1 /* mpi-pow.c - MPI functions
2 * Copyright (C) 1994, 1996, 1998, 2000 Free Software Foundation, Inc.
3 *
4 * This file is part of GnuPG.
5 *
6 * GnuPG is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
9 * (at your option) any later version.
10 *
11 * GnuPG is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, write to the Free Software
18 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
19 *
20 * Note: This code is heavily based on the GNU MP Library.
21 * Actually it's the same code with only minor changes in the
22 * way the data is stored; this is to support the abstraction
23 * of an optional secure memory allocation which may be used
24 * to avoid revealing of sensitive data due to paging etc.
25 * The GNU MP Library itself is published under the LGPL;
26 * however I decided to publish this code under the plain GPL.
27 */
28
29 #include <linux/sched.h>
30 #include <linux/string.h>
31 #include "mpi-internal.h"
32 #include "longlong.h"
33
34 /****************
35 * RES = BASE ^ EXP mod MOD
36 */
mpi_powm(MPI res,MPI base,MPI exp,MPI mod)37 int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
38 {
39 mpi_ptr_t mp_marker = NULL, bp_marker = NULL, ep_marker = NULL;
40 mpi_ptr_t xp_marker = NULL;
41 mpi_ptr_t tspace = NULL;
42 mpi_ptr_t rp, ep, mp, bp;
43 mpi_size_t esize, msize, bsize, rsize;
44 int msign, bsign, rsign;
45 mpi_size_t size;
46 int mod_shift_cnt;
47 int negative_result;
48 int assign_rp = 0;
49 mpi_size_t tsize = 0; /* to avoid compiler warning */
50 /* fixme: we should check that the warning is void */
51 int rc = -ENOMEM;
52
53 esize = exp->nlimbs;
54 msize = mod->nlimbs;
55 size = 2 * msize;
56 msign = mod->sign;
57
58 rp = res->d;
59 ep = exp->d;
60
61 if (!msize)
62 return -EINVAL;
63
64 if (!esize) {
65 /* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0
66 * depending on if MOD equals 1. */
67 res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
68 if (res->nlimbs) {
69 if (mpi_resize(res, 1) < 0)
70 goto enomem;
71 rp = res->d;
72 rp[0] = 1;
73 }
74 res->sign = 0;
75 goto leave;
76 }
77
78 /* Normalize MOD (i.e. make its most significant bit set) as required by
79 * mpn_divrem. This will make the intermediate values in the calculation
80 * slightly larger, but the correct result is obtained after a final
81 * reduction using the original MOD value. */
82 mp = mp_marker = mpi_alloc_limb_space(msize);
83 if (!mp)
84 goto enomem;
85 mod_shift_cnt = count_leading_zeros(mod->d[msize - 1]);
86 if (mod_shift_cnt)
87 mpihelp_lshift(mp, mod->d, msize, mod_shift_cnt);
88 else
89 MPN_COPY(mp, mod->d, msize);
90
91 bsize = base->nlimbs;
92 bsign = base->sign;
93 if (bsize > msize) { /* The base is larger than the module. Reduce it. */
94 /* Allocate (BSIZE + 1) with space for remainder and quotient.
95 * (The quotient is (bsize - msize + 1) limbs.) */
96 bp = bp_marker = mpi_alloc_limb_space(bsize + 1);
97 if (!bp)
98 goto enomem;
99 MPN_COPY(bp, base->d, bsize);
100 /* We don't care about the quotient, store it above the remainder,
101 * at BP + MSIZE. */
102 mpihelp_divrem(bp + msize, 0, bp, bsize, mp, msize);
103 bsize = msize;
104 /* Canonicalize the base, since we are going to multiply with it
105 * quite a few times. */
106 MPN_NORMALIZE(bp, bsize);
107 } else
108 bp = base->d;
109
110 if (!bsize) {
111 res->nlimbs = 0;
112 res->sign = 0;
113 goto leave;
114 }
115
116 if (res->alloced < size) {
117 /* We have to allocate more space for RES. If any of the input
118 * parameters are identical to RES, defer deallocation of the old
119 * space. */
120 if (rp == ep || rp == mp || rp == bp) {
121 rp = mpi_alloc_limb_space(size);
122 if (!rp)
123 goto enomem;
124 assign_rp = 1;
125 } else {
126 if (mpi_resize(res, size) < 0)
127 goto enomem;
128 rp = res->d;
129 }
130 } else { /* Make BASE, EXP and MOD not overlap with RES. */
131 if (rp == bp) {
132 /* RES and BASE are identical. Allocate temp. space for BASE. */
133 BUG_ON(bp_marker);
134 bp = bp_marker = mpi_alloc_limb_space(bsize);
135 if (!bp)
136 goto enomem;
137 MPN_COPY(bp, rp, bsize);
138 }
139 if (rp == ep) {
140 /* RES and EXP are identical. Allocate temp. space for EXP. */
141 ep = ep_marker = mpi_alloc_limb_space(esize);
142 if (!ep)
143 goto enomem;
144 MPN_COPY(ep, rp, esize);
145 }
146 if (rp == mp) {
147 /* RES and MOD are identical. Allocate temporary space for MOD. */
148 BUG_ON(mp_marker);
149 mp = mp_marker = mpi_alloc_limb_space(msize);
150 if (!mp)
151 goto enomem;
152 MPN_COPY(mp, rp, msize);
153 }
154 }
155
156 MPN_COPY(rp, bp, bsize);
157 rsize = bsize;
158 rsign = bsign;
159
160 {
161 mpi_size_t i;
162 mpi_ptr_t xp;
163 int c;
164 mpi_limb_t e;
165 mpi_limb_t carry_limb;
166 struct karatsuba_ctx karactx;
167
168 xp = xp_marker = mpi_alloc_limb_space(2 * (msize + 1));
169 if (!xp)
170 goto enomem;
171
172 memset(&karactx, 0, sizeof karactx);
173 negative_result = (ep[0] & 1) && base->sign;
174
175 i = esize - 1;
176 e = ep[i];
177 c = count_leading_zeros(e);
178 e = (e << c) << 1; /* shift the exp bits to the left, lose msb */
179 c = BITS_PER_MPI_LIMB - 1 - c;
180
181 /* Main loop.
182 *
183 * Make the result be pointed to alternately by XP and RP. This
184 * helps us avoid block copying, which would otherwise be necessary
185 * with the overlap restrictions of mpihelp_divmod. With 50% probability
186 * the result after this loop will be in the area originally pointed
187 * by RP (==RES->d), and with 50% probability in the area originally
188 * pointed to by XP.
189 */
190
191 for (;;) {
192 while (c) {
193 mpi_ptr_t tp;
194 mpi_size_t xsize;
195
196 /*if (mpihelp_mul_n(xp, rp, rp, rsize) < 0) goto enomem */
197 if (rsize < KARATSUBA_THRESHOLD)
198 mpih_sqr_n_basecase(xp, rp, rsize);
199 else {
200 if (!tspace) {
201 tsize = 2 * rsize;
202 tspace =
203 mpi_alloc_limb_space(tsize);
204 if (!tspace)
205 goto enomem;
206 } else if (tsize < (2 * rsize)) {
207 mpi_free_limb_space(tspace);
208 tsize = 2 * rsize;
209 tspace =
210 mpi_alloc_limb_space(tsize);
211 if (!tspace)
212 goto enomem;
213 }
214 mpih_sqr_n(xp, rp, rsize, tspace);
215 }
216
217 xsize = 2 * rsize;
218 if (xsize > msize) {
219 mpihelp_divrem(xp + msize, 0, xp, xsize,
220 mp, msize);
221 xsize = msize;
222 }
223
224 tp = rp;
225 rp = xp;
226 xp = tp;
227 rsize = xsize;
228
229 if ((mpi_limb_signed_t) e < 0) {
230 /*mpihelp_mul( xp, rp, rsize, bp, bsize ); */
231 if (bsize < KARATSUBA_THRESHOLD) {
232 mpi_limb_t tmp;
233 if (mpihelp_mul
234 (xp, rp, rsize, bp, bsize,
235 &tmp) < 0)
236 goto enomem;
237 } else {
238 if (mpihelp_mul_karatsuba_case
239 (xp, rp, rsize, bp, bsize,
240 &karactx) < 0)
241 goto enomem;
242 }
243
244 xsize = rsize + bsize;
245 if (xsize > msize) {
246 mpihelp_divrem(xp + msize, 0,
247 xp, xsize, mp,
248 msize);
249 xsize = msize;
250 }
251
252 tp = rp;
253 rp = xp;
254 xp = tp;
255 rsize = xsize;
256 }
257 e <<= 1;
258 c--;
259 cond_resched();
260 }
261
262 i--;
263 if (i < 0)
264 break;
265 e = ep[i];
266 c = BITS_PER_MPI_LIMB;
267 }
268
269 /* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
270 * steps. Adjust the result by reducing it with the original MOD.
271 *
272 * Also make sure the result is put in RES->d (where it already
273 * might be, see above).
274 */
275 if (mod_shift_cnt) {
276 carry_limb =
277 mpihelp_lshift(res->d, rp, rsize, mod_shift_cnt);
278 rp = res->d;
279 if (carry_limb) {
280 rp[rsize] = carry_limb;
281 rsize++;
282 }
283 } else {
284 MPN_COPY(res->d, rp, rsize);
285 rp = res->d;
286 }
287
288 if (rsize >= msize) {
289 mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
290 rsize = msize;
291 }
292
293 /* Remove any leading zero words from the result. */
294 if (mod_shift_cnt)
295 mpihelp_rshift(rp, rp, rsize, mod_shift_cnt);
296 MPN_NORMALIZE(rp, rsize);
297
298 mpihelp_release_karatsuba_ctx(&karactx);
299 }
300
301 if (negative_result && rsize) {
302 if (mod_shift_cnt)
303 mpihelp_rshift(mp, mp, msize, mod_shift_cnt);
304 mpihelp_sub(rp, mp, msize, rp, rsize);
305 rsize = msize;
306 rsign = msign;
307 MPN_NORMALIZE(rp, rsize);
308 }
309 res->nlimbs = rsize;
310 res->sign = rsign;
311
312 leave:
313 rc = 0;
314 enomem:
315 if (assign_rp)
316 mpi_assign_limb_space(res, rp, size);
317 if (mp_marker)
318 mpi_free_limb_space(mp_marker);
319 if (bp_marker)
320 mpi_free_limb_space(bp_marker);
321 if (ep_marker)
322 mpi_free_limb_space(ep_marker);
323 if (xp_marker)
324 mpi_free_limb_space(xp_marker);
325 if (tspace)
326 mpi_free_limb_space(tspace);
327 return rc;
328 }
329 EXPORT_SYMBOL_GPL(mpi_powm);
330
