/*********************************************************************** * Distributed under the MIT software license, see the accompanying * * file COPYING or https://www.opensource.org/licenses/mit-license.php.* ***********************************************************************/ #ifndef SECP256K1_MODULE_ELLSWIFT_MAIN_H #define SECP256K1_MODULE_ELLSWIFT_MAIN_H #include "../../../include/secp256k1.h" #include "../../../include/secp256k1_ellswift.h" #include "../../eckey.h" #include "../../hash.h" /** c1 = (sqrt(-3)-1)/2 */ static const secp256k1_fe secp256k1_ellswift_c1 = SECP256K1_FE_CONST(0x851695d4, 0x9a83f8ef, 0x919bb861, 0x53cbcb16, 0x630fb68a, 0xed0a766a, 0x3ec693d6, 0x8e6afa40); /** c2 = (-sqrt(-3)-1)/2 = -(c1+1) */ static const secp256k1_fe secp256k1_ellswift_c2 = SECP256K1_FE_CONST(0x7ae96a2b, 0x657c0710, 0x6e64479e, 0xac3434e9, 0x9cf04975, 0x12f58995, 0xc1396c28, 0x719501ee); /** c3 = (-sqrt(-3)+1)/2 = -c1 = c2+1 */ static const secp256k1_fe secp256k1_ellswift_c3 = SECP256K1_FE_CONST(0x7ae96a2b, 0x657c0710, 0x6e64479e, 0xac3434e9, 0x9cf04975, 0x12f58995, 0xc1396c28, 0x719501ef); /** c4 = (sqrt(-3)+1)/2 = -c2 = c1+1 */ static const secp256k1_fe secp256k1_ellswift_c4 = SECP256K1_FE_CONST(0x851695d4, 0x9a83f8ef, 0x919bb861, 0x53cbcb16, 0x630fb68a, 0xed0a766a, 0x3ec693d6, 0x8e6afa41); /** Decode ElligatorSwift encoding (u, t) to a fraction xn/xd representing a curve X coordinate. */ static void secp256k1_ellswift_xswiftec_frac_var(secp256k1_fe *xn, secp256k1_fe *xd, const secp256k1_fe *u, const secp256k1_fe *t) { /* The implemented algorithm is the following (all operations in GF(p)): * * - Let c0 = sqrt(-3) = 0xa2d2ba93507f1df233770c2a797962cc61f6d15da14ecd47d8d27ae1cd5f852. * - If u = 0, set u = 1. * - If t = 0, set t = 1. * - If u^3+7+t^2 = 0, set t = 2*t. * - Let X = (u^3+7-t^2)/(2*t). * - Let Y = (X+t)/(c0*u). * - If x3 = u+4*Y^2 is a valid x coordinate, return it. * - If x2 = (-X/Y-u)/2 is a valid x coordinate, return it. * - Return x1 = (X/Y-u)/2 (which is now guaranteed to be a valid x coordinate). * * Introducing s=t^2, g=u^3+7, and simplifying x1=-(x2+u) we get: * * - Let c0 = ... * - If u = 0, set u = 1. * - If t = 0, set t = 1. * - Let s = t^2 * - Let g = u^3+7 * - If g+s = 0, set t = 2*t, s = 4*s * - Let X = (g-s)/(2*t). * - Let Y = (X+t)/(c0*u) = (g+s)/(2*c0*t*u). * - If x3 = u+4*Y^2 is a valid x coordinate, return it. * - If x2 = (-X/Y-u)/2 is a valid x coordinate, return it. * - Return x1 = -(x2+u). * * Now substitute Y^2 = -(g+s)^2/(12*s*u^2) and X/Y = c0*u*(g-s)/(g+s). This * means X and Y do not need to be evaluated explicitly anymore. * * - ... * - If g+s = 0, set s = 4*s. * - If x3 = u-(g+s)^2/(3*s*u^2) is a valid x coordinate, return it. * - If x2 = (-c0*u*(g-s)/(g+s)-u)/2 is a valid x coordinate, return it. * - Return x1 = -(x2+u). * * Simplifying x2 using 2 additional constants: * * - Let c1 = (c0-1)/2 = 0x851695d49a83f8ef919bb86153cbcb16630fb68aed0a766a3ec693d68e6afa40. * - Let c2 = (-c0-1)/2 = 0x7ae96a2b657c07106e64479eac3434e99cf0497512f58995c1396c28719501ee. * - ... * - If x2 = u*(c1*s+c2*g)/(g+s) is a valid x coordinate, return it. * - ... * * Writing x3 as a fraction: * * - ... * - If x3 = (3*s*u^3-(g+s)^2)/(3*s*u^2) ... * - ... * Overall, we get: * * - Let c1 = 0x851695d49a83f8ef919bb86153cbcb16630fb68aed0a766a3ec693d68e6afa40. * - Let c2 = 0x7ae96a2b657c07106e64479eac3434e99cf0497512f58995c1396c28719501ee. * - If u = 0, set u = 1. * - If t = 0, set s = 1, else set s = t^2. * - Let g = u^3+7. * - If g+s = 0, set s = 4*s. * - If x3 = (3*s*u^3-(g+s)^2)/(3*s*u^2) is a valid x coordinate, return it. * - If x2 = u*(c1*s+c2*g)/(g+s) is a valid x coordinate, return it. * - Return x1 = -(x2+u). */ secp256k1_fe u1, s, g, p, d, n, l; u1 = *u; if (EXPECT(secp256k1_fe_normalizes_to_zero_var(&u1), 0)) u1 = secp256k1_fe_one; secp256k1_fe_sqr(&s, t); if (EXPECT(secp256k1_fe_normalizes_to_zero_var(t), 0)) s = secp256k1_fe_one; secp256k1_fe_sqr(&l, &u1); /* l = u^2 */ secp256k1_fe_mul(&g, &l, &u1); /* g = u^3 */ secp256k1_fe_add_int(&g, SECP256K1_B); /* g = u^3 + 7 */ p = g; /* p = g */ secp256k1_fe_add(&p, &s); /* p = g+s */ if (EXPECT(secp256k1_fe_normalizes_to_zero_var(&p), 0)) { secp256k1_fe_mul_int(&s, 4); /* Recompute p = g+s */ p = g; /* p = g */ secp256k1_fe_add(&p, &s); /* p = g+s */ } secp256k1_fe_mul(&d, &s, &l); /* d = s*u^2 */ secp256k1_fe_mul_int(&d, 3); /* d = 3*s*u^2 */ secp256k1_fe_sqr(&l, &p); /* l = (g+s)^2 */ secp256k1_fe_negate(&l, &l, 1); /* l = -(g+s)^2 */ secp256k1_fe_mul(&n, &d, &u1); /* n = 3*s*u^3 */ secp256k1_fe_add(&n, &l); /* n = 3*s*u^3-(g+s)^2 */ if (secp256k1_ge_x_frac_on_curve_var(&n, &d)) { /* Return x3 = n/d = (3*s*u^3-(g+s)^2)/(3*s*u^2) */ *xn = n; *xd = d; return; } *xd = p; secp256k1_fe_mul(&l, &secp256k1_ellswift_c1, &s); /* l = c1*s */ secp256k1_fe_mul(&n, &secp256k1_ellswift_c2, &g); /* n = c2*g */ secp256k1_fe_add(&n, &l); /* n = c1*s+c2*g */ secp256k1_fe_mul(&n, &n, &u1); /* n = u*(c1*s+c2*g) */ /* Possible optimization: in the invocation below, p^2 = (g+s)^2 is computed, * which we already have computed above. This could be deduplicated. */ if (secp256k1_ge_x_frac_on_curve_var(&n, &p)) { /* Return x2 = n/p = u*(c1*s+c2*g)/(g+s) */ *xn = n; return; } secp256k1_fe_mul(&l, &p, &u1); /* l = u*(g+s) */ secp256k1_fe_add(&n, &l); /* n = u*(c1*s+c2*g)+u*(g+s) */ secp256k1_fe_negate(xn, &n, 2); /* n = -u*(c1*s+c2*g)-u*(g+s) */ VERIFY_CHECK(secp256k1_ge_x_frac_on_curve_var(xn, &p)); /* Return x3 = n/p = -(u*(c1*s+c2*g)/(g+s)+u) */ } /** Decode ElligatorSwift encoding (u, t) to X coordinate. */ static void secp256k1_ellswift_xswiftec_var(secp256k1_fe *x, const secp256k1_fe *u, const secp256k1_fe *t) { secp256k1_fe xn, xd; secp256k1_ellswift_xswiftec_frac_var(&xn, &xd, u, t); secp256k1_fe_inv_var(&xd, &xd); secp256k1_fe_mul(x, &xn, &xd); } /** Decode ElligatorSwift encoding (u, t) to point P. */ static void secp256k1_ellswift_swiftec_var(secp256k1_ge *p, const secp256k1_fe *u, const secp256k1_fe *t) { secp256k1_fe x; secp256k1_ellswift_xswiftec_var(&x, u, t); secp256k1_ge_set_xo_var(p, &x, secp256k1_fe_is_odd(t)); } /* Try to complete an ElligatorSwift encoding (u, t) for X coordinate x, given u and x. * * There may be up to 8 distinct t values such that (u, t) decodes back to x, but also * fewer, or none at all. Each such partial inverse can be accessed individually using a * distinct input argument c (in range 0-7), and some or all of these may return failure. * The following guarantees exist: * - Given (x, u), no two distinct c values give the same successful result t. * - Every successful result maps back to x through secp256k1_ellswift_xswiftec_var. * - Given (x, u), all t values that map back to x can be reached by combining the * successful results from this function over all c values, with the exception of: * - this function cannot be called with u=0 * - no result with t=0 will be returned * - no result for which u^3 + t^2 + 7 = 0 will be returned. * * The rather unusual encoding of bits in c (a large "if" based on the middle bit, and then * using the low and high bits to pick signs of square roots) is to match the paper's * encoding more closely: c=0 through c=3 match branches 1..4 in the paper, while c=4 through * c=7 are copies of those with an additional negation of sqrt(w). */ static int secp256k1_ellswift_xswiftec_inv_var(secp256k1_fe *t, const secp256k1_fe *x_in, const secp256k1_fe *u_in, int c) { /* The implemented algorithm is this (all arithmetic, except involving c, is mod p): * * - If (c & 2) = 0: * - If (-x-u) is a valid X coordinate, fail. * - Let s=-(u^3+7)/(u^2+u*x+x^2). * - If s is not square, fail. * - Let v=x. * - If (c & 2) = 2: * - Let s=x-u. * - If s is not square, fail. * - Let r=sqrt(-s*(4*(u^3+7)+3*u^2*s)); fail if it doesn't exist. * - If (c & 1) = 1 and r = 0, fail. * - If s=0, fail. * - Let v=(r/s-u)/2. * - Let w=sqrt(s). * - If (c & 5) = 0: return -w*(c3*u + v). * - If (c & 5) = 1: return w*(c4*u + v). * - If (c & 5) = 4: return w*(c3*u + v). * - If (c & 5) = 5: return -w*(c4*u + v). */ secp256k1_fe x = *x_in, u = *u_in, g, v, s, m, r, q; int ret; secp256k1_fe_normalize_weak(&x); secp256k1_fe_normalize_weak(&u); VERIFY_CHECK(c >= 0 && c < 8); VERIFY_CHECK(secp256k1_ge_x_on_curve_var(&x)); if (!(c & 2)) { /* c is in {0, 1, 4, 5}. In this case we look for an inverse under the x1 (if c=0 or * c=4) formula, or x2 (if c=1 or c=5) formula. */ /* If -u-x is a valid X coordinate, fail. This would yield an encoding that roundtrips * back under the x3 formula instead (which has priority over x1 and x2, so the decoding * would not match x). */ m = x; /* m = x */ secp256k1_fe_add(&m, &u); /* m = u+x */ secp256k1_fe_negate(&m, &m, 2); /* m = -u-x */ /* Test if (-u-x) is a valid X coordinate. If so, fail. */ if (secp256k1_ge_x_on_curve_var(&m)) return 0; /* Let s = -(u^3 + 7)/(u^2 + u*x + x^2) [first part] */ secp256k1_fe_sqr(&s, &m); /* s = (u+x)^2 */ secp256k1_fe_negate(&s, &s, 1); /* s = -(u+x)^2 */ secp256k1_fe_mul(&m, &u, &x); /* m = u*x */ secp256k1_fe_add(&s, &m); /* s = -(u^2 + u*x + x^2) */ /* Note that at this point, s = 0 is impossible. If it were the case: * s = -(u^2 + u*x + x^2) = 0 * => u^2 + u*x + x^2 = 0 * => (u + 2*x) * (u^2 + u*x + x^2) = 0 * => 2*x^3 + 3*x^2*u + 3*x*u^2 + u^3 = 0 * => (x + u)^3 + x^3 = 0 * => x^3 = -(x + u)^3 * => x^3 + B = (-u - x)^3 + B * * However, we know x^3 + B is square (because x is on the curve) and * that (-u-x)^3 + B is not square (the secp256k1_ge_x_on_curve_var(&m) * test above would have failed). This is a contradiction, and thus the * assumption s=0 is false. */ VERIFY_CHECK(!secp256k1_fe_normalizes_to_zero_var(&s)); /* If s is not square, fail. We have not fully computed s yet, but s is square iff * -(u^3+7)*(u^2+u*x+x^2) is square (because a/b is square iff a*b is square and b is * nonzero). */ secp256k1_fe_sqr(&g, &u); /* g = u^2 */ secp256k1_fe_mul(&g, &g, &u); /* g = u^3 */ secp256k1_fe_add_int(&g, SECP256K1_B); /* g = u^3+7 */ secp256k1_fe_mul(&m, &s, &g); /* m = -(u^3 + 7)*(u^2 + u*x + x^2) */ if (!secp256k1_fe_is_square_var(&m)) return 0; /* Let s = -(u^3 + 7)/(u^2 + u*x + x^2) [second part] */ secp256k1_fe_inv_var(&s, &s); /* s = -1/(u^2 + u*x + x^2) [no div by 0] */ secp256k1_fe_mul(&s, &s, &g); /* s = -(u^3 + 7)/(u^2 + u*x + x^2) */ /* Let v = x. */ v = x; } else { /* c is in {2, 3, 6, 7}. In this case we look for an inverse under the x3 formula. */ /* Let s = x-u. */ secp256k1_fe_negate(&m, &u, 1); /* m = -u */ s = m; /* s = -u */ secp256k1_fe_add(&s, &x); /* s = x-u */ /* If s is not square, fail. */ if (!secp256k1_fe_is_square_var(&s)) return 0; /* Let r = sqrt(-s*(4*(u^3+7)+3*u^2*s)); fail if it doesn't exist. */ secp256k1_fe_sqr(&g, &u); /* g = u^2 */ secp256k1_fe_mul(&q, &s, &g); /* q = s*u^2 */ secp256k1_fe_mul_int(&q, 3); /* q = 3*s*u^2 */ secp256k1_fe_mul(&g, &g, &u); /* g = u^3 */ secp256k1_fe_mul_int(&g, 4); /* g = 4*u^3 */ secp256k1_fe_add_int(&g, 4 * SECP256K1_B); /* g = 4*(u^3+7) */ secp256k1_fe_add(&q, &g); /* q = 4*(u^3+7)+3*s*u^2 */ secp256k1_fe_mul(&q, &q, &s); /* q = s*(4*(u^3+7)+3*u^2*s) */ secp256k1_fe_negate(&q, &q, 1); /* q = -s*(4*(u^3+7)+3*u^2*s) */ if (!secp256k1_fe_is_square_var(&q)) return 0; ret = secp256k1_fe_sqrt(&r, &q); /* r = sqrt(-s*(4*(u^3+7)+3*u^2*s)) */ #ifdef VERIFY VERIFY_CHECK(ret); #else (void)ret; #endif /* If (c & 1) = 1 and r = 0, fail. */ if (EXPECT((c & 1) && secp256k1_fe_normalizes_to_zero_var(&r), 0)) return 0; /* If s = 0, fail. */ if (EXPECT(secp256k1_fe_normalizes_to_zero_var(&s), 0)) return 0; /* Let v = (r/s-u)/2. */ secp256k1_fe_inv_var(&v, &s); /* v = 1/s [no div by 0] */ secp256k1_fe_mul(&v, &v, &r); /* v = r/s */ secp256k1_fe_add(&v, &m); /* v = r/s-u */ secp256k1_fe_half(&v); /* v = (r/s-u)/2 */ } /* Let w = sqrt(s). */ ret = secp256k1_fe_sqrt(&m, &s); /* m = sqrt(s) = w */ VERIFY_CHECK(ret); /* Return logic. */ if ((c & 5) == 0 || (c & 5) == 5) { secp256k1_fe_negate(&m, &m, 1); /* m = -w */ } /* Now m = {-w if c&5=0 or c&5=5; w otherwise}. */ secp256k1_fe_mul(&u, &u, c&1 ? &secp256k1_ellswift_c4 : &secp256k1_ellswift_c3); /* u = {c4 if c&1=1; c3 otherwise}*u */ secp256k1_fe_add(&u, &v); /* u = {c4 if c&1=1; c3 otherwise}*u + v */ secp256k1_fe_mul(t, &m, &u); return 1; } /** Use SHA256 as a PRNG, returning SHA256(hasher || cnt). * * hasher is a SHA256 object to which an incrementing 4-byte counter is written to generate randomness. * Writing 13 bytes (4 bytes for counter, plus 9 bytes for the SHA256 padding) cannot cross a * 64-byte block size boundary (to make sure it only triggers a single SHA256 compression). */ static void secp256k1_ellswift_prng(unsigned char* out32, const secp256k1_sha256 *hasher, uint32_t cnt) { secp256k1_sha256 hash = *hasher; unsigned char buf4[4]; #ifdef VERIFY size_t blocks = hash.bytes >> 6; #endif buf4[0] = cnt; buf4[1] = cnt >> 8; buf4[2] = cnt >> 16; buf4[3] = cnt >> 24; secp256k1_sha256_write(&hash, buf4, 4); secp256k1_sha256_finalize(&hash, out32); /* Writing and finalizing together should trigger exactly one SHA256 compression. */ VERIFY_CHECK(((hash.bytes) >> 6) == (blocks + 1)); } /** Find an ElligatorSwift encoding (u, t) for X coordinate x, and random Y coordinate. * * u32 is the 32-byte big endian encoding of u; t is the output field element t that still * needs encoding. * * hasher is a hasher in the secp256k1_ellswift_prng sense, with the same restrictions. */ static void secp256k1_ellswift_xelligatorswift_var(unsigned char *u32, secp256k1_fe *t, const secp256k1_fe *x, const secp256k1_sha256 *hasher) { /* Pool of 3-bit branch values. */ unsigned char branch_hash[32]; /* Number of 3-bit values in branch_hash left. */ int branches_left = 0; /* Field elements u and branch values are extracted from RNG based on hasher for consecutive * values of cnt. cnt==0 is first used to populate a pool of 64 4-bit branch values. The 64 * cnt values that follow are used to generate field elements u. cnt==65 (and multiples * thereof) are used to repopulate the pool and start over, if that were ever necessary. * On average, 4 iterations are needed. */ uint32_t cnt = 0; while (1) { int branch; secp256k1_fe u; /* If the pool of branch values is empty, populate it. */ if (branches_left == 0) { secp256k1_ellswift_prng(branch_hash, hasher, cnt++); branches_left = 64; } /* Take a 3-bit branch value from the branch pool (top bit is discarded). */ --branches_left; branch = (branch_hash[branches_left >> 1] >> ((branches_left & 1) << 2)) & 7; /* Compute a new u value by hashing. */ secp256k1_ellswift_prng(u32, hasher, cnt++); /* overflow is not a problem (we prefer uniform u32 over uniform u). */ secp256k1_fe_set_b32_mod(&u, u32); /* Since u is the output of a hash, it should practically never be 0. We could apply the * u=0 to u=1 correction here too to deal with that case still, but it's such a low * probability event that we do not bother. */ VERIFY_CHECK(!secp256k1_fe_normalizes_to_zero_var(&u)); /* Find a remainder t, and return it if found. */ if (EXPECT(secp256k1_ellswift_xswiftec_inv_var(t, x, &u, branch), 0)) break; } } /** Find an ElligatorSwift encoding (u, t) for point P. * * This is similar secp256k1_ellswift_xelligatorswift_var, except it takes a full group element p * as input, and returns an encoding that matches the provided Y coordinate rather than a random * one. */ static void secp256k1_ellswift_elligatorswift_var(unsigned char *u32, secp256k1_fe *t, const secp256k1_ge *p, const secp256k1_sha256 *hasher) { secp256k1_ellswift_xelligatorswift_var(u32, t, &p->x, hasher); secp256k1_fe_normalize_var(t); if (secp256k1_fe_is_odd(t) != secp256k1_fe_is_odd(&p->y)) { secp256k1_fe_negate(t, t, 1); secp256k1_fe_normalize_var(t); } } /** Set hash state to the BIP340 tagged hash midstate for "secp256k1_ellswift_encode". */ static void secp256k1_ellswift_sha256_init_encode(secp256k1_sha256* hash) { secp256k1_sha256_initialize(hash); hash->s[0] = 0xd1a6524bul; hash->s[1] = 0x028594b3ul; hash->s[2] = 0x96e42f4eul; hash->s[3] = 0x1037a177ul; hash->s[4] = 0x1b8fcb8bul; hash->s[5] = 0x56023885ul; hash->s[6] = 0x2560ede1ul; hash->s[7] = 0xd626b715ul; hash->bytes = 64; } int secp256k1_ellswift_encode(const secp256k1_context *ctx, unsigned char *ell64, const secp256k1_pubkey *pubkey, const unsigned char *rnd32) { secp256k1_ge p; VERIFY_CHECK(ctx != NULL); ARG_CHECK(ell64 != NULL); ARG_CHECK(pubkey != NULL); ARG_CHECK(rnd32 != NULL); if (secp256k1_pubkey_load(ctx, &p, pubkey)) { secp256k1_fe t; unsigned char p64[64] = {0}; size_t ser_size; int ser_ret; secp256k1_sha256 hash; /* Set up hasher state; the used RNG is H(pubkey || "\x00"*31 || rnd32 || cnt++), using * BIP340 tagged hash with tag "secp256k1_ellswift_encode". */ secp256k1_ellswift_sha256_init_encode(&hash); ser_ret = secp256k1_eckey_pubkey_serialize(&p, p64, &ser_size, 1); #ifdef VERIFY VERIFY_CHECK(ser_ret && ser_size == 33); #else (void)ser_ret; #endif secp256k1_sha256_write(&hash, p64, sizeof(p64)); secp256k1_sha256_write(&hash, rnd32, 32); /* Compute ElligatorSwift encoding and construct output. */ secp256k1_ellswift_elligatorswift_var(ell64, &t, &p, &hash); /* puts u in ell64[0..32] */ secp256k1_fe_get_b32(ell64 + 32, &t); /* puts t in ell64[32..64] */ return 1; } /* Only reached in case the provided pubkey is invalid. */ memset(ell64, 0, 64); return 0; } /** Set hash state to the BIP340 tagged hash midstate for "secp256k1_ellswift_create". */ static void secp256k1_ellswift_sha256_init_create(secp256k1_sha256* hash) { secp256k1_sha256_initialize(hash); hash->s[0] = 0xd29e1bf5ul; hash->s[1] = 0xf7025f42ul; hash->s[2] = 0x9b024773ul; hash->s[3] = 0x094cb7d5ul; hash->s[4] = 0xe59ed789ul; hash->s[5] = 0x03bc9786ul; hash->s[6] = 0x68335b35ul; hash->s[7] = 0x4e363b53ul; hash->bytes = 64; } int secp256k1_ellswift_create(const secp256k1_context *ctx, unsigned char *ell64, const unsigned char *seckey32, const unsigned char *auxrnd32) { secp256k1_ge p; secp256k1_fe t; secp256k1_sha256 hash; secp256k1_scalar seckey_scalar; int ret; static const unsigned char zero32[32] = {0}; /* Sanity check inputs. */ VERIFY_CHECK(ctx != NULL); ARG_CHECK(ell64 != NULL); memset(ell64, 0, 64); ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx)); ARG_CHECK(seckey32 != NULL); /* Compute (affine) public key */ ret = secp256k1_ec_pubkey_create_helper(&ctx->ecmult_gen_ctx, &seckey_scalar, &p, seckey32); secp256k1_declassify(ctx, &p, sizeof(p)); /* not constant time in produced pubkey */ secp256k1_fe_normalize_var(&p.x); secp256k1_fe_normalize_var(&p.y); /* Set up hasher state. The used RNG is H(privkey || "\x00"*32 [|| auxrnd32] || cnt++), * using BIP340 tagged hash with tag "secp256k1_ellswift_create". */ secp256k1_ellswift_sha256_init_create(&hash); secp256k1_sha256_write(&hash, seckey32, 32); secp256k1_sha256_write(&hash, zero32, sizeof(zero32)); secp256k1_declassify(ctx, &hash, sizeof(hash)); /* private key is hashed now */ if (auxrnd32) secp256k1_sha256_write(&hash, auxrnd32, 32); /* Compute ElligatorSwift encoding and construct output. */ secp256k1_ellswift_elligatorswift_var(ell64, &t, &p, &hash); /* puts u in ell64[0..32] */ secp256k1_fe_get_b32(ell64 + 32, &t); /* puts t in ell64[32..64] */ secp256k1_memczero(ell64, 64, !ret); secp256k1_scalar_clear(&seckey_scalar); return ret; } int secp256k1_ellswift_decode(const secp256k1_context *ctx, secp256k1_pubkey *pubkey, const unsigned char *ell64) { secp256k1_fe u, t; secp256k1_ge p; VERIFY_CHECK(ctx != NULL); ARG_CHECK(pubkey != NULL); ARG_CHECK(ell64 != NULL); secp256k1_fe_set_b32_mod(&u, ell64); secp256k1_fe_set_b32_mod(&t, ell64 + 32); secp256k1_fe_normalize_var(&t); secp256k1_ellswift_swiftec_var(&p, &u, &t); secp256k1_pubkey_save(pubkey, &p); return 1; } static int ellswift_xdh_hash_function_prefix(unsigned char *output, const unsigned char *x32, const unsigned char *ell_a64, const unsigned char *ell_b64, void *data) { secp256k1_sha256 sha; secp256k1_sha256_initialize(&sha); secp256k1_sha256_write(&sha, data, 64); secp256k1_sha256_write(&sha, ell_a64, 64); secp256k1_sha256_write(&sha, ell_b64, 64); secp256k1_sha256_write(&sha, x32, 32); secp256k1_sha256_finalize(&sha, output); return 1; } /** Set hash state to the BIP340 tagged hash midstate for "bip324_ellswift_xonly_ecdh". */ static void secp256k1_ellswift_sha256_init_bip324(secp256k1_sha256* hash) { secp256k1_sha256_initialize(hash); hash->s[0] = 0x8c12d730ul; hash->s[1] = 0x827bd392ul; hash->s[2] = 0x9e4fb2eeul; hash->s[3] = 0x207b373eul; hash->s[4] = 0x2292bd7aul; hash->s[5] = 0xaa5441bcul; hash->s[6] = 0x15c3779ful; hash->s[7] = 0xcfb52549ul; hash->bytes = 64; } static int ellswift_xdh_hash_function_bip324(unsigned char* output, const unsigned char *x32, const unsigned char *ell_a64, const unsigned char *ell_b64, void *data) { secp256k1_sha256 sha; (void)data; secp256k1_ellswift_sha256_init_bip324(&sha); secp256k1_sha256_write(&sha, ell_a64, 64); secp256k1_sha256_write(&sha, ell_b64, 64); secp256k1_sha256_write(&sha, x32, 32); secp256k1_sha256_finalize(&sha, output); return 1; } const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_prefix = ellswift_xdh_hash_function_prefix; const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_bip324 = ellswift_xdh_hash_function_bip324; int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output, const unsigned char *ell_a64, const unsigned char *ell_b64, const unsigned char *seckey32, int party, secp256k1_ellswift_xdh_hash_function hashfp, void *data) { int ret = 0; int overflow; secp256k1_scalar s; secp256k1_fe xn, xd, px, u, t; unsigned char sx[32]; const unsigned char* theirs64; VERIFY_CHECK(ctx != NULL); ARG_CHECK(output != NULL); ARG_CHECK(ell_a64 != NULL); ARG_CHECK(ell_b64 != NULL); ARG_CHECK(seckey32 != NULL); ARG_CHECK(hashfp != NULL); /* Load remote public key (as fraction). */ theirs64 = party ? ell_a64 : ell_b64; secp256k1_fe_set_b32_mod(&u, theirs64); secp256k1_fe_set_b32_mod(&t, theirs64 + 32); secp256k1_ellswift_xswiftec_frac_var(&xn, &xd, &u, &t); /* Load private key (using one if invalid). */ secp256k1_scalar_set_b32(&s, seckey32, &overflow); overflow = secp256k1_scalar_is_zero(&s); secp256k1_scalar_cmov(&s, &secp256k1_scalar_one, overflow); /* Compute shared X coordinate. */ secp256k1_ecmult_const_xonly(&px, &xn, &xd, &s, 1); secp256k1_fe_normalize(&px); secp256k1_fe_get_b32(sx, &px); /* Invoke hasher */ ret = hashfp(output, sx, ell_a64, ell_b64, data); memset(sx, 0, 32); secp256k1_fe_clear(&px); secp256k1_scalar_clear(&s); return !!ret & !overflow; } #endif