diff options
Diffstat (limited to 'src/scalar_impl.h')
-rw-r--r-- | src/scalar_impl.h | 250 |
1 files changed, 212 insertions, 38 deletions
diff --git a/src/scalar_impl.h b/src/scalar_impl.h index 2ec04b1ae9..fc75891818 100644 --- a/src/scalar_impl.h +++ b/src/scalar_impl.h @@ -7,6 +7,10 @@ #ifndef SECP256K1_SCALAR_IMPL_H #define SECP256K1_SCALAR_IMPL_H +#ifdef VERIFY +#include <string.h> +#endif + #include "scalar.h" #include "util.h" @@ -252,37 +256,65 @@ static void secp256k1_scalar_inverse_var(secp256k1_scalar *r, const secp256k1_sc #endif } -#ifdef USE_ENDOMORPHISM +/* These parameters are generated using sage/gen_exhaustive_groups.sage. */ #if defined(EXHAUSTIVE_TEST_ORDER) +# if EXHAUSTIVE_TEST_ORDER == 13 +# define EXHAUSTIVE_TEST_LAMBDA 9 +# elif EXHAUSTIVE_TEST_ORDER == 199 +# define EXHAUSTIVE_TEST_LAMBDA 92 +# else +# error No known lambda for the specified exhaustive test group order. +# endif + /** - * Find k1 and k2 given k, such that k1 + k2 * lambda == k mod n; unlike in the - * full case we don't bother making k1 and k2 be small, we just want them to be + * Find r1 and r2 given k, such that r1 + r2 * lambda == k mod n; unlike in the + * full case we don't bother making r1 and r2 be small, we just want them to be * nontrivial to get full test coverage for the exhaustive tests. We therefore - * (arbitrarily) set k2 = k + 5 and k1 = k - k2 * lambda. + * (arbitrarily) set r2 = k + 5 (mod n) and r1 = k - r2 * lambda (mod n). */ -static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a) { - *r2 = (*a + 5) % EXHAUSTIVE_TEST_ORDER; - *r1 = (*a + (EXHAUSTIVE_TEST_ORDER - *r2) * EXHAUSTIVE_TEST_LAMBDA) % EXHAUSTIVE_TEST_ORDER; +static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k) { + *r2 = (*k + 5) % EXHAUSTIVE_TEST_ORDER; + *r1 = (*k + (EXHAUSTIVE_TEST_ORDER - *r2) * EXHAUSTIVE_TEST_LAMBDA) % EXHAUSTIVE_TEST_ORDER; } #else /** * The Secp256k1 curve has an endomorphism, where lambda * (x, y) = (beta * x, y), where - * lambda is {0x53,0x63,0xad,0x4c,0xc0,0x5c,0x30,0xe0,0xa5,0x26,0x1c,0x02,0x88,0x12,0x64,0x5a, - * 0x12,0x2e,0x22,0xea,0x20,0x81,0x66,0x78,0xdf,0x02,0x96,0x7c,0x1b,0x23,0xbd,0x72} + * lambda is: */ +static const secp256k1_scalar secp256k1_const_lambda = SECP256K1_SCALAR_CONST( + 0x5363AD4CUL, 0xC05C30E0UL, 0xA5261C02UL, 0x8812645AUL, + 0x122E22EAUL, 0x20816678UL, 0xDF02967CUL, 0x1B23BD72UL +); + +#ifdef VERIFY +static void secp256k1_scalar_split_lambda_verify(const secp256k1_scalar *r1, const secp256k1_scalar *r2, const secp256k1_scalar *k); +#endif + +/* + * Both lambda and beta are primitive cube roots of unity. That is lamba^3 == 1 mod n and + * beta^3 == 1 mod p, where n is the curve order and p is the field order. * - * "Guide to Elliptic Curve Cryptography" (Hankerson, Menezes, Vanstone) gives an algorithm - * (algorithm 3.74) to find k1 and k2 given k, such that k1 + k2 * lambda == k mod n, and k1 - * and k2 have a small size. - * It relies on constants a1, b1, a2, b2. These constants for the value of lambda above are: + * Futhermore, because (X^3 - 1) = (X - 1)(X^2 + X + 1), the primitive cube roots of unity are + * roots of X^2 + X + 1. Therefore lambda^2 + lamba == -1 mod n and beta^2 + beta == -1 mod p. + * (The other primitive cube roots of unity are lambda^2 and beta^2 respectively.) + * + * Let l = -1/2 + i*sqrt(3)/2, the complex root of X^2 + X + 1. We can define a ring + * homomorphism phi : Z[l] -> Z_n where phi(a + b*l) == a + b*lambda mod n. The kernel of phi + * is a lattice over Z[l] (considering Z[l] as a Z-module). This lattice is generated by a + * reduced basis {a1 + b1*l, a2 + b2*l} where * * - a1 = {0x30,0x86,0xd2,0x21,0xa7,0xd4,0x6b,0xcd,0xe8,0x6c,0x90,0xe4,0x92,0x84,0xeb,0x15} * - b1 = -{0xe4,0x43,0x7e,0xd6,0x01,0x0e,0x88,0x28,0x6f,0x54,0x7f,0xa9,0x0a,0xbf,0xe4,0xc3} * - a2 = {0x01,0x14,0xca,0x50,0xf7,0xa8,0xe2,0xf3,0xf6,0x57,0xc1,0x10,0x8d,0x9d,0x44,0xcf,0xd8} * - b2 = {0x30,0x86,0xd2,0x21,0xa7,0xd4,0x6b,0xcd,0xe8,0x6c,0x90,0xe4,0x92,0x84,0xeb,0x15} * - * The algorithm then computes c1 = round(b1 * k / n) and c2 = round(b2 * k / n), and gives + * "Guide to Elliptic Curve Cryptography" (Hankerson, Menezes, Vanstone) gives an algorithm + * (algorithm 3.74) to find k1 and k2 given k, such that k1 + k2 * lambda == k mod n, and k1 + * and k2 are small in absolute value. + * + * The algorithm computes c1 = round(b2 * k / n) and c2 = round((-b1) * k / n), and gives * k1 = k - (c1*a1 + c2*a2) and k2 = -(c1*b1 + c2*b2). Instead, we use modular arithmetic, and - * compute k1 as k - k2 * lambda, avoiding the need for constants a1 and a2. + * compute r2 = k2 mod n, and r1 = k1 mod n = (k - r2 * lambda) mod n, avoiding the need for + * the constants a1 and a2. * * g1, g2 are precomputed constants used to replace division with a rounded multiplication * when decomposing the scalar for an endomorphism-based point multiplication. @@ -294,21 +326,21 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar * Cryptography on Sensor Networks Using the MSP430X Microcontroller" (Gouvea, Oliveira, Lopez), * Section 4.3 (here we use a somewhat higher-precision estimate): * d = a1*b2 - b1*a2 - * g1 = round((2^272)*b2/d) - * g2 = round((2^272)*b1/d) + * g1 = round(2^384 * b2/d) + * g2 = round(2^384 * (-b1)/d) * - * (Note that 'd' is also equal to the curve order here because [a1,b1] and [a2,b2] are found - * as outputs of the Extended Euclidean Algorithm on inputs 'order' and 'lambda'). + * (Note that d is also equal to the curve order, n, here because [a1,b1] and [a2,b2] + * can be found as outputs of the Extended Euclidean Algorithm on inputs n and lambda). * - * The function below splits a in r1 and r2, such that r1 + lambda * r2 == a (mod order). + * The function below splits k into r1 and r2, such that + * - r1 + lambda * r2 == k (mod n) + * - either r1 < 2^128 or -r1 mod n < 2^128 + * - either r2 < 2^128 or -r2 mod n < 2^128 + * + * See proof below. */ - -static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a) { +static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k) { secp256k1_scalar c1, c2; - static const secp256k1_scalar minus_lambda = SECP256K1_SCALAR_CONST( - 0xAC9C52B3UL, 0x3FA3CF1FUL, 0x5AD9E3FDUL, 0x77ED9BA4UL, - 0xA880B9FCUL, 0x8EC739C2UL, 0xE0CFC810UL, 0xB51283CFUL - ); static const secp256k1_scalar minus_b1 = SECP256K1_SCALAR_CONST( 0x00000000UL, 0x00000000UL, 0x00000000UL, 0x00000000UL, 0xE4437ED6UL, 0x010E8828UL, 0x6F547FA9UL, 0x0ABFE4C3UL @@ -318,25 +350,167 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar 0x8A280AC5UL, 0x0774346DUL, 0xD765CDA8UL, 0x3DB1562CUL ); static const secp256k1_scalar g1 = SECP256K1_SCALAR_CONST( - 0x00000000UL, 0x00000000UL, 0x00000000UL, 0x00003086UL, - 0xD221A7D4UL, 0x6BCDE86CUL, 0x90E49284UL, 0xEB153DABUL + 0x3086D221UL, 0xA7D46BCDUL, 0xE86C90E4UL, 0x9284EB15UL, + 0x3DAA8A14UL, 0x71E8CA7FUL, 0xE893209AUL, 0x45DBB031UL ); static const secp256k1_scalar g2 = SECP256K1_SCALAR_CONST( - 0x00000000UL, 0x00000000UL, 0x00000000UL, 0x0000E443UL, - 0x7ED6010EUL, 0x88286F54UL, 0x7FA90ABFUL, 0xE4C42212UL + 0xE4437ED6UL, 0x010E8828UL, 0x6F547FA9UL, 0x0ABFE4C4UL, + 0x221208ACUL, 0x9DF506C6UL, 0x1571B4AEUL, 0x8AC47F71UL ); - VERIFY_CHECK(r1 != a); - VERIFY_CHECK(r2 != a); + VERIFY_CHECK(r1 != k); + VERIFY_CHECK(r2 != k); /* these _var calls are constant time since the shift amount is constant */ - secp256k1_scalar_mul_shift_var(&c1, a, &g1, 272); - secp256k1_scalar_mul_shift_var(&c2, a, &g2, 272); + secp256k1_scalar_mul_shift_var(&c1, k, &g1, 384); + secp256k1_scalar_mul_shift_var(&c2, k, &g2, 384); secp256k1_scalar_mul(&c1, &c1, &minus_b1); secp256k1_scalar_mul(&c2, &c2, &minus_b2); secp256k1_scalar_add(r2, &c1, &c2); - secp256k1_scalar_mul(r1, r2, &minus_lambda); - secp256k1_scalar_add(r1, r1, a); -} -#endif + secp256k1_scalar_mul(r1, r2, &secp256k1_const_lambda); + secp256k1_scalar_negate(r1, r1); + secp256k1_scalar_add(r1, r1, k); + +#ifdef VERIFY + secp256k1_scalar_split_lambda_verify(r1, r2, k); #endif +} + +#ifdef VERIFY +/* + * Proof for secp256k1_scalar_split_lambda's bounds. + * + * Let + * - epsilon1 = 2^256 * |g1/2^384 - b2/d| + * - epsilon2 = 2^256 * |g2/2^384 - (-b1)/d| + * - c1 = round(k*g1/2^384) + * - c2 = round(k*g2/2^384) + * + * Lemma 1: |c1 - k*b2/d| < 2^-1 + epsilon1 + * + * |c1 - k*b2/d| + * = + * |c1 - k*g1/2^384 + k*g1/2^384 - k*b2/d| + * <= {triangle inequality} + * |c1 - k*g1/2^384| + |k*g1/2^384 - k*b2/d| + * = + * |c1 - k*g1/2^384| + k*|g1/2^384 - b2/d| + * < {rounding in c1 and 0 <= k < 2^256} + * 2^-1 + 2^256 * |g1/2^384 - b2/d| + * = {definition of epsilon1} + * 2^-1 + epsilon1 + * + * Lemma 2: |c2 - k*(-b1)/d| < 2^-1 + epsilon2 + * + * |c2 - k*(-b1)/d| + * = + * |c2 - k*g2/2^384 + k*g2/2^384 - k*(-b1)/d| + * <= {triangle inequality} + * |c2 - k*g2/2^384| + |k*g2/2^384 - k*(-b1)/d| + * = + * |c2 - k*g2/2^384| + k*|g2/2^384 - (-b1)/d| + * < {rounding in c2 and 0 <= k < 2^256} + * 2^-1 + 2^256 * |g2/2^384 - (-b1)/d| + * = {definition of epsilon2} + * 2^-1 + epsilon2 + * + * Let + * - k1 = k - c1*a1 - c2*a2 + * - k2 = - c1*b1 - c2*b2 + * + * Lemma 3: |k1| < (a1 + a2 + 1)/2 < 2^128 + * + * |k1| + * = {definition of k1} + * |k - c1*a1 - c2*a2| + * = {(a1*b2 - b1*a2)/n = 1} + * |k*(a1*b2 - b1*a2)/n - c1*a1 - c2*a2| + * = + * |a1*(k*b2/n - c1) + a2*(k*(-b1)/n - c2)| + * <= {triangle inequality} + * a1*|k*b2/n - c1| + a2*|k*(-b1)/n - c2| + * < {Lemma 1 and Lemma 2} + * a1*(2^-1 + epslion1) + a2*(2^-1 + epsilon2) + * < {rounding up to an integer} + * (a1 + a2 + 1)/2 + * < {rounding up to a power of 2} + * 2^128 + * + * Lemma 4: |k2| < (-b1 + b2)/2 + 1 < 2^128 + * + * |k2| + * = {definition of k2} + * |- c1*a1 - c2*a2| + * = {(b1*b2 - b1*b2)/n = 0} + * |k*(b1*b2 - b1*b2)/n - c1*b1 - c2*b2| + * = + * |b1*(k*b2/n - c1) + b2*(k*(-b1)/n - c2)| + * <= {triangle inequality} + * (-b1)*|k*b2/n - c1| + b2*|k*(-b1)/n - c2| + * < {Lemma 1 and Lemma 2} + * (-b1)*(2^-1 + epslion1) + b2*(2^-1 + epsilon2) + * < {rounding up to an integer} + * (-b1 + b2)/2 + 1 + * < {rounding up to a power of 2} + * 2^128 + * + * Let + * - r2 = k2 mod n + * - r1 = k - r2*lambda mod n. + * + * Notice that r1 is defined such that r1 + r2 * lambda == k (mod n). + * + * Lemma 5: r1 == k1 mod n. + * + * r1 + * == {definition of r1 and r2} + * k - k2*lambda + * == {definition of k2} + * k - (- c1*b1 - c2*b2)*lambda + * == + * k + c1*b1*lambda + c2*b2*lambda + * == {a1 + b1*lambda == 0 mod n and a2 + b2*lambda == 0 mod n} + * k - c1*a1 - c2*a2 + * == {definition of k1} + * k1 + * + * From Lemma 3, Lemma 4, Lemma 5 and the definition of r2, we can conclude that + * + * - either r1 < 2^128 or -r1 mod n < 2^128 + * - either r2 < 2^128 or -r2 mod n < 2^128. + * + * Q.E.D. + */ +static void secp256k1_scalar_split_lambda_verify(const secp256k1_scalar *r1, const secp256k1_scalar *r2, const secp256k1_scalar *k) { + secp256k1_scalar s; + unsigned char buf1[32]; + unsigned char buf2[32]; + + /* (a1 + a2 + 1)/2 is 0xa2a8918ca85bafe22016d0b917e4dd77 */ + static const unsigned char k1_bound[32] = { + 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, + 0xa2, 0xa8, 0x91, 0x8c, 0xa8, 0x5b, 0xaf, 0xe2, 0x20, 0x16, 0xd0, 0xb9, 0x17, 0xe4, 0xdd, 0x77 + }; + + /* (-b1 + b2)/2 + 1 is 0x8a65287bd47179fb2be08846cea267ed */ + static const unsigned char k2_bound[32] = { + 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, + 0x8a, 0x65, 0x28, 0x7b, 0xd4, 0x71, 0x79, 0xfb, 0x2b, 0xe0, 0x88, 0x46, 0xce, 0xa2, 0x67, 0xed + }; + + secp256k1_scalar_mul(&s, &secp256k1_const_lambda, r2); + secp256k1_scalar_add(&s, &s, r1); + VERIFY_CHECK(secp256k1_scalar_eq(&s, k)); + + secp256k1_scalar_negate(&s, r1); + secp256k1_scalar_get_b32(buf1, r1); + secp256k1_scalar_get_b32(buf2, &s); + VERIFY_CHECK(secp256k1_memcmp_var(buf1, k1_bound, 32) < 0 || secp256k1_memcmp_var(buf2, k1_bound, 32) < 0); + + secp256k1_scalar_negate(&s, r2); + secp256k1_scalar_get_b32(buf1, r2); + secp256k1_scalar_get_b32(buf2, &s); + VERIFY_CHECK(secp256k1_memcmp_var(buf1, k2_bound, 32) < 0 || secp256k1_memcmp_var(buf2, k2_bound, 32) < 0); +} +#endif /* VERIFY */ +#endif /* !defined(EXHAUSTIVE_TEST_ORDER) */ #endif /* SECP256K1_SCALAR_IMPL_H */ |