1 /* Written by Lenka Fibikova <fibikova@exp-math.uni-essen.de>
2 * and Bodo Moeller for the OpenSSL project. */
3 /* ====================================================================
4 * Copyright (c) 1998-2000 The OpenSSL Project. All rights reserved.
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 * notice, this list of conditions and the following disclaimer in
15 * the documentation and/or other materials provided with the
18 * 3. All advertising materials mentioning features or use of this
19 * software must display the following acknowledgment:
20 * "This product includes software developed by the OpenSSL Project
21 * for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
23 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
24 * endorse or promote products derived from this software without
25 * prior written permission. For written permission, please contact
26 * openssl-core@openssl.org.
28 * 5. Products derived from this software may not be called "OpenSSL"
29 * nor may "OpenSSL" appear in their names without prior written
30 * permission of the OpenSSL Project.
32 * 6. Redistributions of any form whatsoever must retain the following
34 * "This product includes software developed by the OpenSSL Project
35 * for use in the OpenSSL Toolkit (http://www.openssl.org/)"
37 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
38 * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
39 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
40 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
41 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
42 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
43 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
44 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
45 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
46 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
47 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
48 * OF THE POSSIBILITY OF SUCH DAMAGE.
49 * ====================================================================
51 * This product includes cryptographic software written by Eric Young
52 * (eay@cryptsoft.com). This product includes software written by Tim
53 * Hudson (tjh@cryptsoft.com). */
55 #include <openssl/bn.h>
57 #include <openssl/err.h>
60 /* Returns 'ret' such that
62 * using the Tonelli/Shanks algorithm (cf. Henri Cohen, "A Course
63 * in Algebraic Computational Number Theory", algorithm 1.5.1).
64 * 'p' must be prime! */
65 BIGNUM *BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx) {
69 BIGNUM *A, *b, *q, *t, *x, *y;
72 if (!BN_is_odd(p) || BN_abs_is_word(p, 1)) {
73 if (BN_abs_is_word(p, 2)) {
80 if (!BN_set_word(ret, BN_is_bit_set(a, 0))) {
89 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_P_IS_NOT_PRIME);
93 if (BN_is_zero(a) || BN_is_one(a)) {
100 if (!BN_set_word(ret, BN_is_one(a))) {
128 if (!BN_nnmod(A, a, p, ctx)) {
132 /* now write |p| - 1 as 2^e*q where q is odd */
134 while (!BN_is_bit_set(p, e)) {
137 /* we'll set q later (if needed) */
140 /* The easy case: (|p|-1)/2 is odd, so 2 has an inverse
141 * modulo (|p|-1)/2, and square roots can be computed
142 * directly by modular exponentiation.
144 * 2 * (|p|+1)/4 == 1 (mod (|p|-1)/2),
145 * so we can use exponent (|p|+1)/4, i.e. (|p|-3)/4 + 1.
147 if (!BN_rshift(q, p, 2)) {
151 if (!BN_add_word(q, 1) ||
152 !BN_mod_exp(ret, A, q, p, ctx)) {
162 * In this case 2 is always a non-square since
163 * Legendre(2,p) = (-1)^((p^2-1)/8) for any odd prime.
164 * So if a really is a square, then 2*a is a non-square.
166 * b := (2*a)^((|p|-5)/8),
169 * i^2 = (2*a)^((1 + (|p|-5)/4)*2)
175 * x^2 = a^2 * b^2 * (i^2 - 2*i + 1)
176 * = a^2 * b^2 * (-2*i)
181 * (This is due to A.O.L. Atkin,
183 *http://listserv.nodak.edu/scripts/wa.exe?A2=ind9211&L=nmbrthry&O=T&P=562>,
188 if (!BN_mod_lshift1_quick(t, A, p)) {
192 /* b := (2*a)^((|p|-5)/8) */
193 if (!BN_rshift(q, p, 3)) {
197 if (!BN_mod_exp(b, t, q, p, ctx)) {
202 if (!BN_mod_sqr(y, b, p, ctx)) {
206 /* t := (2*a)*b^2 - 1*/
207 if (!BN_mod_mul(t, t, y, p, ctx) ||
208 !BN_sub_word(t, 1)) {
213 if (!BN_mod_mul(x, A, b, p, ctx) ||
214 !BN_mod_mul(x, x, t, p, ctx)) {
218 if (!BN_copy(ret, x)) {
225 /* e > 2, so we really have to use the Tonelli/Shanks algorithm.
226 * First, find some y that is not a square. */
227 if (!BN_copy(q, p)) {
228 goto end; /* use 'q' as temp */
233 /* For efficiency, try small numbers first;
234 * if this fails, try random numbers.
237 if (!BN_set_word(y, i)) {
241 if (!BN_pseudo_rand(y, BN_num_bits(p), 0, 0)) {
244 if (BN_ucmp(y, p) >= 0) {
245 if (!(p->neg ? BN_add : BN_sub)(y, y, p)) {
249 /* now 0 <= y < |p| */
251 if (!BN_set_word(y, i)) {
257 r = BN_kronecker(y, q, ctx); /* here 'q' is |p| */
263 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_P_IS_NOT_PRIME);
266 } while (r == 1 && ++i < 82);
269 /* Many rounds and still no non-square -- this is more likely
270 * a bug than just bad luck.
271 * Even if p is not prime, we should have found some y
274 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_TOO_MANY_ITERATIONS);
278 /* Here's our actual 'q': */
279 if (!BN_rshift(q, q, e)) {
283 /* Now that we have some non-square, we can find an element
284 * of order 2^e by computing its q'th power. */
285 if (!BN_mod_exp(y, y, q, p, ctx)) {
289 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_P_IS_NOT_PRIME);
293 /* Now we know that (if p is indeed prime) there is an integer
294 * k, 0 <= k < 2^e, such that
296 * a^q * y^k == 1 (mod p).
298 * As a^q is a square and y is not, k must be even.
299 * q+1 is even, too, so there is an element
301 * X := a^((q+1)/2) * y^(k/2),
305 * X^2 = a^q * a * y^k
308 * so it is the square root that we are looking for.
311 /* t := (q-1)/2 (note that q is odd) */
312 if (!BN_rshift1(t, q)) {
316 /* x := a^((q-1)/2) */
317 if (BN_is_zero(t)) /* special case: p = 2^e + 1 */
319 if (!BN_nnmod(t, A, p, ctx)) {
323 /* special case: a == 0 (mod p) */
327 } else if (!BN_one(x)) {
331 if (!BN_mod_exp(x, A, t, p, ctx)) {
335 /* special case: a == 0 (mod p) */
342 /* b := a*x^2 (= a^q) */
343 if (!BN_mod_sqr(b, x, p, ctx) ||
344 !BN_mod_mul(b, b, A, p, ctx)) {
348 /* x := a*x (= a^((q+1)/2)) */
349 if (!BN_mod_mul(x, x, A, p, ctx)) {
354 /* Now b is a^q * y^k for some even k (0 <= k < 2^E
355 * where E refers to the original value of e, which we
356 * don't keep in a variable), and x is a^((q+1)/2) * y^(k/2).
364 if (!BN_copy(ret, x)) {
372 /* find smallest i such that b^(2^i) = 1 */
374 if (!BN_mod_sqr(t, b, p, ctx)) {
377 while (!BN_is_one(t)) {
380 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_NOT_A_SQUARE);
383 if (!BN_mod_mul(t, t, t, p, ctx)) {
389 /* t := y^2^(e - i - 1) */
390 if (!BN_copy(t, y)) {
393 for (j = e - i - 1; j > 0; j--) {
394 if (!BN_mod_sqr(t, t, p, ctx)) {
398 if (!BN_mod_mul(y, t, t, p, ctx) ||
399 !BN_mod_mul(x, x, t, p, ctx) ||
400 !BN_mod_mul(b, b, y, p, ctx)) {
408 /* verify the result -- the input might have been not a square
409 * (test added in 0.9.8) */
411 if (!BN_mod_sqr(x, ret, p, ctx)) {
415 if (!err && 0 != BN_cmp(x, A)) {
416 OPENSSL_PUT_ERROR(BN, BN_mod_sqrt, BN_R_NOT_A_SQUARE);
423 if (ret != NULL && ret != in) {
432 int BN_sqrt(BIGNUM *out_sqrt, const BIGNUM *in, BN_CTX *ctx) {
433 BIGNUM *estimate, *tmp, *delta, *last_delta, *tmp2;
434 int ok = 0, last_delta_valid = 0;
437 OPENSSL_PUT_ERROR(BN, BN_sqrt, BN_R_NEGATIVE_NUMBER);
440 if (BN_is_zero(in)) {
446 if (out_sqrt == in) {
447 estimate = BN_CTX_get(ctx);
451 tmp = BN_CTX_get(ctx);
452 last_delta = BN_CTX_get(ctx);
453 delta = BN_CTX_get(ctx);
454 if (estimate == NULL || tmp == NULL || last_delta == NULL || delta == NULL) {
455 OPENSSL_PUT_ERROR(BN, BN_sqrt, ERR_R_MALLOC_FAILURE);
459 /* We estimate that the square root of an n-bit number is 2^{n/2}. */
460 BN_lshift(estimate, BN_value_one(), BN_num_bits(in)/2);
462 /* This is Newton's method for finding a root of the equation |estimate|^2 -
465 /* |estimate| = 1/2 * (|estimate| + |in|/|estimate|) */
466 if (!BN_div(tmp, NULL, in, estimate, ctx) ||
467 !BN_add(tmp, tmp, estimate) ||
468 !BN_rshift1(estimate, tmp) ||
469 /* |tmp| = |estimate|^2 */
470 !BN_sqr(tmp, estimate, ctx) ||
471 /* |delta| = |in| - |tmp| */
472 !BN_sub(delta, in, tmp)) {
473 OPENSSL_PUT_ERROR(BN, BN_sqrt, ERR_R_BN_LIB);
478 /* The difference between |in| and |estimate| squared is required to always
479 * decrease. This ensures that the loop always terminates, but I don't have
480 * a proof that it always finds the square root for a given square. */
481 if (last_delta_valid && BN_cmp(delta, last_delta) >= 0) {
485 last_delta_valid = 1;
492 if (BN_cmp(tmp, in) != 0) {
493 OPENSSL_PUT_ERROR(BN, BN_sqrt, BN_R_NOT_A_SQUARE);
500 if (ok && out_sqrt == in) {
501 BN_copy(out_sqrt, estimate);