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317 lines
10 KiB
C
317 lines
10 KiB
C
/***********************************************************************
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* Copyright (c) 2013-2015 Pieter Wuille *
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* Distributed under the MIT software license, see the accompanying *
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* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
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***********************************************************************/
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#ifndef SECP256K1_ECDSA_IMPL_H
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#define SECP256K1_ECDSA_IMPL_H
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#include "scalar.h"
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#include "field.h"
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#include "group.h"
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#include "ecmult.h"
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#include "ecmult_gen.h"
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#include "ecdsa.h"
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/** Group order for secp256k1 defined as 'n' in "Standards for Efficient Cryptography" (SEC2) 2.7.1
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* sage: for t in xrange(1023, -1, -1):
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* .. p = 2**256 - 2**32 - t
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* .. if p.is_prime():
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* .. print '%x'%p
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* .. break
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* 'fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f'
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* sage: a = 0
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* sage: b = 7
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* sage: F = FiniteField (p)
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* sage: '%x' % (EllipticCurve ([F (a), F (b)]).order())
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* 'fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141'
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*/
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static const secp256k1_fe secp256k1_ecdsa_const_order_as_fe = SECP256K1_FE_CONST(
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0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFEUL,
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0xBAAEDCE6UL, 0xAF48A03BUL, 0xBFD25E8CUL, 0xD0364141UL
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);
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/** Difference between field and order, values 'p' and 'n' values defined in
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* "Standards for Efficient Cryptography" (SEC2) 2.7.1.
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* sage: p = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F
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* sage: a = 0
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* sage: b = 7
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* sage: F = FiniteField (p)
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* sage: '%x' % (p - EllipticCurve ([F (a), F (b)]).order())
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* '14551231950b75fc4402da1722fc9baee'
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*/
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static const secp256k1_fe secp256k1_ecdsa_const_p_minus_order = SECP256K1_FE_CONST(
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0, 0, 0, 1, 0x45512319UL, 0x50B75FC4UL, 0x402DA172UL, 0x2FC9BAEEUL
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);
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static int secp256k1_der_read_len(size_t *len, const unsigned char **sigp, const unsigned char *sigend) {
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size_t lenleft;
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unsigned char b1;
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VERIFY_CHECK(len != NULL);
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*len = 0;
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if (*sigp >= sigend) {
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return 0;
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}
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b1 = *((*sigp)++);
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if (b1 == 0xFF) {
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/* X.690-0207 8.1.3.5.c the value 0xFF shall not be used. */
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return 0;
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}
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if ((b1 & 0x80) == 0) {
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/* X.690-0207 8.1.3.4 short form length octets */
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*len = b1;
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return 1;
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}
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if (b1 == 0x80) {
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/* Indefinite length is not allowed in DER. */
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return 0;
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}
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/* X.690-207 8.1.3.5 long form length octets */
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lenleft = b1 & 0x7F; /* lenleft is at least 1 */
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if (lenleft > (size_t)(sigend - *sigp)) {
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return 0;
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}
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if (**sigp == 0) {
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/* Not the shortest possible length encoding. */
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return 0;
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}
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if (lenleft > sizeof(size_t)) {
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/* The resulting length would exceed the range of a size_t, so
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* certainly longer than the passed array size.
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*/
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return 0;
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}
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while (lenleft > 0) {
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*len = (*len << 8) | **sigp;
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(*sigp)++;
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lenleft--;
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}
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if (*len > (size_t)(sigend - *sigp)) {
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/* Result exceeds the length of the passed array. */
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return 0;
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}
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if (*len < 128) {
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/* Not the shortest possible length encoding. */
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return 0;
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}
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return 1;
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}
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static int secp256k1_der_parse_integer(secp256k1_scalar *r, const unsigned char **sig, const unsigned char *sigend) {
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int overflow = 0;
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unsigned char ra[32] = {0};
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size_t rlen;
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if (*sig == sigend || **sig != 0x02) {
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/* Not a primitive integer (X.690-0207 8.3.1). */
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return 0;
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}
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(*sig)++;
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if (secp256k1_der_read_len(&rlen, sig, sigend) == 0) {
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return 0;
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}
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if (rlen == 0 || rlen > (size_t)(sigend - *sig)) {
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/* Exceeds bounds or not at least length 1 (X.690-0207 8.3.1). */
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return 0;
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}
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if (**sig == 0x00 && rlen > 1 && (((*sig)[1]) & 0x80) == 0x00) {
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/* Excessive 0x00 padding. */
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return 0;
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}
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if (**sig == 0xFF && rlen > 1 && (((*sig)[1]) & 0x80) == 0x80) {
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/* Excessive 0xFF padding. */
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return 0;
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}
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if ((**sig & 0x80) == 0x80) {
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/* Negative. */
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overflow = 1;
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}
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/* There is at most one leading zero byte:
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* if there were two leading zero bytes, we would have failed and returned 0
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* because of excessive 0x00 padding already. */
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if (rlen > 0 && **sig == 0) {
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/* Skip leading zero byte */
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rlen--;
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(*sig)++;
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}
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if (rlen > 32) {
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overflow = 1;
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}
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if (!overflow) {
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if (rlen) memcpy(ra + 32 - rlen, *sig, rlen);
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secp256k1_scalar_set_b32(r, ra, &overflow);
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}
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if (overflow) {
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secp256k1_scalar_set_int(r, 0);
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}
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(*sig) += rlen;
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return 1;
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}
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static int secp256k1_ecdsa_sig_parse(secp256k1_scalar *rr, secp256k1_scalar *rs, const unsigned char *sig, size_t size) {
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const unsigned char *sigend = sig + size;
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size_t rlen;
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if (sig == sigend || *(sig++) != 0x30) {
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/* The encoding doesn't start with a constructed sequence (X.690-0207 8.9.1). */
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return 0;
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}
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if (secp256k1_der_read_len(&rlen, &sig, sigend) == 0) {
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return 0;
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}
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if (rlen != (size_t)(sigend - sig)) {
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/* Tuple exceeds bounds or garage after tuple. */
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return 0;
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}
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if (!secp256k1_der_parse_integer(rr, &sig, sigend)) {
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return 0;
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}
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if (!secp256k1_der_parse_integer(rs, &sig, sigend)) {
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return 0;
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}
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if (sig != sigend) {
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/* Trailing garbage inside tuple. */
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return 0;
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}
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return 1;
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}
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static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, size_t *size, const secp256k1_scalar* ar, const secp256k1_scalar* as) {
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unsigned char r[33] = {0}, s[33] = {0};
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unsigned char *rp = r, *sp = s;
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size_t lenR = 33, lenS = 33;
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secp256k1_scalar_get_b32(&r[1], ar);
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secp256k1_scalar_get_b32(&s[1], as);
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while (lenR > 1 && rp[0] == 0 && rp[1] < 0x80) { lenR--; rp++; }
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while (lenS > 1 && sp[0] == 0 && sp[1] < 0x80) { lenS--; sp++; }
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if (*size < 6+lenS+lenR) {
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*size = 6 + lenS + lenR;
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return 0;
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}
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*size = 6 + lenS + lenR;
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sig[0] = 0x30;
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sig[1] = 4 + lenS + lenR;
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sig[2] = 0x02;
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sig[3] = lenR;
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memcpy(sig+4, rp, lenR);
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sig[4+lenR] = 0x02;
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sig[5+lenR] = lenS;
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memcpy(sig+lenR+6, sp, lenS);
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return 1;
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}
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static int secp256k1_ecdsa_sig_verify(const secp256k1_scalar *sigr, const secp256k1_scalar *sigs, const secp256k1_ge *pubkey, const secp256k1_scalar *message) {
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unsigned char c[32];
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secp256k1_scalar sn, u1, u2;
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#if !defined(EXHAUSTIVE_TEST_ORDER)
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secp256k1_fe xr;
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#endif
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secp256k1_gej pubkeyj;
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secp256k1_gej pr;
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if (secp256k1_scalar_is_zero(sigr) || secp256k1_scalar_is_zero(sigs)) {
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return 0;
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}
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secp256k1_scalar_inverse_var(&sn, sigs);
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secp256k1_scalar_mul(&u1, &sn, message);
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secp256k1_scalar_mul(&u2, &sn, sigr);
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secp256k1_gej_set_ge(&pubkeyj, pubkey);
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secp256k1_ecmult(&pr, &pubkeyj, &u2, &u1);
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if (secp256k1_gej_is_infinity(&pr)) {
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return 0;
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}
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#if defined(EXHAUSTIVE_TEST_ORDER)
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{
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secp256k1_scalar computed_r;
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secp256k1_ge pr_ge;
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secp256k1_ge_set_gej(&pr_ge, &pr);
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secp256k1_fe_normalize(&pr_ge.x);
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secp256k1_fe_get_b32(c, &pr_ge.x);
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secp256k1_scalar_set_b32(&computed_r, c, NULL);
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return secp256k1_scalar_eq(sigr, &computed_r);
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}
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#else
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secp256k1_scalar_get_b32(c, sigr);
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/* we can ignore the fe_set_b32_limit return value, because we know the input is in range */
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(void)secp256k1_fe_set_b32_limit(&xr, c);
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/** We now have the recomputed R point in pr, and its claimed x coordinate (modulo n)
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* in xr. Naively, we would extract the x coordinate from pr (requiring a inversion modulo p),
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* compute the remainder modulo n, and compare it to xr. However:
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*
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* xr == X(pr) mod n
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* <=> exists h. (xr + h * n < p && xr + h * n == X(pr))
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* [Since 2 * n > p, h can only be 0 or 1]
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* <=> (xr == X(pr)) || (xr + n < p && xr + n == X(pr))
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* [In Jacobian coordinates, X(pr) is pr.x / pr.z^2 mod p]
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* <=> (xr == pr.x / pr.z^2 mod p) || (xr + n < p && xr + n == pr.x / pr.z^2 mod p)
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* [Multiplying both sides of the equations by pr.z^2 mod p]
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* <=> (xr * pr.z^2 mod p == pr.x) || (xr + n < p && (xr + n) * pr.z^2 mod p == pr.x)
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*
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* Thus, we can avoid the inversion, but we have to check both cases separately.
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* secp256k1_gej_eq_x implements the (xr * pr.z^2 mod p == pr.x) test.
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*/
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if (secp256k1_gej_eq_x_var(&xr, &pr)) {
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/* xr * pr.z^2 mod p == pr.x, so the signature is valid. */
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return 1;
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}
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if (secp256k1_fe_cmp_var(&xr, &secp256k1_ecdsa_const_p_minus_order) >= 0) {
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/* xr + n >= p, so we can skip testing the second case. */
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return 0;
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}
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secp256k1_fe_add(&xr, &secp256k1_ecdsa_const_order_as_fe);
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if (secp256k1_gej_eq_x_var(&xr, &pr)) {
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/* (xr + n) * pr.z^2 mod p == pr.x, so the signature is valid. */
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return 1;
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}
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return 0;
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#endif
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}
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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) {
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unsigned char b[32];
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secp256k1_gej rp;
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secp256k1_ge r;
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secp256k1_scalar n;
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int overflow = 0;
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int high;
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secp256k1_ecmult_gen(ctx, &rp, nonce);
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secp256k1_ge_set_gej(&r, &rp);
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secp256k1_fe_normalize(&r.x);
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secp256k1_fe_normalize(&r.y);
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secp256k1_fe_get_b32(b, &r.x);
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secp256k1_scalar_set_b32(sigr, b, &overflow);
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if (recid) {
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/* The overflow condition is cryptographically unreachable as hitting it requires finding the discrete log
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* of some P where P.x >= order, and only 1 in about 2^127 points meet this criteria.
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*/
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*recid = (overflow << 1) | secp256k1_fe_is_odd(&r.y);
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}
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secp256k1_scalar_mul(&n, sigr, seckey);
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secp256k1_scalar_add(&n, &n, message);
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secp256k1_scalar_inverse(sigs, nonce);
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secp256k1_scalar_mul(sigs, sigs, &n);
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secp256k1_scalar_clear(&n);
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secp256k1_gej_clear(&rp);
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secp256k1_ge_clear(&r);
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high = secp256k1_scalar_is_high(sigs);
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secp256k1_scalar_cond_negate(sigs, high);
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if (recid) {
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*recid ^= high;
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}
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/* P.x = order is on the curve, so technically sig->r could end up being zero, which would be an invalid signature.
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* This is cryptographically unreachable as hitting it requires finding the discrete log of P.x = N.
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*/
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return (int)(!secp256k1_scalar_is_zero(sigr)) & (int)(!secp256k1_scalar_is_zero(sigs));
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}
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#endif /* SECP256K1_ECDSA_IMPL_H */
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