From 926836ad36778371a98c9f35d0b84d3cc25c08fd Mon Sep 17 00:00:00 2001 From: Pieter Wuille Date: Sat, 9 Jul 2016 14:23:44 +0200 Subject: [PATCH] Make secp256k1_fe_sqrt constant time --- src/bench_internal.c | 6 +++--- src/field.h | 5 ++++- src/field_impl.h | 13 ++++++++++--- src/group.h | 2 +- src/group_impl.h | 6 +++--- src/tests.c | 6 +++--- 6 files changed, 24 insertions(+), 14 deletions(-) diff --git a/src/bench_internal.c b/src/bench_internal.c index d24ddc2..0809f77 100644 --- a/src/bench_internal.c +++ b/src/bench_internal.c @@ -181,12 +181,12 @@ void bench_field_inverse_var(void* arg) { } } -void bench_field_sqrt_var(void* arg) { +void bench_field_sqrt(void* arg) { int i; bench_inv_t *data = (bench_inv_t*)arg; for (i = 0; i < 20000; i++) { - secp256k1_fe_sqrt_var(&data->fe_x, &data->fe_x); + secp256k1_fe_sqrt(&data->fe_x, &data->fe_x); secp256k1_fe_add(&data->fe_x, &data->fe_y); } } @@ -357,7 +357,7 @@ int main(int argc, char **argv) { if (have_flag(argc, argv, "field") || have_flag(argc, argv, "mul")) run_benchmark("field_mul", bench_field_mul, bench_setup, NULL, &data, 10, 200000); if (have_flag(argc, argv, "field") || have_flag(argc, argv, "inverse")) run_benchmark("field_inverse", bench_field_inverse, bench_setup, NULL, &data, 10, 20000); if (have_flag(argc, argv, "field") || have_flag(argc, argv, "inverse")) run_benchmark("field_inverse_var", bench_field_inverse_var, bench_setup, NULL, &data, 10, 20000); - if (have_flag(argc, argv, "field") || have_flag(argc, argv, "sqrt")) run_benchmark("field_sqrt_var", bench_field_sqrt_var, bench_setup, NULL, &data, 10, 20000); + if (have_flag(argc, argv, "field") || have_flag(argc, argv, "sqrt")) run_benchmark("field_sqrt", bench_field_sqrt, bench_setup, NULL, &data, 10, 20000); if (have_flag(argc, argv, "group") || have_flag(argc, argv, "double")) run_benchmark("group_double_var", bench_group_double_var, bench_setup, NULL, &data, 10, 200000); if (have_flag(argc, argv, "group") || have_flag(argc, argv, "add")) run_benchmark("group_add_var", bench_group_add_var, bench_setup, NULL, &data, 10, 200000); diff --git a/src/field.h b/src/field.h index 47d4685..c5ba074 100644 --- a/src/field.h +++ b/src/field.h @@ -57,6 +57,9 @@ static int secp256k1_fe_is_zero(const secp256k1_fe *a); static int secp256k1_fe_is_odd(const secp256k1_fe *a); /** Compare two field elements. Requires magnitude-1 inputs. */ +static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b); + +/** Same as secp256k1_fe_equal, but may be variable time. */ static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b); /** Compare two field elements. Requires both inputs to be normalized */ @@ -92,7 +95,7 @@ static void secp256k1_fe_sqr(secp256k1_fe *r, const secp256k1_fe *a); * The input's magnitude can be at most 8. The output magnitude is 1 (but not * guaranteed to be normalized). The result in r will always be a square * itself. */ -static int secp256k1_fe_sqrt_var(secp256k1_fe *r, const secp256k1_fe *a); +static int secp256k1_fe_sqrt(secp256k1_fe *r, const secp256k1_fe *a); /** Checks whether a field element is a quadratic residue. */ static int secp256k1_fe_is_quad_var(const secp256k1_fe *a); diff --git a/src/field_impl.h b/src/field_impl.h index 00ba6c8..52cd902 100644 --- a/src/field_impl.h +++ b/src/field_impl.h @@ -21,6 +21,13 @@ #error "Please select field implementation" #endif +SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b) { + secp256k1_fe na; + secp256k1_fe_negate(&na, a, 1); + secp256k1_fe_add(&na, b); + return secp256k1_fe_normalizes_to_zero(&na); +} + SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b) { secp256k1_fe na; secp256k1_fe_negate(&na, a, 1); @@ -28,7 +35,7 @@ SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const return secp256k1_fe_normalizes_to_zero_var(&na); } -static int secp256k1_fe_sqrt_var(secp256k1_fe *r, const secp256k1_fe *a) { +static int secp256k1_fe_sqrt(secp256k1_fe *r, const secp256k1_fe *a) { /** Given that p is congruent to 3 mod 4, we can compute the square root of * a mod p as the (p+1)/4'th power of a. * @@ -123,7 +130,7 @@ static int secp256k1_fe_sqrt_var(secp256k1_fe *r, const secp256k1_fe *a) { /* Check that a square root was actually calculated */ secp256k1_fe_sqr(&t1, r); - return secp256k1_fe_equal_var(&t1, a); + return secp256k1_fe_equal(&t1, a); } static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *a) { @@ -301,7 +308,7 @@ static int secp256k1_fe_is_quad_var(const secp256k1_fe *a) { return secp256k1_num_jacobi(&n, &m) >= 0; #else secp256k1_fe r; - return secp256k1_fe_sqrt_var(&r, a) == 1; + return secp256k1_fe_sqrt(&r, a); #endif } diff --git a/src/group.h b/src/group.h index 852bb64..d515716 100644 --- a/src/group.h +++ b/src/group.h @@ -47,7 +47,7 @@ static void secp256k1_ge_set_xy(secp256k1_ge *r, const secp256k1_fe *x, const se * and a Y coordinate that is a quadratic residue modulo p. The return value * is true iff a coordinate with the given X coordinate exists. */ -static int secp256k1_ge_set_xquad_var(secp256k1_ge *r, const secp256k1_fe *x); +static int secp256k1_ge_set_xquad(secp256k1_ge *r, const secp256k1_fe *x); /** Set a group element (affine) equal to the point with the given X coordinate, and given oddness * for Y. Return value indicates whether the result is valid. */ diff --git a/src/group_impl.h b/src/group_impl.h index 616866a..3e9c4c4 100644 --- a/src/group_impl.h +++ b/src/group_impl.h @@ -163,7 +163,7 @@ static void secp256k1_ge_clear(secp256k1_ge *r) { secp256k1_fe_clear(&r->y); } -static int secp256k1_ge_set_xquad_var(secp256k1_ge *r, const secp256k1_fe *x) { +static int secp256k1_ge_set_xquad(secp256k1_ge *r, const secp256k1_fe *x) { secp256k1_fe x2, x3, c; r->x = *x; secp256k1_fe_sqr(&x2, x); @@ -171,11 +171,11 @@ static int secp256k1_ge_set_xquad_var(secp256k1_ge *r, const secp256k1_fe *x) { r->infinity = 0; secp256k1_fe_set_int(&c, 7); secp256k1_fe_add(&c, &x3); - return secp256k1_fe_sqrt_var(&r->y, &c); + return secp256k1_fe_sqrt(&r->y, &c); } static int secp256k1_ge_set_xo_var(secp256k1_ge *r, const secp256k1_fe *x, int odd) { - if (!secp256k1_ge_set_xquad_var(r, x)) { + if (!secp256k1_ge_set_xquad(r, x)) { return 0; } secp256k1_fe_normalize_var(&r->y); diff --git a/src/tests.c b/src/tests.c index bb3014f..b32cb90 100644 --- a/src/tests.c +++ b/src/tests.c @@ -1574,7 +1574,7 @@ void random_fe_non_zero(secp256k1_fe *nz) { void random_fe_non_square(secp256k1_fe *ns) { secp256k1_fe r; random_fe_non_zero(ns); - if (secp256k1_fe_sqrt_var(&r, ns)) { + if (secp256k1_fe_sqrt(&r, ns)) { secp256k1_fe_negate(ns, ns, 1); } } @@ -1769,7 +1769,7 @@ void run_sqr(void) { void test_sqrt(const secp256k1_fe *a, const secp256k1_fe *k) { secp256k1_fe r1, r2; - int v = secp256k1_fe_sqrt_var(&r1, a); + int v = secp256k1_fe_sqrt(&r1, a); CHECK((v == 0) == (k == NULL)); if (k != NULL) { @@ -2188,7 +2188,7 @@ void test_group_decompress(const secp256k1_fe* x) { secp256k1_fe_normalize_var(&fex); - res_quad = secp256k1_ge_set_xquad_var(&ge_quad, &fex); + res_quad = secp256k1_ge_set_xquad(&ge_quad, &fex); res_even = secp256k1_ge_set_xo_var(&ge_even, &fex, 0); res_odd = secp256k1_ge_set_xo_var(&ge_odd, &fex, 1);