From 2a04ee074e07cb821564c40313f7b44922bc9ac2 Mon Sep 17 00:00:00 2001 From: Pieter Wuille Date: Sun, 3 Jul 2022 13:51:28 -0400 Subject: [PATCH] Add x-only ecmult_const version for x=n/d --- src/ecmult_const.h | 19 +++++++++++++ src/ecmult_const_impl.h | 54 +++++++++++++++++++++++++++++++++++ src/tests.c | 63 +++++++++++++++++++++++++++++++++++++++++ 3 files changed, 136 insertions(+) diff --git a/src/ecmult_const.h b/src/ecmult_const.h index f891f3f306..aae902743b 100644 --- a/src/ecmult_const.h +++ b/src/ecmult_const.h @@ -18,4 +18,23 @@ */ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits); +/** + * Same as secp256k1_ecmult_const, but takes in an x coordinate of the base point + * only, specified as fraction n/d. Only the x coordinate of the result is returned. + * + * If known_on_curve is 0, a verification is performed that n/d is a valid X + * coordinate, and 0 is returned if not. Otherwise, 1 is returned. + * + * d being NULL is interpreted as d=1. + * + * Constant time in the value of q, but not any other inputs. + */ +static int secp256k1_ecmult_const_xonly( + secp256k1_fe* r, + const secp256k1_fe *n, + const secp256k1_fe *d, + const secp256k1_scalar *q, + int bits, + int known_on_curve); + #endif /* SECP256K1_ECMULT_CONST_H */ diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h index 12dbcc6c5b..1940ee7f08 100644 --- a/src/ecmult_const_impl.h +++ b/src/ecmult_const_impl.h @@ -228,4 +228,58 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons secp256k1_fe_mul(&r->z, &r->z, &Z); } +static int secp256k1_ecmult_const_xonly(secp256k1_fe* r, const secp256k1_fe *n, const secp256k1_fe *d, const secp256k1_scalar *q, int bits, int known_on_curve) { + + /* This algorithm is a generalization of Peter Dettman's technique for + * avoiding the square root in a random-basepoint x-only multiplication + * on a Weierstrass curve: + * https://mailarchive.ietf.org/arch/msg/cfrg/7DyYY6gg32wDgHAhgSb6XxMDlJA/ + */ + secp256k1_fe g, i; + secp256k1_ge p; + secp256k1_gej rj; + + /* Compute g = (n^3 + B*d^3). */ + secp256k1_fe_sqr(&g, n); + secp256k1_fe_mul(&g, &g, n); + if (d) { + secp256k1_fe b; + secp256k1_fe_sqr(&b, d); + secp256k1_fe_mul(&b, &b, d); + secp256k1_fe_mul(&b, &b, &secp256k1_fe_const_b); + secp256k1_fe_add(&g, &b); + if (!known_on_curve) { + secp256k1_fe c; + secp256k1_fe_mul(&c, &g, d); + if (secp256k1_fe_jacobi_var(&c) < 0) return 0; + } + } else { + secp256k1_fe_add(&g, &secp256k1_fe_const_b); + if (!known_on_curve) { + if (secp256k1_fe_jacobi_var(&g) < 0) return 0; + } + } + + /* Compute base point P = (n*g, g^2), the effective affine version of + * (n*g, g^2, sqrt(d*g)), which has corresponding affine X coordinate + * n/d. */ + secp256k1_fe_mul(&p.x, &g, n); + secp256k1_fe_sqr(&p.y, &g); + p.infinity = 0; + + /* Perform x-only EC multiplication of P with q. */ + secp256k1_ecmult_const(&rj, &p, q, bits); + + /* The resulting (X, Y, Z) point on the effective-affine isomorphic curve + * corresponds to (X, Y, Z*sqrt(d*g)) on the secp256k1 curve. The affine + * version of that has X coordinate (X / (Z^2*d*g)). */ + secp256k1_fe_sqr(&i, &rj.z); + secp256k1_fe_mul(&i, &i, &g); + if (d) secp256k1_fe_mul(&i, &i, d); + secp256k1_fe_inv(&i, &i); + secp256k1_fe_mul(r, &rj.x, &i); + + return 1; +} + #endif /* SECP256K1_ECMULT_CONST_IMPL_H */ diff --git a/src/tests.c b/src/tests.c index 6d3cf83265..1c7cade636 100644 --- a/src/tests.c +++ b/src/tests.c @@ -4347,6 +4347,68 @@ void ecmult_const_mult_zero_one(void) { ge_equals_ge(&res2, &point); } +void ecmult_const_mult_xonly(void) { + int i; + + /* Test correspondence between secp256k1_ecmult_const and secp256k1_ecmult_const_xonly. */ + for (i = 0; i < 2*count; ++i) { + secp256k1_ge base; + secp256k1_gej basej, resj; + secp256k1_fe n, d, resx, v; + secp256k1_scalar q; + int res; + /* Random base point. */ + random_group_element_test(&base); + /* Random scalar to multiply it with. */ + random_scalar_order_test(&q); + /* If i is odd, n=d*base.x for random non-zero d */ + if (i & 1) { + do { + random_field_element_test(&d); + } while (secp256k1_fe_normalizes_to_zero_var(&d)); + secp256k1_fe_mul(&n, &base.x, &d); + } else { + n = base.x; + } + /* Perform x-only multiplication. */ + res = secp256k1_ecmult_const_xonly(&resx, &n, (i & 1) ? &d : NULL, &q, 256, i & 2); + CHECK(res); + /* Perform normal multiplication. */ + secp256k1_gej_set_ge(&basej, &base); + secp256k1_ecmult(&resj, &basej, &q, NULL); + /* Check that resj's X coordinate corresponds with resx. */ + secp256k1_fe_sqr(&v, &resj.z); + secp256k1_fe_mul(&v, &v, &resx); + CHECK(check_fe_equal(&v, &resj.x)); + } + + /* Test that secp256k1_ecmult_const_xonly correctly rejects X coordinates not on curve. */ + for (i = 0; i < 2*count; ++i) { + secp256k1_fe x, n, d, c, r; + int res; + secp256k1_scalar q; + random_scalar_order_test(&q); + /* Generate random X coordinate not on the curve. */ + do { + random_field_element_test(&x); + secp256k1_fe_sqr(&c, &x); + secp256k1_fe_mul(&c, &c, &x); + secp256k1_fe_add(&c, &secp256k1_fe_const_b); + } while (secp256k1_fe_jacobi_var(&c) >= 0); + /* If i is odd, n=d*x for random non-zero d. */ + if (i & 1) { + do { + random_field_element_test(&d); + } while (secp256k1_fe_normalizes_to_zero_var(&d)); + secp256k1_fe_mul(&n, &x, &d); + } else { + n = x; + } + res = secp256k1_ecmult_const_xonly(&r, &n, (i & 1) ? &d : NULL, &q, 256, 0); + CHECK(res == 0); + } +} + void ecmult_const_chain_multiply(void) { /* Check known result (randomly generated test problem from sage) */ const secp256k1_scalar scalar = SECP256K1_SCALAR_CONST( @@ -4378,6 +4440,7 @@ void run_ecmult_const_tests(void) { ecmult_const_random_mult(); ecmult_const_commutativity(); ecmult_const_chain_multiply(); + ecmult_const_mult_xonly(); } typedef struct {