1 /* crypto/bn/bn_sqrt.c */ 2 /* Written by Lenka Fibikova <fibikova@exp-math.uni-essen.de> 3 * and Bodo Moeller for the OpenSSL project. */ 4 /* ==================================================================== 5 * Copyright (c) 1998-2000 The OpenSSL Project. All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 11 * 1. Redistributions of source code must retain the above copyright 12 * notice, this list of conditions and the following disclaimer. 13 * 14 * 2. Redistributions in binary form must reproduce the above copyright 15 * notice, this list of conditions and the following disclaimer in 16 * the documentation and/or other materials provided with the 17 * distribution. 18 * 19 * 3. All advertising materials mentioning features or use of this 20 * software must display the following acknowledgment: 21 * "This product includes software developed by the OpenSSL Project 22 * for use in the OpenSSL Toolkit. (http://www.openssl.org/)" 23 * 24 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to 25 * endorse or promote products derived from this software without 26 * prior written permission. For written permission, please contact 27 * openssl-core@openssl.org. 28 * 29 * 5. Products derived from this software may not be called "OpenSSL" 30 * nor may "OpenSSL" appear in their names without prior written 31 * permission of the OpenSSL Project. 32 * 33 * 6. Redistributions of any form whatsoever must retain the following 34 * acknowledgment: 35 * "This product includes software developed by the OpenSSL Project 36 * for use in the OpenSSL Toolkit (http://www.openssl.org/)" 37 * 38 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 39 * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 40 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 41 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 42 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 43 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 44 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 45 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 46 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 47 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 48 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 49 * OF THE POSSIBILITY OF SUCH DAMAGE. 50 * ==================================================================== 51 * 52 * This product includes cryptographic software written by Eric Young 53 * (eay@cryptsoft.com). This product includes software written by Tim 54 * Hudson (tjh@cryptsoft.com). 55 * 56 */ 57 58 #include "cryptlib.h" 59 #include "bn_lcl.h" 60 61 62 BIGNUM *BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx) 63 /* Returns 'ret' such that 64 * ret^2 == a (mod p), 65 * using the Tonelli/Shanks algorithm (cf. Henri Cohen, "A Course 66 * in Algebraic Computational Number Theory", algorithm 1.5.1). 67 * 'p' must be prime! 68 */ 69 { 70 BIGNUM *ret = in; 71 int err = 1; 72 int r; 73 BIGNUM *A, *b, *q, *t, *x, *y; 74 int e, i, j; 75 76 if (!BN_is_odd(p) || BN_abs_is_word(p, 1)) 77 { 78 if (BN_abs_is_word(p, 2)) 79 { 80 if (ret == NULL) 81 ret = BN_new(); 82 if (ret == NULL) 83 goto end; 84 if (!BN_set_word(ret, BN_is_bit_set(a, 0))) 85 { 86 if (ret != in) 87 BN_free(ret); 88 return NULL; 89 } 90 bn_check_top(ret); 91 return ret; 92 } 93 94 BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME); 95 return(NULL); 96 } 97 98 if (BN_is_zero(a) || BN_is_one(a)) 99 { 100 if (ret == NULL) 101 ret = BN_new(); 102 if (ret == NULL) 103 goto end; 104 if (!BN_set_word(ret, BN_is_one(a))) 105 { 106 if (ret != in) 107 BN_free(ret); 108 return NULL; 109 } 110 bn_check_top(ret); 111 return ret; 112 } 113 114 BN_CTX_start(ctx); 115 A = BN_CTX_get(ctx); 116 b = BN_CTX_get(ctx); 117 q = BN_CTX_get(ctx); 118 t = BN_CTX_get(ctx); 119 x = BN_CTX_get(ctx); 120 y = BN_CTX_get(ctx); 121 if (y == NULL) goto end; 122 123 if (ret == NULL) 124 ret = BN_new(); 125 if (ret == NULL) goto end; 126 127 /* A = a mod p */ 128 if (!BN_nnmod(A, a, p, ctx)) goto end; 129 130 /* now write |p| - 1 as 2^e*q where q is odd */ 131 e = 1; 132 while (!BN_is_bit_set(p, e)) 133 e++; 134 /* we'll set q later (if needed) */ 135 136 if (e == 1) 137 { 138 /* The easy case: (|p|-1)/2 is odd, so 2 has an inverse 139 * modulo (|p|-1)/2, and square roots can be computed 140 * directly by modular exponentiation. 141 * We have 142 * 2 * (|p|+1)/4 == 1 (mod (|p|-1)/2), 143 * so we can use exponent (|p|+1)/4, i.e. (|p|-3)/4 + 1. 144 */ 145 if (!BN_rshift(q, p, 2)) goto end; 146 q->neg = 0; 147 if (!BN_add_word(q, 1)) goto end; 148 if (!BN_mod_exp(ret, A, q, p, ctx)) goto end; 149 err = 0; 150 goto vrfy; 151 } 152 153 if (e == 2) 154 { 155 /* |p| == 5 (mod 8) 156 * 157 * In this case 2 is always a non-square since 158 * Legendre(2,p) = (-1)^((p^2-1)/8) for any odd prime. 159 * So if a really is a square, then 2*a is a non-square. 160 * Thus for 161 * b := (2*a)^((|p|-5)/8), 162 * i := (2*a)*b^2 163 * we have 164 * i^2 = (2*a)^((1 + (|p|-5)/4)*2) 165 * = (2*a)^((p-1)/2) 166 * = -1; 167 * so if we set 168 * x := a*b*(i-1), 169 * then 170 * x^2 = a^2 * b^2 * (i^2 - 2*i + 1) 171 * = a^2 * b^2 * (-2*i) 172 * = a*(-i)*(2*a*b^2) 173 * = a*(-i)*i 174 * = a. 175 * 176 * (This is due to A.O.L. Atkin, 177 * <URL: http://listserv.nodak.edu/scripts/wa.exe?A2=ind9211&L=nmbrthry&O=T&P=562>, 178 * November 1992.) 179 */ 180 181 /* t := 2*a */ 182 if (!BN_mod_lshift1_quick(t, A, p)) goto end; 183 184 /* b := (2*a)^((|p|-5)/8) */ 185 if (!BN_rshift(q, p, 3)) goto end; 186 q->neg = 0; 187 if (!BN_mod_exp(b, t, q, p, ctx)) goto end; 188 189 /* y := b^2 */ 190 if (!BN_mod_sqr(y, b, p, ctx)) goto end; 191 192 /* t := (2*a)*b^2 - 1*/ 193 if (!BN_mod_mul(t, t, y, p, ctx)) goto end; 194 if (!BN_sub_word(t, 1)) goto end; 195 196 /* x = a*b*t */ 197 if (!BN_mod_mul(x, A, b, p, ctx)) goto end; 198 if (!BN_mod_mul(x, x, t, p, ctx)) goto end; 199 200 if (!BN_copy(ret, x)) goto end; 201 err = 0; 202 goto vrfy; 203 } 204 205 /* e > 2, so we really have to use the Tonelli/Shanks algorithm. 206 * First, find some y that is not a square. */ 207 if (!BN_copy(q, p)) goto end; /* use 'q' as temp */ 208 q->neg = 0; 209 i = 2; 210 do 211 { 212 /* For efficiency, try small numbers first; 213 * if this fails, try random numbers. 214 */ 215 if (i < 22) 216 { 217 if (!BN_set_word(y, i)) goto end; 218 } 219 else 220 { 221 if (!BN_pseudo_rand(y, BN_num_bits(p), 0, 0)) goto end; 222 if (BN_ucmp(y, p) >= 0) 223 { 224 if (!(p->neg ? BN_add : BN_sub)(y, y, p)) goto end; 225 } 226 /* now 0 <= y < |p| */ 227 if (BN_is_zero(y)) 228 if (!BN_set_word(y, i)) goto end; 229 } 230 231 r = BN_kronecker(y, q, ctx); /* here 'q' is |p| */ 232 if (r < -1) goto end; 233 if (r == 0) 234 { 235 /* m divides p */ 236 BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME); 237 goto end; 238 } 239 } 240 while (r == 1 && ++i < 82); 241 242 if (r != -1) 243 { 244 /* Many rounds and still no non-square -- this is more likely 245 * a bug than just bad luck. 246 * Even if p is not prime, we should have found some y 247 * such that r == -1. 248 */ 249 BNerr(BN_F_BN_MOD_SQRT, BN_R_TOO_MANY_ITERATIONS); 250 goto end; 251 } 252 253 /* Here's our actual 'q': */ 254 if (!BN_rshift(q, q, e)) goto end; 255 256 /* Now that we have some non-square, we can find an element 257 * of order 2^e by computing its q'th power. */ 258 if (!BN_mod_exp(y, y, q, p, ctx)) goto end; 259 if (BN_is_one(y)) 260 { 261 BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME); 262 goto end; 263 } 264 265 /* Now we know that (if p is indeed prime) there is an integer 266 * k, 0 <= k < 2^e, such that 267 * 268 * a^q * y^k == 1 (mod p). 269 * 270 * As a^q is a square and y is not, k must be even. 271 * q+1 is even, too, so there is an element 272 * 273 * X := a^((q+1)/2) * y^(k/2), 274 * 275 * and it satisfies 276 * 277 * X^2 = a^q * a * y^k 278 * = a, 279 * 280 * so it is the square root that we are looking for. 281 */ 282 283 /* t := (q-1)/2 (note that q is odd) */ 284 if (!BN_rshift1(t, q)) goto end; 285 286 /* x := a^((q-1)/2) */ 287 if (BN_is_zero(t)) /* special case: p = 2^e + 1 */ 288 { 289 if (!BN_nnmod(t, A, p, ctx)) goto end; 290 if (BN_is_zero(t)) 291 { 292 /* special case: a == 0 (mod p) */ 293 BN_zero(ret); 294 err = 0; 295 goto end; 296 } 297 else 298 if (!BN_one(x)) goto end; 299 } 300 else 301 { 302 if (!BN_mod_exp(x, A, t, p, ctx)) goto end; 303 if (BN_is_zero(x)) 304 { 305 /* special case: a == 0 (mod p) */ 306 BN_zero(ret); 307 err = 0; 308 goto end; 309 } 310 } 311 312 /* b := a*x^2 (= a^q) */ 313 if (!BN_mod_sqr(b, x, p, ctx)) goto end; 314 if (!BN_mod_mul(b, b, A, p, ctx)) goto end; 315 316 /* x := a*x (= a^((q+1)/2)) */ 317 if (!BN_mod_mul(x, x, A, p, ctx)) goto end; 318 319 while (1) 320 { 321 /* Now b is a^q * y^k for some even k (0 <= k < 2^E 322 * where E refers to the original value of e, which we 323 * don't keep in a variable), and x is a^((q+1)/2) * y^(k/2). 324 * 325 * We have a*b = x^2, 326 * y^2^(e-1) = -1, 327 * b^2^(e-1) = 1. 328 */ 329 330 if (BN_is_one(b)) 331 { 332 if (!BN_copy(ret, x)) goto end; 333 err = 0; 334 goto vrfy; 335 } 336 337 338 /* find smallest i such that b^(2^i) = 1 */ 339 i = 1; 340 if (!BN_mod_sqr(t, b, p, ctx)) goto end; 341 while (!BN_is_one(t)) 342 { 343 i++; 344 if (i == e) 345 { 346 BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE); 347 goto end; 348 } 349 if (!BN_mod_mul(t, t, t, p, ctx)) goto end; 350 } 351 352 353 /* t := y^2^(e - i - 1) */ 354 if (!BN_copy(t, y)) goto end; 355 for (j = e - i - 1; j > 0; j--) 356 { 357 if (!BN_mod_sqr(t, t, p, ctx)) goto end; 358 } 359 if (!BN_mod_mul(y, t, t, p, ctx)) goto end; 360 if (!BN_mod_mul(x, x, t, p, ctx)) goto end; 361 if (!BN_mod_mul(b, b, y, p, ctx)) goto end; 362 e = i; 363 } 364 365 vrfy: 366 if (!err) 367 { 368 /* verify the result -- the input might have been not a square 369 * (test added in 0.9.8) */ 370 371 if (!BN_mod_sqr(x, ret, p, ctx)) 372 err = 1; 373 374 if (!err && 0 != BN_cmp(x, A)) 375 { 376 BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE); 377 err = 1; 378 } 379 } 380 381 end: 382 if (err) 383 { 384 if (ret != NULL && ret != in) 385 { 386 BN_clear_free(ret); 387 } 388 ret = NULL; 389 } 390 BN_CTX_end(ctx); 391 bn_check_top(ret); 392 return ret; 393 } 394