From acab934d24ff26289ab9930587c3fc51c30c6a2f Mon Sep 17 00:00:00 2001 From: Russell O'Connor Date: Tue, 22 Sep 2020 11:01:47 -0400 Subject: [PATCH] Detailed comments for secp256k1_scalar_split_lambda --- src/scalar.h | 9 +-- src/scalar_4x64_impl.h | 10 +-- src/scalar_8x32_impl.h | 18 ++--- src/scalar_impl.h | 152 ++++++++++++++++++++++++++++++++++++----- 4 files changed, 154 insertions(+), 35 deletions(-) diff --git a/src/scalar.h b/src/scalar.h index 95d3e32..ed01c7e 100644 --- a/src/scalar.h +++ b/src/scalar.h @@ -103,10 +103,11 @@ static void secp256k1_scalar_order_get_num(secp256k1_num *r); static int secp256k1_scalar_eq(const secp256k1_scalar *a, const secp256k1_scalar *b); #ifdef USE_ENDOMORPHISM -/** Find r1 and r2 such that r1+r2*2^128 = a. */ -static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a); -/** Find r1 and r2 such that r1+r2*lambda = a, and r1 and r2 are maximum 128 bits long (see secp256k1_gej_mul_lambda). */ -static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a); +/** Find r1 and r2 such that r1+r2*2^128 = k. */ +static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k); +/** Find r1 and r2 such that r1+r2*lambda = k, + * where r1 and r2 or their negations are maximum 128 bits long (see secp256k1_ge_mul_lambda). */ +static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k); #endif /** Multiply a and b (without taking the modulus!), divide by 2**shift, and round to the nearest integer. Shift must be at least 256. */ diff --git a/src/scalar_4x64_impl.h b/src/scalar_4x64_impl.h index 7f39927..3d8d2a5 100644 --- a/src/scalar_4x64_impl.h +++ b/src/scalar_4x64_impl.h @@ -913,13 +913,13 @@ static void secp256k1_scalar_sqr(secp256k1_scalar *r, const secp256k1_scalar *a) } #ifdef USE_ENDOMORPHISM -static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a) { - r1->d[0] = a->d[0]; - r1->d[1] = a->d[1]; +static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k) { + r1->d[0] = k->d[0]; + r1->d[1] = k->d[1]; r1->d[2] = 0; r1->d[3] = 0; - r2->d[0] = a->d[2]; - r2->d[1] = a->d[3]; + r2->d[0] = k->d[2]; + r2->d[1] = k->d[3]; r2->d[2] = 0; r2->d[3] = 0; } diff --git a/src/scalar_8x32_impl.h b/src/scalar_8x32_impl.h index f8c7fa7..44561c8 100644 --- a/src/scalar_8x32_impl.h +++ b/src/scalar_8x32_impl.h @@ -673,19 +673,19 @@ static void secp256k1_scalar_sqr(secp256k1_scalar *r, const secp256k1_scalar *a) } #ifdef USE_ENDOMORPHISM -static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *a) { - r1->d[0] = a->d[0]; - r1->d[1] = a->d[1]; - r1->d[2] = a->d[2]; - r1->d[3] = a->d[3]; +static void secp256k1_scalar_split_128(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k) { + r1->d[0] = k->d[0]; + r1->d[1] = k->d[1]; + r1->d[2] = k->d[2]; + r1->d[3] = k->d[3]; r1->d[4] = 0; r1->d[5] = 0; r1->d[6] = 0; r1->d[7] = 0; - r2->d[0] = a->d[4]; - r2->d[1] = a->d[5]; - r2->d[2] = a->d[6]; - r2->d[3] = a->d[7]; + r2->d[0] = k->d[4]; + r2->d[1] = k->d[5]; + r2->d[2] = k->d[6]; + r2->d[3] = k->d[7]; r2->d[4] = 0; r2->d[5] = 0; r2->d[6] = 0; diff --git a/src/scalar_impl.h b/src/scalar_impl.h index 36c279d..e2a0363 100644 --- a/src/scalar_impl.h +++ b/src/scalar_impl.h @@ -279,19 +279,31 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar * 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} * - * "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: + * 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. + * + * 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 have a small size. + * + * 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 k - k2 * lambda (mod n) which is equivalent to k1 (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. @@ -303,16 +315,122 @@ 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((2^384)*b2/d) - * g2 = round((2^384)*(-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 + * + * Proof. + * + * 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(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, @@ -334,16 +452,16 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar 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, 384); - secp256k1_scalar_mul_shift_var(&c2, a, &g2, 384); + 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); + secp256k1_scalar_add(r1, r1, k); } #endif #endif