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
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12 * The above copyright notice and this permission notice shall be
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15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
28 * Make a random integer of the provided size. The size is encoded.
29 * The header word is untouched.
32 mkrand(const br_prng_class **rng, uint16_t *x, uint32_t esize)
37 len = (esize + 15) >> 4;
38 (*rng)->generate(rng, x + 1, len * sizeof(uint16_t));
39 for (u = 1; u < len; u ++) {
46 x[len] &= 0x7FFF >> (15 - m);
51 * This is the big-endian unsigned representation of the product of
52 * all small primes from 13 to 1481.
54 static const unsigned char SMALL_PRIMES[] = {
55 0x2E, 0xAB, 0x92, 0xD1, 0x8B, 0x12, 0x47, 0x31, 0x54, 0x0A,
56 0x99, 0x5D, 0x25, 0x5E, 0xE2, 0x14, 0x96, 0x29, 0x1E, 0xB7,
57 0x78, 0x70, 0xCC, 0x1F, 0xA5, 0xAB, 0x8D, 0x72, 0x11, 0x37,
58 0xFB, 0xD8, 0x1E, 0x3F, 0x5B, 0x34, 0x30, 0x17, 0x8B, 0xE5,
59 0x26, 0x28, 0x23, 0xA1, 0x8A, 0xA4, 0x29, 0xEA, 0xFD, 0x9E,
60 0x39, 0x60, 0x8A, 0xF3, 0xB5, 0xA6, 0xEB, 0x3F, 0x02, 0xB6,
61 0x16, 0xC3, 0x96, 0x9D, 0x38, 0xB0, 0x7D, 0x82, 0x87, 0x0C,
62 0xF7, 0xBE, 0x24, 0xE5, 0x5F, 0x41, 0x04, 0x79, 0x76, 0x40,
63 0xE7, 0x00, 0x22, 0x7E, 0xB5, 0x85, 0x7F, 0x8D, 0x01, 0x50,
64 0xE9, 0xD3, 0x29, 0x42, 0x08, 0xB3, 0x51, 0x40, 0x7B, 0xD7,
65 0x8D, 0xCC, 0x10, 0x01, 0x64, 0x59, 0x28, 0xB6, 0x53, 0xF3,
66 0x50, 0x4E, 0xB1, 0xF2, 0x58, 0xCD, 0x6E, 0xF5, 0x56, 0x3E,
67 0x66, 0x2F, 0xD7, 0x07, 0x7F, 0x52, 0x4C, 0x13, 0x24, 0xDC,
68 0x8E, 0x8D, 0xCC, 0xED, 0x77, 0xC4, 0x21, 0xD2, 0xFD, 0x08,
69 0xEA, 0xD7, 0xC0, 0x5C, 0x13, 0x82, 0x81, 0x31, 0x2F, 0x2B,
70 0x08, 0xE4, 0x80, 0x04, 0x7A, 0x0C, 0x8A, 0x3C, 0xDC, 0x22,
71 0xE4, 0x5A, 0x7A, 0xB0, 0x12, 0x5E, 0x4A, 0x76, 0x94, 0x77,
72 0xC2, 0x0E, 0x92, 0xBA, 0x8A, 0xA0, 0x1F, 0x14, 0x51, 0x1E,
73 0x66, 0x6C, 0x38, 0x03, 0x6C, 0xC7, 0x4A, 0x4B, 0x70, 0x80,
74 0xAF, 0xCA, 0x84, 0x51, 0xD8, 0xD2, 0x26, 0x49, 0xF5, 0xA8,
75 0x5E, 0x35, 0x4B, 0xAC, 0xCE, 0x29, 0x92, 0x33, 0xB7, 0xA2,
76 0x69, 0x7D, 0x0C, 0xE0, 0x9C, 0xDB, 0x04, 0xD6, 0xB4, 0xBC,
77 0x39, 0xD7, 0x7F, 0x9E, 0x9D, 0x78, 0x38, 0x7F, 0x51, 0x54,
78 0x50, 0x8B, 0x9E, 0x9C, 0x03, 0x6C, 0xF5, 0x9D, 0x2C, 0x74,
79 0x57, 0xF0, 0x27, 0x2A, 0xC3, 0x47, 0xCA, 0xB9, 0xD7, 0x5C,
80 0xFF, 0xC2, 0xAC, 0x65, 0x4E, 0xBD
84 * We need temporary values for at least 7 integers of the same size
85 * as a factor (including header word); more space helps with performance
86 * (in modular exponentiations), but we much prefer to remain under
87 * 2 kilobytes in total, to save stack space. The macro TEMPS below
88 * exceeds 1024 (which is a count in 16-bit words) when BR_MAX_RSA_SIZE
89 * is greater than 4350 (default value is 4096, so the 2-kB limit is
90 * maintained unless BR_MAX_RSA_SIZE was modified).
92 #define MAX(x, y) ((x) > (y) ? (x) : (y))
93 #define TEMPS MAX(1024, 7 * ((((BR_MAX_RSA_SIZE + 1) >> 1) + 29) / 15))
96 * Perform trial division on a candidate prime. This computes
97 * y = SMALL_PRIMES mod x, then tries to compute y/y mod x. The
98 * br_i15_moddiv() function will report an error if y is not invertible
99 * modulo x. Returned value is 1 on success (none of the small primes
100 * divides x), 0 on error (a non-trivial GCD is obtained).
102 * This function assumes that x is odd.
105 trial_divisions(const uint16_t *x, uint16_t *t)
111 t += 1 + ((x[0] + 15) >> 4);
112 x0i = br_i15_ninv15(x[1]);
113 br_i15_decode_reduce(y, SMALL_PRIMES, sizeof SMALL_PRIMES, x);
114 return br_i15_moddiv(y, y, x, x0i, t);
118 * Perform n rounds of Miller-Rabin on the candidate prime x. This
119 * function assumes that x = 3 mod 4.
121 * Returned value is 1 on success (all rounds completed successfully),
125 miller_rabin(const br_prng_class **rng, const uint16_t *x, int n,
126 uint16_t *t, size_t tlen)
129 * Since x = 3 mod 4, the Miller-Rabin test is simple:
130 * - get a random base a (such that 1 < a < x-1)
131 * - compute z = a^((x-1)/2) mod x
132 * - if z != 1 and z != x-1, the number x is composite
134 * We generate bases 'a' randomly with a size which is
135 * one bit less than x, which ensures that a < x-1. It
136 * is not useful to verify that a > 1 because the probability
137 * that we get a value a equal to 0 or 1 is much smaller
138 * than the probability of our Miller-Rabin tests not to
139 * detect a composite, which is already quite smaller than the
140 * probability of the hardware misbehaving and return a
141 * composite integer because of some glitch (e.g. bad RAM
142 * or ill-timed cosmic ray).
144 unsigned char *xm1d2;
145 size_t xlen, xm1d2_len, xm1d2_len_u16, u;
151 * Compute (x-1)/2 (encoded).
153 xm1d2 = (unsigned char *)t;
154 xm1d2_len = ((x[0] - (x[0] >> 4)) + 7) >> 3;
155 br_i15_encode(xm1d2, xm1d2_len, x);
157 for (u = 0; u < xm1d2_len; u ++) {
161 xm1d2[u] = (unsigned char)((w >> 1) | cc);
166 * We used some words of the provided buffer for (x-1)/2.
168 xm1d2_len_u16 = (xm1d2_len + 1) >> 1;
170 tlen -= xm1d2_len_u16;
172 xlen = (x[0] + 15) >> 4;
173 asize = x[0] - 1 - EQ0(x[0] & 15);
174 x0i = br_i15_ninv15(x[1]);
180 * Generate a random base. We don't need the base to be
181 * really uniform modulo x, so we just get a random
182 * number which is one bit shorter than x.
187 mkrand(rng, a, asize);
190 * Compute a^((x-1)/2) mod x. We assume here that the
191 * function will not fail (the temporary array is large
194 br_i15_modpow_opt(a, xm1d2, xm1d2_len,
195 x, x0i, t + 1 + xlen, tlen - 1 - xlen);
198 * We must obtain either 1 or x-1. Note that x is odd,
199 * hence x-1 differs from x only in its low word (no
203 eqm1 = a[1] ^ (x[1] - 1);
204 for (u = 2; u <= xlen; u ++) {
209 if ((EQ0(eq1) | EQ0(eqm1)) == 0) {
217 * Create a random prime of the provided size. 'size' is the _encoded_
218 * bit length. The two top bits and the two bottom bits are set to 1.
221 mkprime(const br_prng_class **rng, uint16_t *x, uint32_t esize,
222 uint32_t pubexp, uint16_t *t, size_t tlen)
227 len = (esize + 15) >> 4;
230 uint32_t m3, m5, m7, m11;
234 * Generate random bits. We force the two top bits and the
235 * two bottom bits to 1.
237 mkrand(rng, x, esize);
238 if ((esize & 15) == 0) {
240 } else if ((esize & 15) == 1) {
242 x[len - 1] |= 0x4000;
244 x[len] |= 0x0003 << ((esize & 15) - 2);
249 * Trial division with low primes (3, 5, 7 and 11). We
250 * use the following properties:
261 for (u = 0; u < len; u ++) {
266 m3 = (m3 & 0xFF) + (m3 >> 8);
267 m5 += w << ((4 - u) & 3);
268 m5 = (m5 & 0xFF) + (m5 >> 8);
270 m7 = (m7 & 0x1FF) + (m7 >> 9);
271 m11 += w << (5 & -(u & 1));
272 m11 = (m11 & 0x3FF) + (m11 >> 10);
276 * Maximum values of m* at this point:
281 * We use the same properties to make further reductions.
284 m3 = (m3 & 0x0F) + (m3 >> 4); /* max: 46 */
285 m3 = (m3 & 0x0F) + (m3 >> 4); /* max: 16 */
286 m3 = ((m3 * 43) >> 5) & 3;
288 m5 = (m5 & 0xFF) + (m5 >> 8); /* max: 263 */
289 m5 = (m5 & 0x0F) + (m5 >> 4); /* max: 30 */
290 m5 = (m5 & 0x0F) + (m5 >> 4); /* max: 15 */
291 m5 -= 10 & -GT(m5, 9);
292 m5 -= 5 & -GT(m5, 4);
294 m7 = (m7 & 0x3F) + (m7 >> 6); /* max: 69 */
295 m7 = (m7 & 7) + (m7 >> 3); /* max: 14 */
296 m7 = ((m7 * 147) >> 7) & 7;
299 * 2^5 = 32 = -1 mod 11.
301 m11 = (m11 & 0x1F) + 66 - (m11 >> 5); /* max: 97 */
302 m11 -= 88 & -GT(m11, 87);
303 m11 -= 44 & -GT(m11, 43);
304 m11 -= 22 & -GT(m11, 21);
305 m11 -= 11 & -GT(m11, 10);
308 * If any of these modulo is 0, then the candidate is
309 * not prime. Also, if pubexp is 3, 5, 7 or 11, and the
310 * corresponding modulus is 1, then the candidate must
311 * be rejected, because we need e to be invertible
312 * modulo p-1. We can use simple comparisons here
313 * because they won't leak information on a candidate
314 * that we keep, only on one that we reject (and is thus
317 if (m3 == 0 || m5 == 0 || m7 == 0 || m11 == 0) {
320 if ((pubexp == 3 && m3 == 1)
321 || (pubexp == 5 && m5 == 5)
322 || (pubexp == 7 && m5 == 7)
323 || (pubexp == 11 && m5 == 11))
329 * More trial divisions.
331 if (!trial_divisions(x, t)) {
336 * Miller-Rabin algorithm. Since we selected a random
337 * integer, not a maliciously crafted integer, we can use
338 * relatively few rounds to lower the risk of a false
339 * positive (i.e. declaring prime a non-prime) under
340 * 2^(-80). It is not useful to lower the probability much
341 * below that, since that would be substantially below
342 * the probability of the hardware misbehaving. Sufficient
343 * numbers of rounds are extracted from the Handbook of
344 * Applied Cryptography, note 4.49 (page 149).
346 * Since we work on the encoded size (esize), we need to
347 * compare with encoded thresholds.
351 } else if (esize < 480) {
353 } else if (esize < 693) {
355 } else if (esize < 906) {
357 } else if (esize < 1386) {
363 if (miller_rabin(rng, x, rounds, t, tlen)) {
370 * Let p be a prime (p > 2^33, p = 3 mod 4). Let m = (p-1)/2, provided
371 * as parameter (with announced bit length equal to that of p). This
372 * function computes d = 1/e mod p-1 (for an odd integer e). Returned
373 * value is 1 on success, 0 on error (an error is reported if e is not
374 * invertible modulo p-1).
376 * The temporary buffer (t) must have room for at least 4 integers of
380 invert_pubexp(uint16_t *d, const uint16_t *m, uint32_t e, uint16_t *t)
386 t += 1 + ((m[0] + 15) >> 4);
389 * Compute d = 1/e mod m. Since p = 3 mod 4, m is odd.
391 br_i15_zero(d, m[0]);
393 br_i15_zero(f, m[0]);
395 f[2] = (e >> 15) & 0x7FFF;
397 r = br_i15_moddiv(d, f, m, br_i15_ninv15(m[1]), t);
400 * We really want d = 1/e mod p-1, with p = 2m. By the CRT,
401 * the result is either the d we got, or d + m.
403 * Let's write e*d = 1 + k*m, for some integer k. Integers e
404 * and m are odd. If d is odd, then e*d is odd, which implies
405 * that k must be even; in that case, e*d = 1 + (k/2)*2m, and
406 * thus d is already fine. Conversely, if d is even, then k
407 * is odd, and we must add m to d in order to get the correct
410 br_i15_add(d, m, (uint32_t)(1 - (d[1] & 1)));
416 * Swap two buffers in RAM. They must be disjoint.
419 bufswap(void *b1, void *b2, size_t len)
422 unsigned char *buf1, *buf2;
426 for (u = 0; u < len; u ++) {
435 /* see bearssl_rsa.h */
437 br_rsa_i15_keygen(const br_prng_class **rng,
438 br_rsa_private_key *sk, void *kbuf_priv,
439 br_rsa_public_key *pk, void *kbuf_pub,
440 unsigned size, uint32_t pubexp)
442 uint32_t esize_p, esize_q;
443 size_t plen, qlen, tlen;
448 if (size < BR_MIN_RSA_SIZE || size > BR_MAX_RSA_SIZE) {
453 } else if (pubexp == 1 || (pubexp & 1) == 0) {
457 esize_p = (size + 1) >> 1;
458 esize_q = size - esize_p;
461 sk->plen = (esize_p + 7) >> 3;
462 sk->q = sk->p + sk->plen;
463 sk->qlen = (esize_q + 7) >> 3;
464 sk->dp = sk->q + sk->qlen;
465 sk->dplen = sk->plen;
466 sk->dq = sk->dp + sk->dplen;
467 sk->dqlen = sk->qlen;
468 sk->iq = sk->dq + sk->dqlen;
469 sk->iqlen = sk->plen;
473 pk->nlen = (size + 7) >> 3;
474 pk->e = pk->n + pk->nlen;
476 br_enc32be(pk->e, pubexp);
477 while (*pk->e == 0) {
484 * We now switch to encoded sizes.
486 * floor((x * 17477) / (2^18)) is equal to floor(x/15) for all
487 * integers x from 0 to 23833.
489 esize_p += MUL15(esize_p, 17477) >> 18;
490 esize_q += MUL15(esize_q, 17477) >> 18;
491 plen = (esize_p + 15) >> 4;
492 qlen = (esize_q + 15) >> 4;
496 tlen = ((sizeof tmp) / sizeof(uint16_t)) - (2 + plen + qlen);
499 * When looking for primes p and q, we temporarily divide
500 * candidates by 2, in order to compute the inverse of the
505 mkprime(rng, p, esize_p, pubexp, t, tlen);
507 if (invert_pubexp(t, p, pubexp, t + 1 + plen)) {
510 br_i15_encode(sk->p, sk->plen, p);
511 br_i15_encode(sk->dp, sk->dplen, t);
517 mkprime(rng, q, esize_q, pubexp, t, tlen);
519 if (invert_pubexp(t, q, pubexp, t + 1 + qlen)) {
522 br_i15_encode(sk->q, sk->qlen, q);
523 br_i15_encode(sk->dq, sk->dqlen, t);
529 * If p and q have the same size, then it is possible that q > p
530 * (when the target modulus size is odd, we generate p with a
531 * greater bit length than q). If q > p, we want to swap p and q
532 * (and also dp and dq) for two reasons:
533 * - The final step below (inversion of q modulo p) is easier if
535 * - While BearSSL's RSA code is perfectly happy with RSA keys such
536 * that p < q, some other implementations have restrictions and
539 * Note that we can do a simple non-constant-time swap here,
540 * because the only information we leak here is that we insist on
541 * returning p and q such that p > q, which is not a secret.
543 if (esize_p == esize_q && br_i15_sub(p, q, 0) == 1) {
544 bufswap(p, q, (1 + plen) * sizeof *p);
545 bufswap(sk->p, sk->q, sk->plen);
546 bufswap(sk->dp, sk->dq, sk->dplen);
550 * We have produced p, q, dp and dq. We can now compute iq = 1/d mod p.
552 * We ensured that p >= q, so this is just a matter of updating the
553 * header word for q (and possibly adding an extra word).
555 * Theoretically, the call below may fail, in case we were
556 * extraordinarily unlucky, and p = q. Another failure case is if
557 * Miller-Rabin failed us _twice_, and p and q are non-prime and
558 * have a factor is common. We report the error mostly because it
559 * is cheap and we can, but in practice this never happens (or, at
560 * least, it happens way less often than hardware glitches).
568 br_i15_zero(t, p[0]);
570 r = br_i15_moddiv(t, q, p, br_i15_ninv15(p[1]), t + 1 + plen);
571 br_i15_encode(sk->iq, sk->iqlen, t);
574 * Compute the public modulus too, if required.
577 br_i15_zero(t, p[0]);
578 br_i15_mulacc(t, p, q);
579 br_i15_encode(pk->n, pk->nlen, t);