/********************************************************************** * Copyright (c) 2013-2015 Pieter Wuille * * Distributed under the MIT software license, see the accompanying * * file COPYING or http://www.opensource.org/licenses/mit-license.php.* **********************************************************************/ #ifndef _SECP256K1_ECDSA_IMPL_H_ #define _SECP256K1_ECDSA_IMPL_H_ #include "scalar.h" #include "field.h" #include "group.h" #include "ecmult.h" #include "ecmult_gen.h" #include "ecdsa.h" /** Group order for secp256k1 defined as 'n' in "Standards for Efficient Cryptography" (SEC2) 2.7.1 * sage: for t in xrange(1023, -1, -1): * .. p = 2**256 - 2**32 - t * .. if p.is_prime(): * .. print '%x'%p * .. break * 'fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f' * sage: a = 0 * sage: b = 7 * sage: F = FiniteField (p) * sage: '%x' % (EllipticCurve ([F (a), F (b)]).order()) * 'fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141' */ static const secp256k1_fe secp256k1_ecdsa_const_order_as_fe = SECP256K1_FE_CONST( 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFEUL, 0xBAAEDCE6UL, 0xAF48A03BUL, 0xBFD25E8CUL, 0xD0364141UL ); /** Difference between field and order, values 'p' and 'n' values defined in * "Standards for Efficient Cryptography" (SEC2) 2.7.1. * sage: p = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F * sage: a = 0 * sage: b = 7 * sage: F = FiniteField (p) * sage: '%x' % (p - EllipticCurve ([F (a), F (b)]).order()) * '14551231950b75fc4402da1722fc9baee' */ static const secp256k1_fe secp256k1_ecdsa_const_p_minus_order = SECP256K1_FE_CONST( 0, 0, 0, 1, 0x45512319UL, 0x50B75FC4UL, 0x402DA172UL, 0x2FC9BAEEUL ); static int secp256k1_der_read_len(const unsigned char **sigp, const unsigned char *sigend) { int lenleft, b1; size_t ret = 0; if (*sigp >= sigend) { return -1; } b1 = *((*sigp)++); if (b1 == 0xFF) { /* X.690-0207 8.1.3.5.c the value 0xFF shall not be used. */ return -1; } if ((b1 & 0x80) == 0) { /* X.690-0207 8.1.3.4 short form length octets */ return b1; } if (b1 == 0x80) { /* Indefinite length is not allowed in DER. */ return -1; } /* X.690-207 8.1.3.5 long form length octets */ lenleft = b1 & 0x7F; if (lenleft > sigend - *sigp) { return -1; } if (**sigp == 0) { /* Not the shortest possible length encoding. */ return -1; } if ((size_t)lenleft > sizeof(size_t)) { /* The resulthing length would exceed the range of a size_t, so certainly longer than the passed array size. */ return -1; } while (lenleft > 0) { if ((ret >> ((sizeof(size_t) - 1) * 8)) != 0) { } ret = (ret << 8) | **sigp; if (ret + lenleft > (size_t)(sigend - *sigp)) { /* Result exceeds the length of the passed array. */ return -1; } (*sigp)++; lenleft--; } if (ret < 128) { /* Not the shortest possible length encoding. */ return -1; } return ret; } static int secp256k1_der_parse_integer(secp256k1_scalar *r, const unsigned char **sig, const unsigned char *sigend) { int overflow = 0; unsigned char ra[32] = {0}; int rlen; if (*sig == sigend || **sig != 0x02) { /* Not a primitive integer (X.690-0207 8.3.1). */ return 0; } (*sig)++; rlen = secp256k1_der_read_len(sig, sigend); if (rlen <= 0 || (*sig) + rlen > sigend) { /* Exceeds bounds or not at least length 1 (X.690-0207 8.3.1). */ return 0; } if (**sig == 0x00 && rlen > 1 && (((*sig)[1]) & 0x80) == 0x00) { /* Excessive 0x00 padding. */ return 0; } if (**sig == 0xFF && rlen > 1 && (((*sig)[1]) & 0x80) == 0x80) { /* Excessive 0xFF padding. */ return 0; } if ((**sig & 0x80) == 0x80) { /* Negative. */ overflow = 1; } while (rlen > 0 && **sig == 0) { /* Skip leading zero bytes */ rlen--; (*sig)++; } if (rlen > 32) { overflow = 1; } if (!overflow) { memcpy(ra + 32 - rlen, *sig, rlen); secp256k1_scalar_set_b32(r, ra, &overflow); } if (overflow) { secp256k1_scalar_set_int(r, 0); } (*sig) += rlen; return 1; } static int secp256k1_ecdsa_sig_parse(secp256k1_scalar *rr, secp256k1_scalar *rs, const unsigned char *sig, size_t size) { const unsigned char *sigend = sig + size; int rlen; if (sig == sigend || *(sig++) != 0x30) { /* The encoding doesn't start with a constructed sequence (X.690-0207 8.9.1). */ return 0; } rlen = secp256k1_der_read_len(&sig, sigend); if (rlen < 0 || sig + rlen > sigend) { /* Tuple exceeds bounds */ return 0; } if (sig + rlen != sigend) { /* Garbage after tuple. */ return 0; } if (!secp256k1_der_parse_integer(rr, &sig, sigend)) { return 0; } if (!secp256k1_der_parse_integer(rs, &sig, sigend)) { return 0; } if (sig != sigend) { /* Trailing garbage inside tuple. */ return 0; } return 1; } static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, size_t *size, const secp256k1_scalar* ar, const secp256k1_scalar* as) { unsigned char r[33] = {0}, s[33] = {0}; unsigned char *rp = r, *sp = s; size_t lenR = 33, lenS = 33; secp256k1_scalar_get_b32(&r[1], ar); secp256k1_scalar_get_b32(&s[1], as); while (lenR > 1 && rp[0] == 0 && rp[1] < 0x80) { lenR--; rp++; } while (lenS > 1 && sp[0] == 0 && sp[1] < 0x80) { lenS--; sp++; } if (*size < 6+lenS+lenR) { *size = 6 + lenS + lenR; return 0; } *size = 6 + lenS + lenR; sig[0] = 0x30; sig[1] = 4 + lenS + lenR; sig[2] = 0x02; sig[3] = lenR; memcpy(sig+4, rp, lenR); sig[4+lenR] = 0x02; sig[5+lenR] = lenS; memcpy(sig+lenR+6, sp, lenS); return 1; } static int secp256k1_ecdsa_sig_verify(const secp256k1_ecmult_context *ctx, const secp256k1_scalar *sigr, const secp256k1_scalar *sigs, const secp256k1_ge *pubkey, const secp256k1_scalar *message) { unsigned char c[32]; secp256k1_scalar sn, u1, u2; secp256k1_fe xr; secp256k1_gej pubkeyj; secp256k1_gej pr; if (secp256k1_scalar_is_zero(sigr) || secp256k1_scalar_is_zero(sigs)) { return 0; } secp256k1_scalar_inverse_var(&sn, sigs); secp256k1_scalar_mul(&u1, &sn, message); secp256k1_scalar_mul(&u2, &sn, sigr); secp256k1_gej_set_ge(&pubkeyj, pubkey); secp256k1_ecmult(ctx, &pr, &pubkeyj, &u2, &u1); if (secp256k1_gej_is_infinity(&pr)) { return 0; } secp256k1_scalar_get_b32(c, sigr); secp256k1_fe_set_b32(&xr, c); /** We now have the recomputed R point in pr, and its claimed x coordinate (modulo n) * in xr. Naively, we would extract the x coordinate from pr (requiring a inversion modulo p), * compute the remainder modulo n, and compare it to xr. However: * * xr == X(pr) mod n * <=> exists h. (xr + h * n < p && xr + h * n == X(pr)) * [Since 2 * n > p, h can only be 0 or 1] * <=> (xr == X(pr)) || (xr + n < p && xr + n == X(pr)) * [In Jacobian coordinates, X(pr) is pr.x / pr.z^2 mod p] * <=> (xr == pr.x / pr.z^2 mod p) || (xr + n < p && xr + n == pr.x / pr.z^2 mod p) * [Multiplying both sides of the equations by pr.z^2 mod p] * <=> (xr * pr.z^2 mod p == pr.x) || (xr + n < p && (xr + n) * pr.z^2 mod p == pr.x) * * Thus, we can avoid the inversion, but we have to check both cases separately. * secp256k1_gej_eq_x implements the (xr * pr.z^2 mod p == pr.x) test. */ if (secp256k1_gej_eq_x_var(&xr, &pr)) { /* xr * pr.z^2 mod p == pr.x, so the signature is valid. */ return 1; } if (secp256k1_fe_cmp_var(&xr, &secp256k1_ecdsa_const_p_minus_order) >= 0) { /* xr + n >= p, so we can skip testing the second case. */ return 0; } secp256k1_fe_add(&xr, &secp256k1_ecdsa_const_order_as_fe); if (secp256k1_gej_eq_x_var(&xr, &pr)) { /* (xr + n) * pr.z^2 mod p == pr.x, so the signature is valid. */ return 1; } return 0; } static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, secp256k1_scalar *sigr, secp256k1_scalar *sigs, const secp256k1_scalar *seckey, const secp256k1_scalar *message, const secp256k1_scalar *nonce, int *recid) { unsigned char b[32]; secp256k1_gej rp; secp256k1_ge r; secp256k1_scalar n; int overflow = 0; secp256k1_ecmult_gen(ctx, &rp, nonce); secp256k1_ge_set_gej(&r, &rp); secp256k1_fe_normalize(&r.x); secp256k1_fe_normalize(&r.y); secp256k1_fe_get_b32(b, &r.x); secp256k1_scalar_set_b32(sigr, b, &overflow); if (secp256k1_scalar_is_zero(sigr)) { /* P.x = order is on the curve, so technically sig->r could end up zero, which would be an invalid signature. */ /* This branch is cryptographically unreachable as hitting it requires finding the discrete log of P.x = N. */ secp256k1_gej_clear(&rp); secp256k1_ge_clear(&r); return 0; } if (recid) { *recid = (overflow ? 2 : 0) | (secp256k1_fe_is_odd(&r.y) ? 1 : 0); } secp256k1_scalar_mul(&n, sigr, seckey); secp256k1_scalar_add(&n, &n, message); secp256k1_scalar_inverse(sigs, nonce); secp256k1_scalar_mul(sigs, sigs, &n); secp256k1_scalar_clear(&n); secp256k1_gej_clear(&rp); secp256k1_ge_clear(&r); if (secp256k1_scalar_is_zero(sigs)) { return 0; } if (secp256k1_scalar_is_high(sigs)) { secp256k1_scalar_negate(sigs, sigs); if (recid) { *recid ^= 1; } } return 1; } #endif