1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
|
/*
* Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL license (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
#include "internal/cryptlib.h"
#include "bn_local.h"
BIGNUM *BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
/*
* Returns 'ret' such that ret^2 == a (mod p), using the Tonelli/Shanks
* algorithm (cf. Henri Cohen, "A Course in Algebraic Computational Number
* Theory", algorithm 1.5.1). 'p' must be prime!
*/
{
BIGNUM *ret = in;
int err = 1;
int r;
BIGNUM *A, *b, *q, *t, *x, *y;
int e, i, j;
if (!BN_is_odd(p) || BN_abs_is_word(p, 1)) {
if (BN_abs_is_word(p, 2)) {
if (ret == NULL)
ret = BN_new();
if (ret == NULL)
goto end;
if (!BN_set_word(ret, BN_is_bit_set(a, 0))) {
if (ret != in)
BN_free(ret);
return NULL;
}
bn_check_top(ret);
return ret;
}
BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME);
return NULL;
}
if (BN_is_zero(a) || BN_is_one(a)) {
if (ret == NULL)
ret = BN_new();
if (ret == NULL)
goto end;
if (!BN_set_word(ret, BN_is_one(a))) {
if (ret != in)
BN_free(ret);
return NULL;
}
bn_check_top(ret);
return ret;
}
BN_CTX_start(ctx);
A = BN_CTX_get(ctx);
b = BN_CTX_get(ctx);
q = BN_CTX_get(ctx);
t = BN_CTX_get(ctx);
x = BN_CTX_get(ctx);
y = BN_CTX_get(ctx);
if (y == NULL)
goto end;
if (ret == NULL)
ret = BN_new();
if (ret == NULL)
goto end;
/* A = a mod p */
if (!BN_nnmod(A, a, p, ctx))
goto end;
/* now write |p| - 1 as 2^e*q where q is odd */
e = 1;
while (!BN_is_bit_set(p, e))
e++;
/* we'll set q later (if needed) */
if (e == 1) {
/*-
* The easy case: (|p|-1)/2 is odd, so 2 has an inverse
* modulo (|p|-1)/2, and square roots can be computed
* directly by modular exponentiation.
* We have
* 2 * (|p|+1)/4 == 1 (mod (|p|-1)/2),
* so we can use exponent (|p|+1)/4, i.e. (|p|-3)/4 + 1.
*/
if (!BN_rshift(q, p, 2))
goto end;
q->neg = 0;
if (!BN_add_word(q, 1))
goto end;
if (!BN_mod_exp(ret, A, q, p, ctx))
goto end;
err = 0;
goto vrfy;
}
if (e == 2) {
/*-
* |p| == 5 (mod 8)
*
* In this case 2 is always a non-square since
* Legendre(2,p) = (-1)^((p^2-1)/8) for any odd prime.
* So if a really is a square, then 2*a is a non-square.
* Thus for
* b := (2*a)^((|p|-5)/8),
* i := (2*a)*b^2
* we have
* i^2 = (2*a)^((1 + (|p|-5)/4)*2)
* = (2*a)^((p-1)/2)
* = -1;
* so if we set
* x := a*b*(i-1),
* then
* x^2 = a^2 * b^2 * (i^2 - 2*i + 1)
* = a^2 * b^2 * (-2*i)
* = a*(-i)*(2*a*b^2)
* = a*(-i)*i
* = a.
*
* (This is due to A.O.L. Atkin,
* Subject: Square Roots and Cognate Matters modulo p=8n+5.
* URL: https://listserv.nodak.edu/cgi-bin/wa.exe?A2=ind9211&L=NMBRTHRY&P=4026
* November 1992.)
*/
/* t := 2*a */
if (!BN_mod_lshift1_quick(t, A, p))
goto end;
/* b := (2*a)^((|p|-5)/8) */
if (!BN_rshift(q, p, 3))
goto end;
q->neg = 0;
if (!BN_mod_exp(b, t, q, p, ctx))
goto end;
/* y := b^2 */
if (!BN_mod_sqr(y, b, p, ctx))
goto end;
/* t := (2*a)*b^2 - 1 */
if (!BN_mod_mul(t, t, y, p, ctx))
goto end;
if (!BN_sub_word(t, 1))
goto end;
/* x = a*b*t */
if (!BN_mod_mul(x, A, b, p, ctx))
goto end;
if (!BN_mod_mul(x, x, t, p, ctx))
goto end;
if (!BN_copy(ret, x))
goto end;
err = 0;
goto vrfy;
}
/*
* e > 2, so we really have to use the Tonelli/Shanks algorithm. First,
* find some y that is not a square.
*/
if (!BN_copy(q, p))
goto end; /* use 'q' as temp */
q->neg = 0;
i = 2;
do {
/*
* For efficiency, try small numbers first; if this fails, try random
* numbers.
*/
if (i < 22) {
if (!BN_set_word(y, i))
goto end;
} else {
if (!BN_priv_rand(y, BN_num_bits(p), 0, 0))
goto end;
if (BN_ucmp(y, p) >= 0) {
if (!(p->neg ? BN_add : BN_sub) (y, y, p))
goto end;
}
/* now 0 <= y < |p| */
if (BN_is_zero(y))
if (!BN_set_word(y, i))
goto end;
}
r = BN_kronecker(y, q, ctx); /* here 'q' is |p| */
if (r < -1)
goto end;
if (r == 0) {
/* m divides p */
BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME);
goto end;
}
}
while (r == 1 && ++i < 82);
if (r != -1) {
/*
* Many rounds and still no non-square -- this is more likely a bug
* than just bad luck. Even if p is not prime, we should have found
* some y such that r == -1.
*/
BNerr(BN_F_BN_MOD_SQRT, BN_R_TOO_MANY_ITERATIONS);
goto end;
}
/* Here's our actual 'q': */
if (!BN_rshift(q, q, e))
goto end;
/*
* Now that we have some non-square, we can find an element of order 2^e
* by computing its q'th power.
*/
if (!BN_mod_exp(y, y, q, p, ctx))
goto end;
if (BN_is_one(y)) {
BNerr(BN_F_BN_MOD_SQRT, BN_R_P_IS_NOT_PRIME);
goto end;
}
/*-
* Now we know that (if p is indeed prime) there is an integer
* k, 0 <= k < 2^e, such that
*
* a^q * y^k == 1 (mod p).
*
* As a^q is a square and y is not, k must be even.
* q+1 is even, too, so there is an element
*
* X := a^((q+1)/2) * y^(k/2),
*
* and it satisfies
*
* X^2 = a^q * a * y^k
* = a,
*
* so it is the square root that we are looking for.
*/
/* t := (q-1)/2 (note that q is odd) */
if (!BN_rshift1(t, q))
goto end;
/* x := a^((q-1)/2) */
if (BN_is_zero(t)) { /* special case: p = 2^e + 1 */
if (!BN_nnmod(t, A, p, ctx))
goto end;
if (BN_is_zero(t)) {
/* special case: a == 0 (mod p) */
BN_zero(ret);
err = 0;
goto end;
} else if (!BN_one(x))
goto end;
} else {
if (!BN_mod_exp(x, A, t, p, ctx))
goto end;
if (BN_is_zero(x)) {
/* special case: a == 0 (mod p) */
BN_zero(ret);
err = 0;
goto end;
}
}
/* b := a*x^2 (= a^q) */
if (!BN_mod_sqr(b, x, p, ctx))
goto end;
if (!BN_mod_mul(b, b, A, p, ctx))
goto end;
/* x := a*x (= a^((q+1)/2)) */
if (!BN_mod_mul(x, x, A, p, ctx))
goto end;
while (1) {
/*-
* Now b is a^q * y^k for some even k (0 <= k < 2^E
* where E refers to the original value of e, which we
* don't keep in a variable), and x is a^((q+1)/2) * y^(k/2).
*
* We have a*b = x^2,
* y^2^(e-1) = -1,
* b^2^(e-1) = 1.
*/
if (BN_is_one(b)) {
if (!BN_copy(ret, x))
goto end;
err = 0;
goto vrfy;
}
/* find smallest i such that b^(2^i) = 1 */
i = 1;
if (!BN_mod_sqr(t, b, p, ctx))
goto end;
while (!BN_is_one(t)) {
i++;
if (i == e) {
BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE);
goto end;
}
if (!BN_mod_mul(t, t, t, p, ctx))
goto end;
}
/* t := y^2^(e - i - 1) */
if (!BN_copy(t, y))
goto end;
for (j = e - i - 1; j > 0; j--) {
if (!BN_mod_sqr(t, t, p, ctx))
goto end;
}
if (!BN_mod_mul(y, t, t, p, ctx))
goto end;
if (!BN_mod_mul(x, x, t, p, ctx))
goto end;
if (!BN_mod_mul(b, b, y, p, ctx))
goto end;
e = i;
}
vrfy:
if (!err) {
/*
* verify the result -- the input might have been not a square (test
* added in 0.9.8)
*/
if (!BN_mod_sqr(x, ret, p, ctx))
err = 1;
if (!err && 0 != BN_cmp(x, A)) {
BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE);
err = 1;
}
}
end:
if (err) {
if (ret != in)
BN_clear_free(ret);
ret = NULL;
}
BN_CTX_end(ctx);
bn_check_top(ret);
return ret;
}
|
