2 * Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
27 #if BR_INT128 || BR_UMUL128
33 static const unsigned char P256_G[] = {
34 0x04, 0x6B, 0x17, 0xD1, 0xF2, 0xE1, 0x2C, 0x42, 0x47, 0xF8,
35 0xBC, 0xE6, 0xE5, 0x63, 0xA4, 0x40, 0xF2, 0x77, 0x03, 0x7D,
36 0x81, 0x2D, 0xEB, 0x33, 0xA0, 0xF4, 0xA1, 0x39, 0x45, 0xD8,
37 0x98, 0xC2, 0x96, 0x4F, 0xE3, 0x42, 0xE2, 0xFE, 0x1A, 0x7F,
38 0x9B, 0x8E, 0xE7, 0xEB, 0x4A, 0x7C, 0x0F, 0x9E, 0x16, 0x2B,
39 0xCE, 0x33, 0x57, 0x6B, 0x31, 0x5E, 0xCE, 0xCB, 0xB6, 0x40,
40 0x68, 0x37, 0xBF, 0x51, 0xF5
43 static const unsigned char P256_N[] = {
44 0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF,
45 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xBC, 0xE6, 0xFA, 0xAD,
46 0xA7, 0x17, 0x9E, 0x84, 0xF3, 0xB9, 0xCA, 0xC2, 0xFC, 0x63,
50 static const unsigned char *
51 api_generator(int curve, size_t *len)
58 static const unsigned char *
59 api_order(int curve, size_t *len)
67 api_xoff(int curve, size_t *len)
75 * A field element is encoded as five 64-bit integers, in basis 2^52.
76 * Limbs may occasionally exceed 2^52.
78 * A _partially reduced_ value is such that the following hold:
79 * - top limb is less than 2^48 + 2^30
80 * - the other limbs fit on 53 bits each
81 * In particular, such a value is less than twice the modulus p.
84 #define BIT(n) ((uint64_t)1 << (n))
85 #define MASK48 (BIT(48) - BIT(0))
86 #define MASK52 (BIT(52) - BIT(0))
89 static const uint64_t F256_R[] = {
90 0x0000000000010, 0xF000000000000, 0xFFFFFFFFFFFFF,
91 0xFFEFFFFFFFFFF, 0x00000000FFFFF
94 /* Curve equation is y^2 = x^3 - 3*x + B. This constant is B*R mod p
95 (Montgomery representation of B). */
96 static const uint64_t P256_B_MONTY[] = {
97 0xDF6229C4BDDFD, 0xCA8843090D89C, 0x212ED6ACF005C,
98 0x83415A220ABF7, 0x0C30061DD4874
102 * Addition in the field. Carry propagation is not performed.
103 * On input, limbs may be up to 63 bits each; on output, they will
104 * be up to one bit more than on input.
107 f256_add(uint64_t *d, const uint64_t *a, const uint64_t *b)
117 * Partially reduce the provided value.
118 * Input: limbs can go up to 61 bits each.
119 * Output: partially reduced.
122 f256_partial_reduce(uint64_t *a)
143 s = a[4] >> 48; /* s < 2^14 */
144 a[0] += s; /* a[0] < 2^52 + 2^14 */
145 w = a[1] - (s << 44);
146 a[1] = w & MASK52; /* a[1] < 2^52 */
147 cc = -(w >> 52) & 0xFFF; /* cc < 16 */
149 a[2] = w & MASK52; /* a[2] < 2^52 */
150 cc = w >> 63; /* cc = 0 or 1 */
151 w = a[3] - cc - (s << 36);
152 a[3] = w & MASK52; /* a[3] < 2^52 */
153 cc = w >> 63; /* cc = 0 or 1 */
155 a[4] = w + (s << 16) - cc; /* a[4] < 2^48 + 2^30 */
159 * Subtraction in the field.
160 * Input: limbs must fit on 60 bits each; in particular, the complete
161 * integer will be less than 2^268 + 2^217.
162 * Output: partially reduced.
165 f256_sub(uint64_t *d, const uint64_t *a, const uint64_t *b)
167 uint64_t t[5], w, s, cc;
170 * We compute d = 2^13*p + a - b; this ensures a positive
171 * intermediate value.
173 * Each individual addition/subtraction may yield a positive or
174 * negative result; thus, we need to handle a signed carry, thus
175 * with sign extension. We prefer not to use signed types (int64_t)
176 * because conversion from unsigned to signed is cumbersome (a
177 * direct cast with the top bit set is undefined behavior; instead,
178 * we have to use pointer aliasing, using the guaranteed properties
179 * of exact-width types, but this requires the compiler to optimize
180 * away the writes and reads from RAM), and right-shifting a
181 * signed negative value is implementation-defined. Therefore,
182 * we use a custom sign extension.
185 w = a[0] - b[0] - BIT(13);
188 cc |= -(cc & BIT(11));
189 w = a[1] - b[1] + cc;
192 cc |= -(cc & BIT(11));
193 w = a[2] - b[2] + cc;
194 t[2] = (w & MASK52) + BIT(5);
196 cc |= -(cc & BIT(11));
197 w = a[3] - b[3] + cc;
198 t[3] = (w & MASK52) + BIT(49);
200 cc |= -(cc & BIT(11));
201 t[4] = (BIT(61) - BIT(29)) + a[4] - b[4] + cc;
204 * Perform partial reduction. Rule is:
205 * 2^256 = 2^224 - 2^192 - 2^96 + 1 mod p
208 * 0 <= t[0] <= 2^52 - 1
209 * 0 <= t[1] <= 2^52 - 1
210 * 2^5 <= t[2] <= 2^52 + 2^5 - 1
211 * 2^49 <= t[3] <= 2^52 + 2^49 - 1
212 * 2^59 < t[4] <= 2^61 + 2^60 - 2^29
214 * Thus, the value 's' (t[4] / 2^48) will be necessarily
215 * greater than 2048, and less than 12288.
219 d[0] = t[0] + s; /* d[0] <= 2^52 + 12287 */
220 w = t[1] - (s << 44);
221 d[1] = w & MASK52; /* d[1] <= 2^52 - 1 */
222 cc = -(w >> 52) & 0xFFF; /* cc <= 48 */
224 cc = w >> 63; /* cc = 0 or 1 */
225 d[2] = w + (cc << 52); /* d[2] <= 2^52 + 31 */
226 w = t[3] - cc - (s << 36);
227 cc = w >> 63; /* cc = 0 or 1 */
228 d[3] = w + (cc << 52); /* t[3] <= 2^52 + 2^49 - 1 */
229 d[4] = (t[4] & MASK48) + (s << 16) - cc; /* d[4] < 2^48 + 2^30 */
232 * If s = 0, then none of the limbs is modified, and there cannot
233 * be an overflow; if s != 0, then (s << 16) > cc, and there is
234 * no overflow either.
239 * Montgomery multiplication in the field.
240 * Input: limbs must fit on 56 bits each.
241 * Output: partially reduced.
244 f256_montymul(uint64_t *d, const uint64_t *a, const uint64_t *b)
256 for (i = 0; i < 5; i ++) {
257 uint64_t x, f, cc, w, s;
261 * Since limbs of a[] and b[] fit on 56 bits each,
262 * each individual product fits on 112 bits. Also,
263 * the factor f fits on 52 bits, so f<<48 fits on
264 * 112 bits too. This guarantees that carries (cc)
265 * will fit on 62 bits, thus no overflow.
267 * The operations below compute:
268 * t <- (t + x*b + f*p) / 2^64
271 z = (unsigned __int128)b[0] * (unsigned __int128)x
272 + (unsigned __int128)t[0];
273 f = (uint64_t)z & MASK52;
274 cc = (uint64_t)(z >> 52);
275 z = (unsigned __int128)b[1] * (unsigned __int128)x
276 + (unsigned __int128)t[1] + cc
277 + ((unsigned __int128)f << 44);
278 t[0] = (uint64_t)z & MASK52;
279 cc = (uint64_t)(z >> 52);
280 z = (unsigned __int128)b[2] * (unsigned __int128)x
281 + (unsigned __int128)t[2] + cc;
282 t[1] = (uint64_t)z & MASK52;
283 cc = (uint64_t)(z >> 52);
284 z = (unsigned __int128)b[3] * (unsigned __int128)x
285 + (unsigned __int128)t[3] + cc
286 + ((unsigned __int128)f << 36);
287 t[2] = (uint64_t)z & MASK52;
288 cc = (uint64_t)(z >> 52);
289 z = (unsigned __int128)b[4] * (unsigned __int128)x
290 + (unsigned __int128)t[4] + cc
291 + ((unsigned __int128)f << 48)
292 - ((unsigned __int128)f << 16);
293 t[3] = (uint64_t)z & MASK52;
294 t[4] = (uint64_t)(z >> 52);
297 * t[4] may be up to 62 bits here; we need to do a
298 * partial reduction. Note that limbs t[0] to t[3]
299 * fit on 52 bits each.
301 s = t[4] >> 48; /* s < 2^14 */
302 t[0] += s; /* t[0] < 2^52 + 2^14 */
303 w = t[1] - (s << 44);
304 t[1] = w & MASK52; /* t[1] < 2^52 */
305 cc = -(w >> 52) & 0xFFF; /* cc < 16 */
307 t[2] = w & MASK52; /* t[2] < 2^52 */
308 cc = w >> 63; /* cc = 0 or 1 */
309 w = t[3] - cc - (s << 36);
310 t[3] = w & MASK52; /* t[3] < 2^52 */
311 cc = w >> 63; /* cc = 0 or 1 */
313 t[4] = w + (s << 16) - cc; /* t[4] < 2^48 + 2^30 */
316 * The final t[4] cannot overflow because cc is 0 or 1,
317 * and cc can be 1 only if s != 0.
337 for (i = 0; i < 5; i ++) {
338 uint64_t x, f, cc, w, s, zh, zl;
342 * Since limbs of a[] and b[] fit on 56 bits each,
343 * each individual product fits on 112 bits. Also,
344 * the factor f fits on 52 bits, so f<<48 fits on
345 * 112 bits too. This guarantees that carries (cc)
346 * will fit on 62 bits, thus no overflow.
348 * The operations below compute:
349 * t <- (t + x*b + f*p) / 2^64
352 zl = _umul128(b[0], x, &zh);
353 k = _addcarry_u64(0, t[0], zl, &zl);
354 (void)_addcarry_u64(k, 0, zh, &zh);
356 cc = (zl >> 52) | (zh << 12);
358 zl = _umul128(b[1], x, &zh);
359 k = _addcarry_u64(0, t[1], zl, &zl);
360 (void)_addcarry_u64(k, 0, zh, &zh);
361 k = _addcarry_u64(0, cc, zl, &zl);
362 (void)_addcarry_u64(k, 0, zh, &zh);
363 k = _addcarry_u64(0, f << 44, zl, &zl);
364 (void)_addcarry_u64(k, f >> 20, zh, &zh);
366 cc = (zl >> 52) | (zh << 12);
368 zl = _umul128(b[2], x, &zh);
369 k = _addcarry_u64(0, t[2], zl, &zl);
370 (void)_addcarry_u64(k, 0, zh, &zh);
371 k = _addcarry_u64(0, cc, zl, &zl);
372 (void)_addcarry_u64(k, 0, zh, &zh);
374 cc = (zl >> 52) | (zh << 12);
376 zl = _umul128(b[3], x, &zh);
377 k = _addcarry_u64(0, t[3], zl, &zl);
378 (void)_addcarry_u64(k, 0, zh, &zh);
379 k = _addcarry_u64(0, cc, zl, &zl);
380 (void)_addcarry_u64(k, 0, zh, &zh);
381 k = _addcarry_u64(0, f << 36, zl, &zl);
382 (void)_addcarry_u64(k, f >> 28, zh, &zh);
384 cc = (zl >> 52) | (zh << 12);
386 zl = _umul128(b[4], x, &zh);
387 k = _addcarry_u64(0, t[4], zl, &zl);
388 (void)_addcarry_u64(k, 0, zh, &zh);
389 k = _addcarry_u64(0, cc, zl, &zl);
390 (void)_addcarry_u64(k, 0, zh, &zh);
391 k = _addcarry_u64(0, f << 48, zl, &zl);
392 (void)_addcarry_u64(k, f >> 16, zh, &zh);
393 k = _subborrow_u64(0, zl, f << 16, &zl);
394 (void)_subborrow_u64(k, zh, f >> 48, &zh);
396 t[4] = (zl >> 52) | (zh << 12);
399 * t[4] may be up to 62 bits here; we need to do a
400 * partial reduction. Note that limbs t[0] to t[3]
401 * fit on 52 bits each.
403 s = t[4] >> 48; /* s < 2^14 */
404 t[0] += s; /* t[0] < 2^52 + 2^14 */
405 w = t[1] - (s << 44);
406 t[1] = w & MASK52; /* t[1] < 2^52 */
407 cc = -(w >> 52) & 0xFFF; /* cc < 16 */
409 t[2] = w & MASK52; /* t[2] < 2^52 */
410 cc = w >> 63; /* cc = 0 or 1 */
411 w = t[3] - cc - (s << 36);
412 t[3] = w & MASK52; /* t[3] < 2^52 */
413 cc = w >> 63; /* cc = 0 or 1 */
415 t[4] = w + (s << 16) - cc; /* t[4] < 2^48 + 2^30 */
418 * The final t[4] cannot overflow because cc is 0 or 1,
419 * and cc can be 1 only if s != 0.
433 * Montgomery squaring in the field; currently a basic wrapper around
434 * multiplication (inline, should be optimized away).
435 * TODO: see if some extra speed can be gained here.
438 f256_montysquare(uint64_t *d, const uint64_t *a)
440 f256_montymul(d, a, a);
444 * Convert to Montgomery representation.
447 f256_tomonty(uint64_t *d, const uint64_t *a)
451 * If R = 2^260 mod p, then R2 = R^2 mod p; and the Montgomery
452 * multiplication of a by R2 is: a*R2/R = a*R mod p, i.e. the
453 * conversion to Montgomery representation.
455 static const uint64_t R2[] = {
456 0x0000000000300, 0xFFFFFFFF00000, 0xFFFFEFFFFFFFB,
457 0xFDFFFFFFFFFFF, 0x0000004FFFFFF
460 f256_montymul(d, a, R2);
464 * Convert from Montgomery representation.
467 f256_frommonty(uint64_t *d, const uint64_t *a)
470 * Montgomery multiplication by 1 is division by 2^260 modulo p.
472 static const uint64_t one[] = { 1, 0, 0, 0, 0 };
474 f256_montymul(d, a, one);
478 * Inversion in the field. If the source value is 0 modulo p, then this
479 * returns 0 or p. This function uses Montgomery representation.
482 f256_invert(uint64_t *d, const uint64_t *a)
485 * We compute a^(p-2) mod p. The exponent pattern (from high to
487 * - 32 bits of value 1
488 * - 31 bits of value 0
490 * - 96 bits of value 0
491 * - 94 bits of value 1
494 * To speed up the square-and-multiply algorithm, we precompute
501 memcpy(t, a, sizeof t);
502 for (i = 0; i < 30; i ++) {
503 f256_montysquare(t, t);
504 f256_montymul(t, t, a);
507 memcpy(r, t, sizeof t);
508 for (i = 224; i >= 0; i --) {
509 f256_montysquare(r, r);
515 f256_montymul(r, r, a);
520 f256_montymul(r, r, t);
524 memcpy(d, r, sizeof r);
528 * Finalize reduction.
529 * Input value should be partially reduced.
530 * On output, limbs a[0] to a[3] fit on 52 bits each, limb a[4] fits
531 * on 48 bits, and the integer is less than p.
534 f256_final_reduce(uint64_t *a)
536 uint64_t r[5], t[5], w, cc;
540 * Propagate carries to ensure that limbs 0 to 3 fit on 52 bits.
543 for (i = 0; i < 5; i ++) {
550 * We compute t = r + (2^256 - p) = r + 2^224 - 2^192 - 2^96 + 1.
551 * If t < 2^256, then r < p, and we return r. Otherwise, we
552 * want to return r - p = t - 2^256.
556 * Add 2^224 + 1, and propagate carries to ensure that limbs
557 * t[0] to t[3] fit in 52 bits each.
571 t[4] = r[4] + cc + BIT(16);
574 * Subtract 2^192 + 2^96. Since we just added 2^224 + 1, the
575 * result cannot be negative.
589 * If the top limb t[4] fits on 48 bits, then r[] is already
590 * in the proper range. Otherwise, t[] is the value to return
591 * (truncated to 256 bits).
595 for (i = 0; i < 5; i ++) {
596 a[i] = r[i] ^ (cc & (r[i] ^ t[i]));
601 * Points in affine and Jacobian coordinates.
603 * - In affine coordinates, the point-at-infinity cannot be encoded.
604 * - Jacobian coordinates (X,Y,Z) correspond to affine (X/Z^2,Y/Z^3);
605 * if Z = 0 then this is the point-at-infinity.
619 * Decode a field element (unsigned big endian notation).
622 f256_decode(uint64_t *a, const unsigned char *buf)
624 uint64_t w0, w1, w2, w3;
626 w3 = br_dec64be(buf + 0);
627 w2 = br_dec64be(buf + 8);
628 w1 = br_dec64be(buf + 16);
629 w0 = br_dec64be(buf + 24);
631 a[1] = ((w0 >> 52) | (w1 << 12)) & MASK52;
632 a[2] = ((w1 >> 40) | (w2 << 24)) & MASK52;
633 a[3] = ((w2 >> 28) | (w3 << 36)) & MASK52;
638 * Encode a field element (unsigned big endian notation). The field
639 * element MUST be fully reduced.
642 f256_encode(unsigned char *buf, const uint64_t *a)
644 uint64_t w0, w1, w2, w3;
646 w0 = a[0] | (a[1] << 52);
647 w1 = (a[1] >> 12) | (a[2] << 40);
648 w2 = (a[2] >> 24) | (a[3] << 28);
649 w3 = (a[3] >> 36) | (a[4] << 16);
650 br_enc64be(buf + 0, w3);
651 br_enc64be(buf + 8, w2);
652 br_enc64be(buf + 16, w1);
653 br_enc64be(buf + 24, w0);
657 * Decode a point. The returned point is in Jacobian coordinates, but
658 * with z = 1. If the encoding is invalid, or encodes a point which is
659 * not on the curve, or encodes the point at infinity, then this function
660 * returns 0. Otherwise, 1 is returned.
662 * The buffer is assumed to have length exactly 65 bytes.
665 point_decode(p256_jacobian *P, const unsigned char *buf)
667 uint64_t x[5], y[5], t[5], x3[5], tt;
671 * Header byte shall be 0x04.
673 r = EQ(buf[0], 0x04);
676 * Decode X and Y coordinates, and convert them into
677 * Montgomery representation.
679 f256_decode(x, buf + 1);
680 f256_decode(y, buf + 33);
685 * Verify y^2 = x^3 + A*x + B. In curve P-256, A = -3.
686 * Note that the Montgomery representation of 0 is 0. We must
687 * take care to apply the final reduction to make sure we have
690 f256_montysquare(t, y);
691 f256_montysquare(x3, x);
692 f256_montymul(x3, x3, x);
697 f256_sub(t, t, P256_B_MONTY);
698 f256_final_reduce(t);
699 tt = t[0] | t[1] | t[2] | t[3] | t[4];
700 r &= EQ((uint32_t)(tt | (tt >> 32)), 0);
703 * Return the point in Jacobian coordinates (and Montgomery
706 memcpy(P->x, x, sizeof x);
707 memcpy(P->y, y, sizeof y);
708 memcpy(P->z, F256_R, sizeof F256_R);
713 * Final conversion for a point:
714 * - The point is converted back to affine coordinates.
715 * - Final reduction is performed.
716 * - The point is encoded into the provided buffer.
718 * If the point is the point-at-infinity, all operations are performed,
719 * but the buffer contents are indeterminate, and 0 is returned. Otherwise,
720 * the encoded point is written in the buffer, and 1 is returned.
723 point_encode(unsigned char *buf, const p256_jacobian *P)
725 uint64_t t1[5], t2[5], z;
727 /* Set t1 = 1/z^2 and t2 = 1/z^3. */
728 f256_invert(t2, P->z);
729 f256_montysquare(t1, t2);
730 f256_montymul(t2, t2, t1);
732 /* Compute affine coordinates x (in t1) and y (in t2). */
733 f256_montymul(t1, P->x, t1);
734 f256_montymul(t2, P->y, t2);
736 /* Convert back from Montgomery representation, and finalize
738 f256_frommonty(t1, t1);
739 f256_frommonty(t2, t2);
740 f256_final_reduce(t1);
741 f256_final_reduce(t2);
745 f256_encode(buf + 1, t1);
746 f256_encode(buf + 33, t2);
748 /* Return success if and only if P->z != 0. */
749 z = P->z[0] | P->z[1] | P->z[2] | P->z[3] | P->z[4];
750 return NEQ((uint32_t)(z | z >> 32), 0);
754 * Point doubling in Jacobian coordinates: point P is doubled.
755 * Note: if the source point is the point-at-infinity, then the result is
756 * still the point-at-infinity, which is correct. Moreover, if the three
757 * coordinates were zero, then they still are zero in the returned value.
760 p256_double(p256_jacobian *P)
763 * Doubling formulas are:
766 * m = 3*(x + z^2)*(x - z^2)
768 * y' = m*(s - x') - 8*y^4
771 * These formulas work for all points, including points of order 2
772 * and points at infinity:
773 * - If y = 0 then z' = 0. But there is no such point in P-256
775 * - If z = 0 then z' = 0.
777 uint64_t t1[5], t2[5], t3[5], t4[5];
782 f256_montysquare(t1, P->z);
785 * Compute x-z^2 in t2 and x+z^2 in t1.
787 f256_add(t2, P->x, t1);
788 f256_sub(t1, P->x, t1);
791 * Compute 3*(x+z^2)*(x-z^2) in t1.
793 f256_montymul(t3, t1, t2);
794 f256_add(t1, t3, t3);
795 f256_add(t1, t3, t1);
798 * Compute 4*x*y^2 (in t2) and 2*y^2 (in t3).
800 f256_montysquare(t3, P->y);
801 f256_add(t3, t3, t3);
802 f256_montymul(t2, P->x, t3);
803 f256_add(t2, t2, t2);
806 * Compute x' = m^2 - 2*s.
808 f256_montysquare(P->x, t1);
809 f256_sub(P->x, P->x, t2);
810 f256_sub(P->x, P->x, t2);
813 * Compute z' = 2*y*z.
815 f256_montymul(t4, P->y, P->z);
816 f256_add(P->z, t4, t4);
817 f256_partial_reduce(P->z);
820 * Compute y' = m*(s - x') - 8*y^4. Note that we already have
823 f256_sub(t2, t2, P->x);
824 f256_montymul(P->y, t1, t2);
825 f256_montysquare(t4, t3);
826 f256_add(t4, t4, t4);
827 f256_sub(P->y, P->y, t4);
831 * Point addition (Jacobian coordinates): P1 is replaced with P1+P2.
832 * This function computes the wrong result in the following cases:
834 * - If P1 == 0 but P2 != 0
835 * - If P1 != 0 but P2 == 0
838 * In all three cases, P1 is set to the point at infinity.
840 * Returned value is 0 if one of the following occurs:
842 * - P1 and P2 have the same Y coordinate.
843 * - P1 == 0 and P2 == 0.
844 * - The Y coordinate of one of the points is 0 and the other point is
845 * the point at infinity.
847 * The third case cannot actually happen with valid points, since a point
848 * with Y == 0 is a point of order 2, and there is no point of order 2 on
851 * Therefore, assuming that P1 != 0 and P2 != 0 on input, then the caller
852 * can apply the following:
854 * - If the result is not the point at infinity, then it is correct.
855 * - Otherwise, if the returned value is 1, then this is a case of
856 * P1+P2 == 0, so the result is indeed the point at infinity.
857 * - Otherwise, P1 == P2, so a "double" operation should have been
860 * Note that you can get a returned value of 0 with a correct result,
861 * e.g. if P1 and P2 have the same Y coordinate, but distinct X coordinates.
864 p256_add(p256_jacobian *P1, const p256_jacobian *P2)
867 * Addtions formulas are:
875 * x3 = r^2 - h^3 - 2 * u1 * h^2
876 * y3 = r * (u1 * h^2 - x3) - s1 * h^3
879 uint64_t t1[5], t2[5], t3[5], t4[5], t5[5], t6[5], t7[5], tt;
883 * Compute u1 = x1*z2^2 (in t1) and s1 = y1*z2^3 (in t3).
885 f256_montysquare(t3, P2->z);
886 f256_montymul(t1, P1->x, t3);
887 f256_montymul(t4, P2->z, t3);
888 f256_montymul(t3, P1->y, t4);
891 * Compute u2 = x2*z1^2 (in t2) and s2 = y2*z1^3 (in t4).
893 f256_montysquare(t4, P1->z);
894 f256_montymul(t2, P2->x, t4);
895 f256_montymul(t5, P1->z, t4);
896 f256_montymul(t4, P2->y, t5);
899 * Compute h = h2 - u1 (in t2) and r = s2 - s1 (in t4).
900 * We need to test whether r is zero, so we will do some extra
903 f256_sub(t2, t2, t1);
904 f256_sub(t4, t4, t3);
905 f256_final_reduce(t4);
906 tt = t4[0] | t4[1] | t4[2] | t4[3] | t4[4];
907 ret = (uint32_t)(tt | (tt >> 32));
908 ret = (ret | -ret) >> 31;
911 * Compute u1*h^2 (in t6) and h^3 (in t5);
913 f256_montysquare(t7, t2);
914 f256_montymul(t6, t1, t7);
915 f256_montymul(t5, t7, t2);
918 * Compute x3 = r^2 - h^3 - 2*u1*h^2.
920 f256_montysquare(P1->x, t4);
921 f256_sub(P1->x, P1->x, t5);
922 f256_sub(P1->x, P1->x, t6);
923 f256_sub(P1->x, P1->x, t6);
926 * Compute y3 = r*(u1*h^2 - x3) - s1*h^3.
928 f256_sub(t6, t6, P1->x);
929 f256_montymul(P1->y, t4, t6);
930 f256_montymul(t1, t5, t3);
931 f256_sub(P1->y, P1->y, t1);
934 * Compute z3 = h*z1*z2.
936 f256_montymul(t1, P1->z, P2->z);
937 f256_montymul(P1->z, t1, t2);
943 * Point addition (mixed coordinates): P1 is replaced with P1+P2.
944 * This is a specialised function for the case when P2 is a non-zero point
945 * in affine coordinates.
947 * This function computes the wrong result in the following cases:
952 * In both cases, P1 is set to the point at infinity.
954 * Returned value is 0 if one of the following occurs:
956 * - P1 and P2 have the same Y (affine) coordinate.
957 * - The Y coordinate of P2 is 0 and P1 is the point at infinity.
959 * The second case cannot actually happen with valid points, since a point
960 * with Y == 0 is a point of order 2, and there is no point of order 2 on
963 * Therefore, assuming that P1 != 0 on input, then the caller
964 * can apply the following:
966 * - If the result is not the point at infinity, then it is correct.
967 * - Otherwise, if the returned value is 1, then this is a case of
968 * P1+P2 == 0, so the result is indeed the point at infinity.
969 * - Otherwise, P1 == P2, so a "double" operation should have been
972 * Again, a value of 0 may be returned in some cases where the addition
976 p256_add_mixed(p256_jacobian *P1, const p256_affine *P2)
979 * Addtions formulas are:
987 * x3 = r^2 - h^3 - 2 * u1 * h^2
988 * y3 = r * (u1 * h^2 - x3) - s1 * h^3
991 uint64_t t1[5], t2[5], t3[5], t4[5], t5[5], t6[5], t7[5], tt;
995 * Compute u1 = x1 (in t1) and s1 = y1 (in t3).
997 memcpy(t1, P1->x, sizeof t1);
998 memcpy(t3, P1->y, sizeof t3);
1001 * Compute u2 = x2*z1^2 (in t2) and s2 = y2*z1^3 (in t4).
1003 f256_montysquare(t4, P1->z);
1004 f256_montymul(t2, P2->x, t4);
1005 f256_montymul(t5, P1->z, t4);
1006 f256_montymul(t4, P2->y, t5);
1009 * Compute h = h2 - u1 (in t2) and r = s2 - s1 (in t4).
1010 * We need to test whether r is zero, so we will do some extra
1013 f256_sub(t2, t2, t1);
1014 f256_sub(t4, t4, t3);
1015 f256_final_reduce(t4);
1016 tt = t4[0] | t4[1] | t4[2] | t4[3] | t4[4];
1017 ret = (uint32_t)(tt | (tt >> 32));
1018 ret = (ret | -ret) >> 31;
1021 * Compute u1*h^2 (in t6) and h^3 (in t5);
1023 f256_montysquare(t7, t2);
1024 f256_montymul(t6, t1, t7);
1025 f256_montymul(t5, t7, t2);
1028 * Compute x3 = r^2 - h^3 - 2*u1*h^2.
1030 f256_montysquare(P1->x, t4);
1031 f256_sub(P1->x, P1->x, t5);
1032 f256_sub(P1->x, P1->x, t6);
1033 f256_sub(P1->x, P1->x, t6);
1036 * Compute y3 = r*(u1*h^2 - x3) - s1*h^3.
1038 f256_sub(t6, t6, P1->x);
1039 f256_montymul(P1->y, t4, t6);
1040 f256_montymul(t1, t5, t3);
1041 f256_sub(P1->y, P1->y, t1);
1044 * Compute z3 = h*z1*z2.
1046 f256_montymul(P1->z, P1->z, t2);
1054 * Point addition (mixed coordinates, complete): P1 is replaced with P1+P2.
1055 * This is a specialised function for the case when P2 is a non-zero point
1056 * in affine coordinates.
1058 * This function returns the correct result in all cases.
1061 p256_add_complete_mixed(p256_jacobian *P1, const p256_affine *P2)
1064 * Addtions formulas, in the general case, are:
1072 * x3 = r^2 - h^3 - 2 * u1 * h^2
1073 * y3 = r * (u1 * h^2 - x3) - s1 * h^3
1076 * These formulas mishandle the two following cases:
1078 * - If P1 is the point-at-infinity (z1 = 0), then z3 is
1079 * incorrectly set to 0.
1081 * - If P1 = P2, then u1 = u2 and s1 = s2, and x3, y3 and z3
1084 * However, if P1 + P2 = 0, then u1 = u2 but s1 != s2, and then
1085 * we correctly get z3 = 0 (the point-at-infinity).
1087 * To fix the case P1 = 0, we perform at the end a copy of P2
1088 * over P1, conditional to z1 = 0.
1090 * For P1 = P2: in that case, both h and r are set to 0, and
1091 * we get x3, y3 and z3 equal to 0. We can test for that
1092 * occurrence to make a mask which will be all-one if P1 = P2,
1093 * or all-zero otherwise; then we can compute the double of P2
1094 * and add it, combined with the mask, to (x3,y3,z3).
1096 * Using the doubling formulas in p256_double() on (x2,y2),
1097 * simplifying since P2 is affine (i.e. z2 = 1, implicitly),
1100 * m = 3*(x2 + 1)*(x2 - 1)
1102 * y' = m*(s - x') - 8*y2^4
1104 * which requires only 6 multiplications. Added to the 11
1105 * multiplications of the normal mixed addition in Jacobian
1106 * coordinates, we get a cost of 17 multiplications in total.
1108 uint64_t t1[5], t2[5], t3[5], t4[5], t5[5], t6[5], t7[5], tt, zz;
1112 * Set zz to -1 if P1 is the point at infinity, 0 otherwise.
1114 zz = P1->z[0] | P1->z[1] | P1->z[2] | P1->z[3] | P1->z[4];
1115 zz = ((zz | -zz) >> 63) - (uint64_t)1;
1118 * Compute u1 = x1 (in t1) and s1 = y1 (in t3).
1120 memcpy(t1, P1->x, sizeof t1);
1121 memcpy(t3, P1->y, sizeof t3);
1124 * Compute u2 = x2*z1^2 (in t2) and s2 = y2*z1^3 (in t4).
1126 f256_montysquare(t4, P1->z);
1127 f256_montymul(t2, P2->x, t4);
1128 f256_montymul(t5, P1->z, t4);
1129 f256_montymul(t4, P2->y, t5);
1132 * Compute h = h2 - u1 (in t2) and r = s2 - s1 (in t4).
1135 f256_sub(t2, t2, t1);
1136 f256_sub(t4, t4, t3);
1139 * If both h = 0 and r = 0, then P1 = P2, and we want to set
1140 * the mask tt to -1; otherwise, the mask will be 0.
1142 f256_final_reduce(t2);
1143 f256_final_reduce(t4);
1144 tt = t2[0] | t2[1] | t2[2] | t2[3] | t2[4]
1145 | t4[0] | t4[1] | t4[2] | t4[3] | t4[4];
1146 tt = ((tt | -tt) >> 63) - (uint64_t)1;
1149 * Compute u1*h^2 (in t6) and h^3 (in t5);
1151 f256_montysquare(t7, t2);
1152 f256_montymul(t6, t1, t7);
1153 f256_montymul(t5, t7, t2);
1156 * Compute x3 = r^2 - h^3 - 2*u1*h^2.
1158 f256_montysquare(P1->x, t4);
1159 f256_sub(P1->x, P1->x, t5);
1160 f256_sub(P1->x, P1->x, t6);
1161 f256_sub(P1->x, P1->x, t6);
1164 * Compute y3 = r*(u1*h^2 - x3) - s1*h^3.
1166 f256_sub(t6, t6, P1->x);
1167 f256_montymul(P1->y, t4, t6);
1168 f256_montymul(t1, t5, t3);
1169 f256_sub(P1->y, P1->y, t1);
1172 * Compute z3 = h*z1.
1174 f256_montymul(P1->z, P1->z, t2);
1177 * The "double" result, in case P1 = P2.
1181 * Compute z' = 2*y2 (in t1).
1183 f256_add(t1, P2->y, P2->y);
1184 f256_partial_reduce(t1);
1187 * Compute 2*(y2^2) (in t2) and s = 4*x2*(y2^2) (in t3).
1189 f256_montysquare(t2, P2->y);
1190 f256_add(t2, t2, t2);
1191 f256_add(t3, t2, t2);
1192 f256_montymul(t3, P2->x, t3);
1195 * Compute m = 3*(x2^2 - 1) (in t4).
1197 f256_montysquare(t4, P2->x);
1198 f256_sub(t4, t4, F256_R);
1199 f256_add(t5, t4, t4);
1200 f256_add(t4, t4, t5);
1203 * Compute x' = m^2 - 2*s (in t5).
1205 f256_montysquare(t5, t4);
1210 * Compute y' = m*(s - x') - 8*y2^4 (in t6).
1212 f256_sub(t6, t3, t5);
1213 f256_montymul(t6, t6, t4);
1214 f256_montysquare(t7, t2);
1215 f256_sub(t6, t6, t7);
1216 f256_sub(t6, t6, t7);
1219 * We now have the alternate (doubling) coordinates in (t5,t6,t1).
1220 * We combine them with (x3,y3,z3).
1222 for (i = 0; i < 5; i ++) {
1223 P1->x[i] |= tt & t5[i];
1224 P1->y[i] |= tt & t6[i];
1225 P1->z[i] |= tt & t1[i];
1229 * If P1 = 0, then we get z3 = 0 (which is invalid); if z1 is 0,
1230 * then we want to replace the result with a copy of P2. The
1231 * test on z1 was done at the start, in the zz mask.
1233 for (i = 0; i < 5; i ++) {
1234 P1->x[i] ^= zz & (P1->x[i] ^ P2->x[i]);
1235 P1->y[i] ^= zz & (P1->y[i] ^ P2->y[i]);
1236 P1->z[i] ^= zz & (P1->z[i] ^ F256_R[i]);
1242 * Inner function for computing a point multiplication. A window is
1243 * provided, with points 1*P to 15*P in affine coordinates.
1246 * - All provided points are valid points on the curve.
1247 * - Multiplier is non-zero, and smaller than the curve order.
1248 * - Everything is in Montgomery representation.
1251 point_mul_inner(p256_jacobian *R, const p256_affine *W,
1252 const unsigned char *k, size_t klen)
1257 memset(&Q, 0, sizeof Q);
1259 while (klen -- > 0) {
1264 for (i = 0; i < 2; i ++) {
1277 bits = (bk >> 4) & 0x0F;
1281 * Lookup point in window. If the bits are 0,
1282 * we get something invalid, which is not a
1283 * problem because we will use it only if the
1284 * bits are non-zero.
1286 memset(&T, 0, sizeof T);
1287 for (n = 0; n < 15; n ++) {
1288 m = -(uint64_t)EQ(bits, n + 1);
1289 T.x[0] |= m & W[n].x[0];
1290 T.x[1] |= m & W[n].x[1];
1291 T.x[2] |= m & W[n].x[2];
1292 T.x[3] |= m & W[n].x[3];
1293 T.x[4] |= m & W[n].x[4];
1294 T.y[0] |= m & W[n].y[0];
1295 T.y[1] |= m & W[n].y[1];
1296 T.y[2] |= m & W[n].y[2];
1297 T.y[3] |= m & W[n].y[3];
1298 T.y[4] |= m & W[n].y[4];
1302 p256_add_mixed(&U, &T);
1305 * If qz is still 1, then Q was all-zeros, and this
1306 * is conserved through p256_double().
1308 m = -(uint64_t)(bnz & qz);
1309 for (j = 0; j < 5; j ++) {
1310 Q.x[j] ^= m & (Q.x[j] ^ T.x[j]);
1311 Q.y[j] ^= m & (Q.y[j] ^ T.y[j]);
1312 Q.z[j] ^= m & (Q.z[j] ^ F256_R[j]);
1314 CCOPY(bnz & ~qz, &Q, &U, sizeof Q);
1323 * Convert a window from Jacobian to affine coordinates. A single
1324 * field inversion is used. This function works for windows up to
1327 * The destination array (aff[]) and the source array (jac[]) may
1328 * overlap, provided that the start of aff[] is not after the start of
1329 * jac[]. Even if the arrays do _not_ overlap, the source array is
1333 window_to_affine(p256_affine *aff, p256_jacobian *jac, int num)
1336 * Convert the window points to affine coordinates. We use the
1337 * following trick to mutualize the inversion computation: if
1338 * we have z1, z2, z3, and z4, and want to invert all of them,
1339 * we compute u = 1/(z1*z2*z3*z4), and then we have:
1345 * The partial products are computed recursively:
1347 * - on input (z_1,z_2), return (z_2,z_1) and z_1*z_2
1348 * - on input (z_1,z_2,... z_n):
1349 * recurse on (z_1,z_2,... z_(n/2)) -> r1 and m1
1350 * recurse on (z_(n/2+1),z_(n/2+2)... z_n) -> r2 and m2
1351 * multiply elements of r1 by m2 -> s1
1352 * multiply elements of r2 by m1 -> s2
1353 * return r1||r2 and m1*m2
1355 * In the example below, we suppose that we have 14 elements.
1356 * Let z1, z2,... zE be the 14 values to invert (index noted in
1357 * hexadecimal, starting at 1).
1360 * swap(z1, z2); z12 = z1*z2
1361 * swap(z3, z4); z34 = z3*z4
1362 * swap(z5, z6); z56 = z5*z6
1363 * swap(z7, z8); z78 = z7*z8
1364 * swap(z9, zA); z9A = z9*zA
1365 * swap(zB, zC); zBC = zB*zC
1366 * swap(zD, zE); zDE = zD*zE
1369 * z1 <- z1*z34, z2 <- z2*z34, z3 <- z3*z12, z4 <- z4*z12
1371 * z5 <- z5*z78, z6 <- z6*z78, z7 <- z7*z56, z8 <- z8*z56
1373 * z9 <- z9*zBC, zA <- zA*zBC, zB <- zB*z9A, zC <- zC*z9A
1377 * z1 <- z1*z5678, z2 <- z2*z5678, z3 <- z3*z5678, z4 <- z4*z5678
1378 * z5 <- z5*z1234, z6 <- z6*z1234, z7 <- z7*z1234, z8 <- z8*z1234
1379 * z12345678 = z1234*z5678
1380 * z9 <- z9*zDE, zA <- zA*zDE, zB <- zB*zDE, zC <- zC*zDE
1381 * zD <- zD*z9ABC, zE*z9ABC
1382 * z9ABCDE = z9ABC*zDE
1385 * multiply z1..z8 by z9ABCDE
1386 * multiply z9..zE by z12345678
1387 * final z = z12345678*z9ABCDE
1397 * First recursion step (pairwise swapping and multiplication).
1398 * If there is an odd number of elements, then we "invent" an
1399 * extra one with coordinate Z = 1 (in Montgomery representation).
1401 for (i = 0; (i + 1) < num; i += 2) {
1402 memcpy(zt, jac[i].z, sizeof zt);
1403 memcpy(jac[i].z, jac[i + 1].z, sizeof zt);
1404 memcpy(jac[i + 1].z, zt, sizeof zt);
1405 f256_montymul(z[i >> 1], jac[i].z, jac[i + 1].z);
1407 if ((num & 1) != 0) {
1408 memcpy(z[num >> 1], jac[num - 1].z, sizeof zt);
1409 memcpy(jac[num - 1].z, F256_R, sizeof F256_R);
1413 * Perform further recursion steps. At the entry of each step,
1414 * the process has been done for groups of 's' points. The
1415 * integer k is the log2 of s.
1417 for (k = 1, s = 2; s < num; k ++, s <<= 1) {
1420 for (i = 0; i < num; i ++) {
1421 f256_montymul(jac[i].z, jac[i].z, z[(i >> k) ^ 1]);
1423 n = (num + s - 1) >> k;
1424 for (i = 0; i < (n >> 1); i ++) {
1425 f256_montymul(z[i], z[i << 1], z[(i << 1) + 1]);
1428 memmove(z[n >> 1], z[n], sizeof zt);
1433 * Invert the final result, and convert all points.
1435 f256_invert(zt, z[0]);
1436 for (i = 0; i < num; i ++) {
1437 f256_montymul(zv, jac[i].z, zt);
1438 f256_montysquare(zu, zv);
1439 f256_montymul(zv, zv, zu);
1440 f256_montymul(aff[i].x, jac[i].x, zu);
1441 f256_montymul(aff[i].y, jac[i].y, zv);
1446 * Multiply the provided point by an integer.
1448 * - Source point is a valid curve point.
1449 * - Source point is not the point-at-infinity.
1450 * - Integer is not 0, and is lower than the curve order.
1451 * If these conditions are not met, then the result is indeterminate
1452 * (but the process is still constant-time).
1455 p256_mul(p256_jacobian *P, const unsigned char *k, size_t klen)
1458 p256_affine aff[15];
1459 p256_jacobian jac[15];
1464 * Compute window, in Jacobian coordinates.
1467 for (i = 2; i < 16; i ++) {
1468 window.jac[i - 1] = window.jac[(i >> 1) - 1];
1470 p256_double(&window.jac[i - 1]);
1472 p256_add(&window.jac[i - 1], &window.jac[i >> 1]);
1477 * Convert the window points to affine coordinates. Point
1478 * window[0] is the source point, already in affine coordinates.
1480 window_to_affine(window.aff, window.jac, 15);
1483 * Perform point multiplication.
1485 point_mul_inner(P, window.aff, k, klen);
1489 * Precomputed window for the conventional generator: P256_Gwin[n]
1490 * contains (n+1)*G (affine coordinates, in Montgomery representation).
1492 static const p256_affine P256_Gwin[] = {
1494 { 0x30D418A9143C1, 0xC4FEDB60179E7, 0x62251075BA95F,
1495 0x5C669FB732B77, 0x08905F76B5375 },
1496 { 0x5357CE95560A8, 0x43A19E45CDDF2, 0x21F3258B4AB8E,
1497 0xD8552E88688DD, 0x0571FF18A5885 }
1500 { 0x46D410DDD64DF, 0x0B433827D8500, 0x1490D9AA6AE3C,
1501 0xA3A832205038D, 0x06BB32E52DCF3 },
1502 { 0x48D361BEE1A57, 0xB7B236FF82F36, 0x042DBE152CD7C,
1503 0xA3AA9A8FB0E92, 0x08C577517A5B8 }
1506 { 0x3F904EEBC1272, 0x9E87D81FBFFAC, 0xCBBC98B027F84,
1507 0x47E46AD77DD87, 0x06936A3FD6FF7 },
1508 { 0x5C1FC983A7EBD, 0xC3861FE1AB04C, 0x2EE98E583E47A,
1509 0xC06A88208311A, 0x05F06A2AB587C }
1512 { 0xB50D46918DCC5, 0xD7623C17374B0, 0x100AF24650A6E,
1513 0x76ABCDAACACE8, 0x077362F591B01 },
1514 { 0xF24CE4CBABA68, 0x17AD6F4472D96, 0xDDD22E1762847,
1515 0x862EB6C36DEE5, 0x04B14C39CC5AB }
1518 { 0x8AAEC45C61F5C, 0x9D4B9537DBE1B, 0x76C20C90EC649,
1519 0x3C7D41CB5AAD0, 0x0907960649052 },
1520 { 0x9B4AE7BA4F107, 0xF75EB882BEB30, 0x7A1F6873C568E,
1521 0x915C540A9877E, 0x03A076BB9DD1E }
1524 { 0x47373E77664A1, 0xF246CEE3E4039, 0x17A3AD55AE744,
1525 0x673C50A961A5B, 0x03074B5964213 },
1526 { 0x6220D377E44BA, 0x30DFF14B593D3, 0x639F11299C2B5,
1527 0x75F5424D44CEF, 0x04C9916DEA07F }
1530 { 0x354EA0173B4F1, 0x3C23C00F70746, 0x23BB082BD2021,
1531 0xE03E43EAAB50C, 0x03BA5119D3123 },
1532 { 0xD0303F5B9D4DE, 0x17DA67BDD2847, 0xC941956742F2F,
1533 0x8670F933BDC77, 0x0AEDD9164E240 }
1536 { 0x4CD19499A78FB, 0x4BF9B345527F1, 0x2CFC6B462AB5C,
1537 0x30CDF90F02AF0, 0x0763891F62652 },
1538 { 0xA3A9532D49775, 0xD7F9EBA15F59D, 0x60BBF021E3327,
1539 0xF75C23C7B84BE, 0x06EC12F2C706D }
1542 { 0x6E8F264E20E8E, 0xC79A7A84175C9, 0xC8EB00ABE6BFE,
1543 0x16A4CC09C0444, 0x005B3081D0C4E },
1544 { 0x777AA45F33140, 0xDCE5D45E31EB7, 0xB12F1A56AF7BE,
1545 0xF9B2B6E019A88, 0x086659CDFD835 }
1548 { 0xDBD19DC21EC8C, 0x94FCF81392C18, 0x250B4998F9868,
1549 0x28EB37D2CD648, 0x0C61C947E4B34 },
1550 { 0x407880DD9E767, 0x0C83FBE080C2B, 0x9BE5D2C43A899,
1551 0xAB4EF7D2D6577, 0x08719A555B3B4 }
1554 { 0x260A6245E4043, 0x53E7FDFE0EA7D, 0xAC1AB59DE4079,
1555 0x072EFF3A4158D, 0x0E7090F1949C9 },
1556 { 0x85612B944E886, 0xE857F61C81A76, 0xAD643D250F939,
1557 0x88DAC0DAA891E, 0x089300244125B }
1560 { 0x1AA7D26977684, 0x58A345A3304B7, 0x37385EABDEDEF,
1561 0x155E409D29DEE, 0x0EE1DF780B83E },
1562 { 0x12D91CBB5B437, 0x65A8956370CAC, 0xDE6D66170ED2F,
1563 0xAC9B8228CFA8A, 0x0FF57C95C3238 }
1566 { 0x25634B2ED7097, 0x9156FD30DCCC4, 0x9E98110E35676,
1567 0x7594CBCD43F55, 0x038477ACC395B },
1568 { 0x2B90C00EE17FF, 0xF842ED2E33575, 0x1F5BC16874838,
1569 0x7968CD06422BD, 0x0BC0876AB9E7B }
1572 { 0xA35BB0CF664AF, 0x68F9707E3A242, 0x832660126E48F,
1573 0x72D2717BF54C6, 0x0AAE7333ED12C },
1574 { 0x2DB7995D586B1, 0xE732237C227B5, 0x65E7DBBE29569,
1575 0xBBBD8E4193E2A, 0x052706DC3EAA1 }
1578 { 0xD8B7BC60055BE, 0xD76E27E4B72BC, 0x81937003CC23E,
1579 0xA090E337424E4, 0x02AA0E43EAD3D },
1580 { 0x524F6383C45D2, 0x422A41B2540B8, 0x8A4797D766355,
1581 0xDF444EFA6DE77, 0x0042170A9079A }
1586 * Multiply the conventional generator of the curve by the provided
1587 * integer. Return is written in *P.
1590 * - Integer is not 0, and is lower than the curve order.
1591 * If this conditions is not met, then the result is indeterminate
1592 * (but the process is still constant-time).
1595 p256_mulgen(p256_jacobian *P, const unsigned char *k, size_t klen)
1597 point_mul_inner(P, P256_Gwin, k, klen);
1601 * Return 1 if all of the following hold:
1604 * - k is lower than the curve order
1605 * Otherwise, return 0.
1607 * Constant-time behaviour: only klen may be observable.
1610 check_scalar(const unsigned char *k, size_t klen)
1620 for (u = 0; u < klen; u ++) {
1625 for (u = 0; u < klen; u ++) {
1626 c |= -(int32_t)EQ0(c) & CMP(k[u], P256_N[u]);
1631 return NEQ(z, 0) & LT0(c);
1635 api_mul(unsigned char *G, size_t Glen,
1636 const unsigned char *k, size_t klen, int curve)
1645 r = check_scalar(k, klen);
1646 r &= point_decode(&P, G);
1647 p256_mul(&P, k, klen);
1648 r &= point_encode(G, &P);
1653 api_mulgen(unsigned char *R,
1654 const unsigned char *k, size_t klen, int curve)
1659 p256_mulgen(&P, k, klen);
1660 point_encode(R, &P);
1665 api_muladd(unsigned char *A, const unsigned char *B, size_t len,
1666 const unsigned char *x, size_t xlen,
1667 const unsigned char *y, size_t ylen, int curve)
1670 * We might want to use Shamir's trick here: make a composite
1671 * window of u*P+v*Q points, to merge the two doubling-ladders
1672 * into one. This, however, has some complications:
1674 * - During the computation, we may hit the point-at-infinity.
1675 * Thus, we would need p256_add_complete_mixed() (complete
1676 * formulas for point addition), with a higher cost (17 muls
1679 * - A 4-bit window would be too large, since it would involve
1680 * 16*16-1 = 255 points. For the same window size as in the
1681 * p256_mul() case, we would need to reduce the window size
1682 * to 2 bits, and thus perform twice as many non-doubling
1685 * - The window may itself contain the point-at-infinity, and
1686 * thus cannot be in all generality be made of affine points.
1687 * Instead, we would need to make it a window of points in
1688 * Jacobian coordinates. Even p256_add_complete_mixed() would
1691 * For these reasons, the code below performs two separate
1692 * point multiplications, then computes the final point addition
1693 * (which is both a "normal" addition, and a doubling, to handle
1705 r = point_decode(&P, A);
1706 p256_mul(&P, x, xlen);
1708 p256_mulgen(&Q, y, ylen);
1710 r &= point_decode(&Q, B);
1711 p256_mul(&Q, y, ylen);
1715 * The final addition may fail in case both points are equal.
1717 t = p256_add(&P, &Q);
1718 f256_final_reduce(P.z);
1719 z = P.z[0] | P.z[1] | P.z[2] | P.z[3] | P.z[4];
1720 s = EQ((uint32_t)(z | (z >> 32)), 0);
1724 * If s is 1 then either P+Q = 0 (t = 1) or P = Q (t = 0). So we
1725 * have the following:
1727 * s = 0, t = 0 return P (normal addition)
1728 * s = 0, t = 1 return P (normal addition)
1729 * s = 1, t = 0 return Q (a 'double' case)
1730 * s = 1, t = 1 report an error (P+Q = 0)
1732 CCOPY(s & ~t, &P, &Q, sizeof Q);
1733 point_encode(A, &P);
1738 /* see bearssl_ec.h */
1739 const br_ec_impl br_ec_p256_m62 = {
1740 (uint32_t)0x00800000,
1749 /* see bearssl_ec.h */
1751 br_ec_p256_m62_get(void)
1753 return &br_ec_p256_m62;
1758 /* see bearssl_ec.h */
1760 br_ec_p256_m62_get(void)