8adf21430e8a15d70734898cc7d38dc6e8ed0b10
[openssl.git] / crypto / bn / bn_div.c
1 /* crypto/bn/bn_div.c */
2 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
3  * All rights reserved.
4  *
5  * This package is an SSL implementation written
6  * by Eric Young (eay@cryptsoft.com).
7  * The implementation was written so as to conform with Netscapes SSL.
8  * 
9  * This library is free for commercial and non-commercial use as long as
10  * the following conditions are aheared to.  The following conditions
11  * apply to all code found in this distribution, be it the RC4, RSA,
12  * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
13  * included with this distribution is covered by the same copyright terms
14  * except that the holder is Tim Hudson (tjh@cryptsoft.com).
15  * 
16  * Copyright remains Eric Young's, and as such any Copyright notices in
17  * the code are not to be removed.
18  * If this package is used in a product, Eric Young should be given attribution
19  * as the author of the parts of the library used.
20  * This can be in the form of a textual message at program startup or
21  * in documentation (online or textual) provided with the package.
22  * 
23  * Redistribution and use in source and binary forms, with or without
24  * modification, are permitted provided that the following conditions
25  * are met:
26  * 1. Redistributions of source code must retain the copyright
27  *    notice, this list of conditions and the following disclaimer.
28  * 2. Redistributions in binary form must reproduce the above copyright
29  *    notice, this list of conditions and the following disclaimer in the
30  *    documentation and/or other materials provided with the distribution.
31  * 3. All advertising materials mentioning features or use of this software
32  *    must display the following acknowledgement:
33  *    "This product includes cryptographic software written by
34  *     Eric Young (eay@cryptsoft.com)"
35  *    The word 'cryptographic' can be left out if the rouines from the library
36  *    being used are not cryptographic related :-).
37  * 4. If you include any Windows specific code (or a derivative thereof) from 
38  *    the apps directory (application code) you must include an acknowledgement:
39  *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
40  * 
41  * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
42  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
43  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
44  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
45  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
46  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
47  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
48  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
49  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
50  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
51  * SUCH DAMAGE.
52  * 
53  * The licence and distribution terms for any publically available version or
54  * derivative of this code cannot be changed.  i.e. this code cannot simply be
55  * copied and put under another distribution licence
56  * [including the GNU Public Licence.]
57  */
58
59 #include <stdio.h>
60 #include <openssl/bn.h>
61 #include "cryptlib.h"
62 #include "bn_lcl.h"
63
64
65 /* The old slow way */
66 #if 0
67 int BN_div(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m, const BIGNUM *d,
68            BN_CTX *ctx)
69         {
70         int i,nm,nd;
71         int ret = 0;
72         BIGNUM *D;
73
74         bn_check_top(m);
75         bn_check_top(d);
76         if (BN_is_zero(d))
77                 {
78                 BNerr(BN_F_BN_DIV,BN_R_DIV_BY_ZERO);
79                 return(0);
80                 }
81
82         if (BN_ucmp(m,d) < 0)
83                 {
84                 if (rem != NULL)
85                         { if (BN_copy(rem,m) == NULL) return(0); }
86                 if (dv != NULL) BN_zero(dv);
87                 return(1);
88                 }
89
90         BN_CTX_start(ctx);
91         D = BN_CTX_get(ctx);
92         if (dv == NULL) dv = BN_CTX_get(ctx);
93         if (rem == NULL) rem = BN_CTX_get(ctx);
94         if (D == NULL || dv == NULL || rem == NULL)
95                 goto end;
96
97         nd=BN_num_bits(d);
98         nm=BN_num_bits(m);
99         if (BN_copy(D,d) == NULL) goto end;
100         if (BN_copy(rem,m) == NULL) goto end;
101
102         /* The next 2 are needed so we can do a dv->d[0]|=1 later
103          * since BN_lshift1 will only work once there is a value :-) */
104         BN_zero(dv);
105         bn_wexpand(dv,1);
106         dv->top=1;
107
108         if (!BN_lshift(D,D,nm-nd)) goto end;
109         for (i=nm-nd; i>=0; i--)
110                 {
111                 if (!BN_lshift1(dv,dv)) goto end;
112                 if (BN_ucmp(rem,D) >= 0)
113                         {
114                         dv->d[0]|=1;
115                         if (!BN_usub(rem,rem,D)) goto end;
116                         }
117 /* CAN IMPROVE (and have now :=) */
118                 if (!BN_rshift1(D,D)) goto end;
119                 }
120         rem->neg=BN_is_zero(rem)?0:m->neg;
121         dv->neg=m->neg^d->neg;
122         ret = 1;
123  end:
124         BN_CTX_end(ctx);
125         return(ret);
126         }
127
128 #else
129
130 #if !defined(OPENSSL_NO_ASM) && !defined(OPENSSL_NO_INLINE_ASM) \
131     && !defined(PEDANTIC) && !defined(BN_DIV3W)
132 # if defined(__GNUC__) && __GNUC__>=2
133 #  if defined(__i386) || defined (__i386__)
134    /*
135     * There were two reasons for implementing this template:
136     * - GNU C generates a call to a function (__udivdi3 to be exact)
137     *   in reply to ((((BN_ULLONG)n0)<<BN_BITS2)|n1)/d0 (I fail to
138     *   understand why...);
139     * - divl doesn't only calculate quotient, but also leaves
140     *   remainder in %edx which we can definitely use here:-)
141     *
142     *                                   <appro@fy.chalmers.se>
143     */
144 #  define bn_div_words(n0,n1,d0)                \
145         ({  asm volatile (                      \
146                 "divl   %4"                     \
147                 : "=a"(q), "=d"(rem)            \
148                 : "a"(n1), "d"(n0), "g"(d0)     \
149                 : "cc");                        \
150             q;                                  \
151         })
152 #  define REMAINDER_IS_ALREADY_CALCULATED
153 #  elif defined(__x86_64) && defined(SIXTY_FOUR_BIT_LONG)
154    /*
155     * Same story here, but it's 128-bit by 64-bit division. Wow!
156     *                                   <appro@fy.chalmers.se>
157     */
158 #  define bn_div_words(n0,n1,d0)                \
159         ({  asm volatile (                      \
160                 "divq   %4"                     \
161                 : "=a"(q), "=d"(rem)            \
162                 : "a"(n1), "d"(n0), "g"(d0)     \
163                 : "cc");                        \
164             q;                                  \
165         })
166 #  define REMAINDER_IS_ALREADY_CALCULATED
167 #  endif /* __<cpu> */
168 # endif /* __GNUC__ */
169 #endif /* OPENSSL_NO_ASM */
170
171
172 /* BN_div[_no_branch] computes  dv := num / divisor,  rounding towards
173  * zero, and sets up rm  such that  dv*divisor + rm = num  holds.
174  * Thus:
175  *     dv->neg == num->neg ^ divisor->neg  (unless the result is zero)
176  *     rm->neg == num->neg                 (unless the remainder is zero)
177  * If 'dv' or 'rm' is NULL, the respective value is not returned.
178  */
179 static int BN_div_no_branch(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num,
180         const BIGNUM *divisor, BN_CTX *ctx);
181 int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
182            BN_CTX *ctx)
183         {
184         int norm_shift,i,loop;
185         BIGNUM *tmp,wnum,*snum,*sdiv,*res;
186         BN_ULONG *resp,*wnump;
187         BN_ULONG d0,d1;
188         int num_n,div_n;
189
190         if ((BN_get_flags(num, BN_FLG_CONSTTIME) != 0) || (BN_get_flags(divisor, BN_FLG_CONSTTIME) != 0))
191                 {
192                 return BN_div_no_branch(dv, rm, num, divisor, ctx);
193                 }
194
195         bn_check_top(dv);
196         bn_check_top(rm);
197         bn_check_top(num);
198         bn_check_top(divisor);
199
200         if (BN_is_zero(divisor))
201                 {
202                 BNerr(BN_F_BN_DIV,BN_R_DIV_BY_ZERO);
203                 return(0);
204                 }
205
206         if (BN_ucmp(num,divisor) < 0)
207                 {
208                 if (rm != NULL)
209                         { if (BN_copy(rm,num) == NULL) return(0); }
210                 if (dv != NULL) BN_zero(dv);
211                 return(1);
212                 }
213
214         BN_CTX_start(ctx);
215         tmp=BN_CTX_get(ctx);
216         snum=BN_CTX_get(ctx);
217         sdiv=BN_CTX_get(ctx);
218         if (dv == NULL)
219                 res=BN_CTX_get(ctx);
220         else    res=dv;
221         if (sdiv == NULL || res == NULL) goto err;
222
223         /* First we normalise the numbers */
224         norm_shift=BN_BITS2-((BN_num_bits(divisor))%BN_BITS2);
225         if (!(BN_lshift(sdiv,divisor,norm_shift))) goto err;
226         sdiv->neg=0;
227         norm_shift+=BN_BITS2;
228         if (!(BN_lshift(snum,num,norm_shift))) goto err;
229         snum->neg=0;
230         div_n=sdiv->top;
231         num_n=snum->top;
232         loop=num_n-div_n;
233         /* Lets setup a 'window' into snum
234          * This is the part that corresponds to the current
235          * 'area' being divided */
236         wnum.neg   = 0;
237         wnum.d     = &(snum->d[loop]);
238         wnum.top   = div_n;
239         /* only needed when BN_ucmp messes up the values between top and max */
240         wnum.dmax  = snum->dmax - loop; /* so we don't step out of bounds */
241
242         /* Get the top 2 words of sdiv */
243         /* div_n=sdiv->top; */
244         d0=sdiv->d[div_n-1];
245         d1=(div_n == 1)?0:sdiv->d[div_n-2];
246
247         /* pointer to the 'top' of snum */
248         wnump= &(snum->d[num_n-1]);
249
250         /* Setup to 'res' */
251         res->neg= (num->neg^divisor->neg);
252         if (!bn_wexpand(res,(loop+1))) goto err;
253         res->top=loop;
254         resp= &(res->d[loop-1]);
255
256         /* space for temp */
257         if (!bn_wexpand(tmp,(div_n+1))) goto err;
258
259         if (BN_ucmp(&wnum,sdiv) >= 0)
260                 {
261                 /* If BN_DEBUG_RAND is defined BN_ucmp changes (via
262                  * bn_pollute) the const bignum arguments =>
263                  * clean the values between top and max again */
264                 bn_clear_top2max(&wnum);
265                 bn_sub_words(wnum.d, wnum.d, sdiv->d, div_n);
266                 *resp=1;
267                 }
268         else
269                 res->top--;
270         /* if res->top == 0 then clear the neg value otherwise decrease
271          * the resp pointer */
272         if (res->top == 0)
273                 res->neg = 0;
274         else
275                 resp--;
276
277         for (i=0; i<loop-1; i++, wnump--, resp--)
278                 {
279                 BN_ULONG q,l0;
280                 /* the first part of the loop uses the top two words of
281                  * snum and sdiv to calculate a BN_ULONG q such that
282                  * | wnum - sdiv * q | < sdiv */
283 #if defined(BN_DIV3W) && !defined(OPENSSL_NO_ASM)
284                 BN_ULONG bn_div_3_words(BN_ULONG*,BN_ULONG,BN_ULONG);
285                 q=bn_div_3_words(wnump,d1,d0);
286 #else
287                 BN_ULONG n0,n1,rem=0;
288
289                 n0=wnump[0];
290                 n1=wnump[-1];
291                 if (n0 == d0)
292                         q=BN_MASK2;
293                 else                    /* n0 < d0 */
294                         {
295 #ifdef BN_LLONG
296                         BN_ULLONG t2;
297
298 #if defined(BN_LLONG) && defined(BN_DIV2W) && !defined(bn_div_words)
299                         q=(BN_ULONG)(((((BN_ULLONG)n0)<<BN_BITS2)|n1)/d0);
300 #else
301                         q=bn_div_words(n0,n1,d0);
302 #ifdef BN_DEBUG_LEVITTE
303                         fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
304 X) -> 0x%08X\n",
305                                 n0, n1, d0, q);
306 #endif
307 #endif
308
309 #ifndef REMAINDER_IS_ALREADY_CALCULATED
310                         /*
311                          * rem doesn't have to be BN_ULLONG. The least we
312                          * know it's less that d0, isn't it?
313                          */
314                         rem=(n1-q*d0)&BN_MASK2;
315 #endif
316                         t2=(BN_ULLONG)d1*q;
317
318                         for (;;)
319                                 {
320                                 if (t2 <= ((((BN_ULLONG)rem)<<BN_BITS2)|wnump[-2]))
321                                         break;
322                                 q--;
323                                 rem += d0;
324                                 if (rem < d0) break; /* don't let rem overflow */
325                                 t2 -= d1;
326                                 }
327 #else /* !BN_LLONG */
328                         BN_ULONG t2l,t2h;
329
330                         q=bn_div_words(n0,n1,d0);
331 #ifdef BN_DEBUG_LEVITTE
332                         fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
333 X) -> 0x%08X\n",
334                                 n0, n1, d0, q);
335 #endif
336 #ifndef REMAINDER_IS_ALREADY_CALCULATED
337                         rem=(n1-q*d0)&BN_MASK2;
338 #endif
339
340 #if defined(BN_UMULT_LOHI)
341                         BN_UMULT_LOHI(t2l,t2h,d1,q);
342 #elif defined(BN_UMULT_HIGH)
343                         t2l = d1 * q;
344                         t2h = BN_UMULT_HIGH(d1,q);
345 #else
346                         {
347                         BN_ULONG ql, qh;
348                         t2l=LBITS(d1); t2h=HBITS(d1);
349                         ql =LBITS(q);  qh =HBITS(q);
350                         mul64(t2l,t2h,ql,qh); /* t2=(BN_ULLONG)d1*q; */
351                         }
352 #endif
353
354                         for (;;)
355                                 {
356                                 if ((t2h < rem) ||
357                                         ((t2h == rem) && (t2l <= wnump[-2])))
358                                         break;
359                                 q--;
360                                 rem += d0;
361                                 if (rem < d0) break; /* don't let rem overflow */
362                                 if (t2l < d1) t2h--; t2l -= d1;
363                                 }
364 #endif /* !BN_LLONG */
365                         }
366 #endif /* !BN_DIV3W */
367
368                 l0=bn_mul_words(tmp->d,sdiv->d,div_n,q);
369                 tmp->d[div_n]=l0;
370                 wnum.d--;
371                 /* ingore top values of the bignums just sub the two 
372                  * BN_ULONG arrays with bn_sub_words */
373                 if (bn_sub_words(wnum.d, wnum.d, tmp->d, div_n+1))
374                         {
375                         /* Note: As we have considered only the leading
376                          * two BN_ULONGs in the calculation of q, sdiv * q
377                          * might be greater than wnum (but then (q-1) * sdiv
378                          * is less or equal than wnum)
379                          */
380                         q--;
381                         if (bn_add_words(wnum.d, wnum.d, sdiv->d, div_n))
382                                 /* we can't have an overflow here (assuming
383                                  * that q != 0, but if q == 0 then tmp is
384                                  * zero anyway) */
385                                 (*wnump)++;
386                         }
387                 /* store part of the result */
388                 *resp = q;
389                 }
390         bn_correct_top(snum);
391         if (rm != NULL)
392                 {
393                 /* Keep a copy of the neg flag in num because if rm==num
394                  * BN_rshift() will overwrite it.
395                  */
396                 int neg = num->neg;
397                 BN_rshift(rm,snum,norm_shift);
398                 if (!BN_is_zero(rm))
399                         rm->neg = neg;
400                 bn_check_top(rm);
401                 }
402         BN_CTX_end(ctx);
403         return(1);
404 err:
405         bn_check_top(rm);
406         BN_CTX_end(ctx);
407         return(0);
408         }
409
410
411 /* BN_div_no_branch is a special version of BN_div. It does not contain
412  * branches that may leak sensitive information.
413  */
414 static int BN_div_no_branch(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, 
415         const BIGNUM *divisor, BN_CTX *ctx)
416         {
417         int norm_shift,i,loop;
418         BIGNUM *tmp,wnum,*snum,*sdiv,*res;
419         BN_ULONG *resp,*wnump;
420         BN_ULONG d0,d1;
421         int num_n,div_n;
422
423         bn_check_top(dv);
424         bn_check_top(rm);
425         bn_check_top(num);
426         bn_check_top(divisor);
427
428         if (BN_is_zero(divisor))
429                 {
430                 BNerr(BN_F_BN_DIV_NO_BRANCH,BN_R_DIV_BY_ZERO);
431                 return(0);
432                 }
433
434         BN_CTX_start(ctx);
435         tmp=BN_CTX_get(ctx);
436         snum=BN_CTX_get(ctx);
437         sdiv=BN_CTX_get(ctx);
438         if (dv == NULL)
439                 res=BN_CTX_get(ctx);
440         else    res=dv;
441         if (sdiv == NULL || res == NULL) goto err;
442
443         /* First we normalise the numbers */
444         norm_shift=BN_BITS2-((BN_num_bits(divisor))%BN_BITS2);
445         if (!(BN_lshift(sdiv,divisor,norm_shift))) goto err;
446         sdiv->neg=0;
447         norm_shift+=BN_BITS2;
448         if (!(BN_lshift(snum,num,norm_shift))) goto err;
449         snum->neg=0;
450
451         /* Since we don't know whether snum is larger than sdiv,
452          * we pad snum with enough zeroes without changing its
453          * value. 
454          */
455         if (snum->top <= sdiv->top+1) 
456                 {
457                 if (bn_wexpand(snum, sdiv->top + 2) == NULL) goto err;
458                 for (i = snum->top; i < sdiv->top + 2; i++) snum->d[i] = 0;
459                 snum->top = sdiv->top + 2;
460                 }
461         else
462                 {
463                 if (bn_wexpand(snum, snum->top + 1) == NULL) goto err;
464                 snum->d[snum->top] = 0;
465                 snum->top ++;
466                 }
467
468         div_n=sdiv->top;
469         num_n=snum->top;
470         loop=num_n-div_n;
471         /* Lets setup a 'window' into snum
472          * This is the part that corresponds to the current
473          * 'area' being divided */
474         wnum.neg   = 0;
475         wnum.d     = &(snum->d[loop]);
476         wnum.top   = div_n;
477         /* only needed when BN_ucmp messes up the values between top and max */
478         wnum.dmax  = snum->dmax - loop; /* so we don't step out of bounds */
479
480         /* Get the top 2 words of sdiv */
481         /* div_n=sdiv->top; */
482         d0=sdiv->d[div_n-1];
483         d1=(div_n == 1)?0:sdiv->d[div_n-2];
484
485         /* pointer to the 'top' of snum */
486         wnump= &(snum->d[num_n-1]);
487
488         /* Setup to 'res' */
489         res->neg= (num->neg^divisor->neg);
490         if (!bn_wexpand(res,(loop+1))) goto err;
491         res->top=loop-1;
492         resp= &(res->d[loop-1]);
493
494         /* space for temp */
495         if (!bn_wexpand(tmp,(div_n+1))) goto err;
496
497         /* if res->top == 0 then clear the neg value otherwise decrease
498          * the resp pointer */
499         if (res->top == 0)
500                 res->neg = 0;
501         else
502                 resp--;
503
504         for (i=0; i<loop-1; i++, wnump--, resp--)
505                 {
506                 BN_ULONG q,l0;
507                 /* the first part of the loop uses the top two words of
508                  * snum and sdiv to calculate a BN_ULONG q such that
509                  * | wnum - sdiv * q | < sdiv */
510 #if defined(BN_DIV3W) && !defined(OPENSSL_NO_ASM)
511                 BN_ULONG bn_div_3_words(BN_ULONG*,BN_ULONG,BN_ULONG);
512                 q=bn_div_3_words(wnump,d1,d0);
513 #else
514                 BN_ULONG n0,n1,rem=0;
515
516                 n0=wnump[0];
517                 n1=wnump[-1];
518                 if (n0 == d0)
519                         q=BN_MASK2;
520                 else                    /* n0 < d0 */
521                         {
522 #ifdef BN_LLONG
523                         BN_ULLONG t2;
524
525 #if defined(BN_LLONG) && defined(BN_DIV2W) && !defined(bn_div_words)
526                         q=(BN_ULONG)(((((BN_ULLONG)n0)<<BN_BITS2)|n1)/d0);
527 #else
528                         q=bn_div_words(n0,n1,d0);
529 #ifdef BN_DEBUG_LEVITTE
530                         fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
531 X) -> 0x%08X\n",
532                                 n0, n1, d0, q);
533 #endif
534 #endif
535
536 #ifndef REMAINDER_IS_ALREADY_CALCULATED
537                         /*
538                          * rem doesn't have to be BN_ULLONG. The least we
539                          * know it's less that d0, isn't it?
540                          */
541                         rem=(n1-q*d0)&BN_MASK2;
542 #endif
543                         t2=(BN_ULLONG)d1*q;
544
545                         for (;;)
546                                 {
547                                 if (t2 <= ((((BN_ULLONG)rem)<<BN_BITS2)|wnump[-2]))
548                                         break;
549                                 q--;
550                                 rem += d0;
551                                 if (rem < d0) break; /* don't let rem overflow */
552                                 t2 -= d1;
553                                 }
554 #else /* !BN_LLONG */
555                         BN_ULONG t2l,t2h;
556
557                         q=bn_div_words(n0,n1,d0);
558 #ifdef BN_DEBUG_LEVITTE
559                         fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
560 X) -> 0x%08X\n",
561                                 n0, n1, d0, q);
562 #endif
563 #ifndef REMAINDER_IS_ALREADY_CALCULATED
564                         rem=(n1-q*d0)&BN_MASK2;
565 #endif
566
567 #if defined(BN_UMULT_LOHI)
568                         BN_UMULT_LOHI(t2l,t2h,d1,q);
569 #elif defined(BN_UMULT_HIGH)
570                         t2l = d1 * q;
571                         t2h = BN_UMULT_HIGH(d1,q);
572 #else
573                         {
574                         BN_ULONG ql, qh;
575                         t2l=LBITS(d1); t2h=HBITS(d1);
576                         ql =LBITS(q);  qh =HBITS(q);
577                         mul64(t2l,t2h,ql,qh); /* t2=(BN_ULLONG)d1*q; */
578                         }
579 #endif
580
581                         for (;;)
582                                 {
583                                 if ((t2h < rem) ||
584                                         ((t2h == rem) && (t2l <= wnump[-2])))
585                                         break;
586                                 q--;
587                                 rem += d0;
588                                 if (rem < d0) break; /* don't let rem overflow */
589                                 if (t2l < d1) t2h--; t2l -= d1;
590                                 }
591 #endif /* !BN_LLONG */
592                         }
593 #endif /* !BN_DIV3W */
594
595                 l0=bn_mul_words(tmp->d,sdiv->d,div_n,q);
596                 tmp->d[div_n]=l0;
597                 wnum.d--;
598                 /* ingore top values of the bignums just sub the two 
599                  * BN_ULONG arrays with bn_sub_words */
600                 if (bn_sub_words(wnum.d, wnum.d, tmp->d, div_n+1))
601                         {
602                         /* Note: As we have considered only the leading
603                          * two BN_ULONGs in the calculation of q, sdiv * q
604                          * might be greater than wnum (but then (q-1) * sdiv
605                          * is less or equal than wnum)
606                          */
607                         q--;
608                         if (bn_add_words(wnum.d, wnum.d, sdiv->d, div_n))
609                                 /* we can't have an overflow here (assuming
610                                  * that q != 0, but if q == 0 then tmp is
611                                  * zero anyway) */
612                                 (*wnump)++;
613                         }
614                 /* store part of the result */
615                 *resp = q;
616                 }
617         bn_correct_top(snum);
618         if (rm != NULL)
619                 {
620                 /* Keep a copy of the neg flag in num because if rm==num
621                  * BN_rshift() will overwrite it.
622                  */
623                 int neg = num->neg;
624                 BN_rshift(rm,snum,norm_shift);
625                 if (!BN_is_zero(rm))
626                         rm->neg = neg;
627                 bn_check_top(rm);
628                 }
629         bn_correct_top(res);
630         BN_CTX_end(ctx);
631         return(1);
632 err:
633         bn_check_top(rm);
634         BN_CTX_end(ctx);
635         return(0);
636         }
637
638 #endif