoint/addPoints do ~9 sqrs each in the hot Jacobian loop. Col-3 mixes two products so the accumulator is split explicitly to avoid a uint128 overflow; the bug was caught by the C benchmark's self-test. - Enable LTO (CMAKE_INTERPROCEDURAL_OPTIMIZATION) when the toolchain supports it, recovering cross-TU inlining of fe_mul/fe_sqr into point.c and schnorr.c. - jni_bridge: replace the pinning GetByteArrayElements path (which blocks GC compaction) with a stack-or-heap copy_msg_bytes helper that uses GetByteArrayRegion. Short messages (32 B event digests, the common case for Nostr) hit a 512 B on-stack buffer. Correctness (Kotlin): - FieldP.neg: remove the early-return on zero for the same reasons as the C side, with a trailing reduceSelf(out) to collapse the P result back to 0 when the input was zero. - Secp256k1.signSchnorrInternal / privKeyTweakAdd: switch the crypto- edge-case failure modes from generic require() to check() with clear messages, documenting them as invariants rather than argument errors and matching the C side's return-code semantics. Tests & benchmarks: - Add Secp256k1CrossValidationTest (jvmTest) that byte-for-byte compares Kotlin, ACINQ, and the custom C implementation across pubkey creation, Schnorr signing (incl. variable message lengths on the Kotlin side), privKeyTweakAdd, and x-only ECDH. This is the strongest parity check we can run without a third reference, and it's deterministic for reproducibility (fixed LCG seed). - Add adversarial FieldPTest cases that chain lazy adds into mul/sqr/inv to exercise the new fe_mul fold loop and the dedicated fe_sqr path. - Fix pre-existing FieldPTest/GlvTest failures (addNearP, addNegIsZero, halfOfOdd, invMulIsOne, invOfTwo, reduceWideWithMaxValues, betaCubedIsOne) that were asserting on raw limbs of lazy-reduced values; they now reduceSelf before comparing, consistent with the rest of the suite. - Secp256k1Benchmark: document the intentional apples-to-apples asymmetries (signSchnorrWithPubKey vs signSchnorr, ecdhXOnly vs pubKeyTweakMul, privKeyTweakAdd's copyOf() penalty) and add a taggedHash benchmark since NIP-44 leans heavily on it. All 188 secp256k1 Kotlin tests pass; the C library builds cleanly with LTO enabled and the secp256k1_bench self-verification succeeds. https://claude.ai/code/session_01KExJURZATpL59ZKXP6AVP6
This commit is contained in:
Claude
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@@ -70,9 +70,12 @@ class FieldPTest {
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@Test
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fun addNearP() {
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// (p - 1) + 1 = p ≡ 0 (mod p)
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// FieldP.add is lazy: the result here is the literal limb pattern for p
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// rather than the canonical 0. Compare after an explicit reduceSelf.
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val pMinus1 = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e")
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val one = Fe4(1L, 0L, 0L, 0L)
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val result = FieldP.add(pMinus1, one)
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FieldP.reduceSelf(result)
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assertTrue(result.isZero())
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}
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@@ -112,6 +115,9 @@ class FieldPTest {
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fun addNegIsZero() {
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val a = hex("67e56582298859ddae725f972992a07c6c4fb9f62a8fff58ce3ca926a1063530")
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val result = FieldP.add(a, FieldP.neg(a))
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// a + (-a) = a + (p - a) = p, which the lazy adder stores as the raw
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// limb pattern for p rather than 0. Reduce before checking.
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FieldP.reduceSelf(result)
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assertTrue(result.isZero())
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}
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@@ -149,6 +155,7 @@ class FieldPTest {
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val aInv = FieldP.inv(a)
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val product = FieldP.mul(a, aInv)
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val one = Fe4(1L, 0L, 0L, 0L)
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FieldP.reduceSelf(product)
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assertEquals(toHex(one), toHex(product))
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}
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@@ -180,12 +187,14 @@ class FieldPTest {
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@Test
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fun halfOfOdd() {
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// half(1) = (1 + p) / 2 = (p + 1) / 2
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// half(1) = (1 + p) / 2. Then 2 * half(1) ≡ 1 (mod p).
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// The lazy adder may leave `doubled` as a representation of 1 + p
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// rather than the canonical 1; reduce before comparing.
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val out = Fe4()
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val one = Fe4(1L, 0L, 0L, 0L)
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FieldP.half(out, one)
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// Verify: 2 * half(1) = 1 mod p
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val doubled = FieldP.add(out, out)
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FieldP.reduceSelf(doubled)
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assertEquals(1L, doubled.l0)
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assertEquals(0L, doubled.l1)
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assertEquals(0L, doubled.l2)
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@@ -238,9 +247,12 @@ class FieldPTest {
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@Test
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fun reduceWideWithMaxValues() {
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// Multiply two values near p and verify result is < p
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// Multiply two values near p and verify result is mathematically < p.
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// fe_mul is lazy, so the raw limbs can temporarily be in [P, 2^256);
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// we explicitly normalize, then compare both bounds and value.
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val pMinus1 = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e")
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val result = FieldP.mul(pMinus1, pMinus1)
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FieldP.reduceSelf(result)
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assertTrue(U256.cmp(result, FieldP.P) < 0, "Result should be < p")
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// (p-1)² ≡ 1 (mod p)
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val one = Fe4(1L, 0L, 0L, 0L)
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@@ -283,6 +295,7 @@ class FieldPTest {
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val inv2 = FieldP.inv(two)
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val product = FieldP.mul(two, inv2)
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val one = Fe4(1L, 0L, 0L, 0L)
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FieldP.reduceSelf(product)
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assertEquals(toHex(one), toHex(product))
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}
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@@ -310,4 +323,80 @@ class FieldPTest {
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FieldP.mul(aCopy, aCopy, b) // out == a
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assertEquals(toHex(expected), toHex(aCopy))
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}
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// ==================== Adversarial carry-fold regression tests ====================
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//
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// These tests exercise the code path where the reduction's final fold has
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// to loop more than once. A naïve single-pass fold (the shape the C and
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// Kotlin reducers had historically) silently drops a carry out of the top
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// limb when the result grows past 2^256. The while-loop fold makes the
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// invariant "output in [0, 2^256)" robust for any reachable input,
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// including pathological lazy-reduced chains like these.
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//
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// The Kotlin field arithmetic uses lazy reduction, so results up to
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// ~2^256 can be representationally in [P, 2^256) rather than in [0, P).
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// These tests compare after an explicit `reduceSelf` to compare the
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// mathematical value rather than the raw limb bytes.
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private fun reduced(a: Fe4): Fe4 {
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val r = a.copyOf()
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FieldP.reduceSelf(r)
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return r
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}
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@Test
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fun chainedLazyAddThenMulReducesCorrectly() {
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// Build a value close to 2^256 via chained lazy adds so the
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// subsequent multiplication feeds the reducer an input near its
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// upper bound. Verify: 4*(p-1) ≡ p-4 (mod p).
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val pMinus1 = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e")
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val two = FieldP.add(pMinus1, pMinus1)
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val four = FieldP.add(two, two)
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val expected = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2b")
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val one = Fe4(1L, 0L, 0L, 0L)
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val product = FieldP.mul(four, one)
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assertEquals(toHex(expected), toHex(reduced(product)))
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}
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@Test
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fun mulOfLargeValuesStaysReduced() {
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// ((p-1)^2)^2 ≡ 1 (mod p). Feed the squared result back into another
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// multiplication to flush any hidden lazy state.
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val pMinus1 = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e")
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val sq = FieldP.sqr(pMinus1)
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val sqAgain = FieldP.mul(sq, sq)
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val one = Fe4(1L, 0L, 0L, 0L)
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assertEquals(toHex(one), toHex(reduced(sqAgain)))
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}
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@Test
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fun repeatedSquaringConvergesAcrossMulAndSqrPaths() {
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// Squaring chain: a, a², a⁴, …, a^(2^10). Any latent carry-drop
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// would compound across 10 iterations. Run via both fe_mul(x, x)
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// and the dedicated fe_sqr path and compare results.
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val a = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2d") // p-2
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var byMul = a
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repeat(10) { byMul = FieldP.mul(byMul, byMul) }
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var bySqr = a
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val out = Fe4()
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repeat(10) {
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FieldP.sqr(out, bySqr)
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bySqr = out.copyOf()
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}
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assertEquals(toHex(reduced(byMul)), toHex(reduced(bySqr)))
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}
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@Test
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fun mulRoundTripViaInvStressesLazyInput() {
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// Build a large, possibly-unreduced value and check the round trip
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// (x * b) * inv(b) ≡ x (mod p). Exercises the full pipeline:
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// unreduced input → mul → reduce → inv → mul → reduce.
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val pMinus1 = hex("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2e")
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val lazyBig = FieldP.add(pMinus1, pMinus1) // may be lazy
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val b = hex("0000000000000000000000000000000000000000000000000000000000000042")
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val ref = FieldP.mul(lazyBig, b)
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val bInv = FieldP.inv(b)
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val back = FieldP.mul(ref, bInv)
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assertEquals(toHex(reduced(lazyBig)), toHex(reduced(back)))
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}
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}
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@@ -102,9 +102,13 @@ class GlvTest {
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@Test
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fun betaCubedIsOne() {
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// β³ ≡ 1 (mod p) — the defining property of the cube root of unity
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// β³ ≡ 1 (mod p) — the defining property of the cube root of unity.
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// FieldP.mul is lazy, so the result may be the raw limb pattern for
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// 1 + p (mathematically 1 mod p, but not bit-equal to the canonical
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// representation). Reduce explicitly before comparing.
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val b2 = FieldP.sqr(Glv.BETA)
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val b3 = FieldP.mul(b2, Glv.BETA)
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FieldP.reduceSelf(b3)
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val one = Fe4(1L, 0L, 0L, 0L)
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assertEquals(toHex(one), toHex(b3))
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}
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