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bn_lib.c

/* crypto/bn/bn_lib.c */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 * 
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 * 
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from 
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 * 
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */

#ifndef BN_DEBUG
# undef NDEBUG /* avoid conflicting definitions */
# define NDEBUG
#endif

#include <assert.h>
#include <limits.h>
#include <stdio.h>
#include "cryptlib.h"
#include "bn_lcl.h"

const char BN_version[]="Big Number" OPENSSL_VERSION_PTEXT;

/* This stuff appears to be completely unused, so is deprecated */
#ifndef OPENSSL_NO_DEPRECATED
/* For a 32 bit machine
 * 2 -   4 ==  128
 * 3 -   8 ==  256
 * 4 -  16 ==  512
 * 5 -  32 == 1024
 * 6 -  64 == 2048
 * 7 - 128 == 4096
 * 8 - 256 == 8192
 */
static int bn_limit_bits=0;
static int bn_limit_num=8;        /* (1<<bn_limit_bits) */
static int bn_limit_bits_low=0;
static int bn_limit_num_low=8;    /* (1<<bn_limit_bits_low) */
static int bn_limit_bits_high=0;
static int bn_limit_num_high=8;   /* (1<<bn_limit_bits_high) */
static int bn_limit_bits_mont=0;
static int bn_limit_num_mont=8;   /* (1<<bn_limit_bits_mont) */

void BN_set_params(int mult, int high, int low, int mont)
      {
      if (mult >= 0)
            {
            if (mult > (int)(sizeof(int)*8)-1)
                  mult=sizeof(int)*8-1;
            bn_limit_bits=mult;
            bn_limit_num=1<<mult;
            }
      if (high >= 0)
            {
            if (high > (int)(sizeof(int)*8)-1)
                  high=sizeof(int)*8-1;
            bn_limit_bits_high=high;
            bn_limit_num_high=1<<high;
            }
      if (low >= 0)
            {
            if (low > (int)(sizeof(int)*8)-1)
                  low=sizeof(int)*8-1;
            bn_limit_bits_low=low;
            bn_limit_num_low=1<<low;
            }
      if (mont >= 0)
            {
            if (mont > (int)(sizeof(int)*8)-1)
                  mont=sizeof(int)*8-1;
            bn_limit_bits_mont=mont;
            bn_limit_num_mont=1<<mont;
            }
      }

int BN_get_params(int which)
      {
      if      (which == 0) return(bn_limit_bits);
      else if (which == 1) return(bn_limit_bits_high);
      else if (which == 2) return(bn_limit_bits_low);
      else if (which == 3) return(bn_limit_bits_mont);
      else return(0);
      }
#endif

const BIGNUM *BN_value_one(void)
      {
      static BN_ULONG data_one=1L;
      static BIGNUM const_one={&data_one,1,1,0,BN_FLG_STATIC_DATA};

      return(&const_one);
      }

char *BN_options(void)
      {
      static int init=0;
      static char data[16];

      if (!init)
            {
            init++;
#ifdef BN_LLONG
            BIO_snprintf(data,sizeof data,"bn(%d,%d)",
                       (int)sizeof(BN_ULLONG)*8,(int)sizeof(BN_ULONG)*8);
#else
            BIO_snprintf(data,sizeof data,"bn(%d,%d)",
                       (int)sizeof(BN_ULONG)*8,(int)sizeof(BN_ULONG)*8);
#endif
            }
      return(data);
      }

int BN_num_bits_word(BN_ULONG l)
      {
      static const char bits[256]={
            0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4,
            5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
            6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
            6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
            7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
            7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
            7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
            7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
            };

#if defined(SIXTY_FOUR_BIT_LONG)
      if (l & 0xffffffff00000000L)
            {
            if (l & 0xffff000000000000L)
                  {
                  if (l & 0xff00000000000000L)
                        {
                        return(bits[(int)(l>>56)]+56);
                        }
                  else  return(bits[(int)(l>>48)]+48);
                  }
            else
                  {
                  if (l & 0x0000ff0000000000L)
                        {
                        return(bits[(int)(l>>40)]+40);
                        }
                  else  return(bits[(int)(l>>32)]+32);
                  }
            }
      else
#else
#ifdef SIXTY_FOUR_BIT
      if (l & 0xffffffff00000000LL)
            {
            if (l & 0xffff000000000000LL)
                  {
                  if (l & 0xff00000000000000LL)
                        {
                        return(bits[(int)(l>>56)]+56);
                        }
                  else  return(bits[(int)(l>>48)]+48);
                  }
            else
                  {
                  if (l & 0x0000ff0000000000LL)
                        {
                        return(bits[(int)(l>>40)]+40);
                        }
                  else  return(bits[(int)(l>>32)]+32);
                  }
            }
      else
#endif
#endif
            {
#if defined(THIRTY_TWO_BIT) || defined(SIXTY_FOUR_BIT) || defined(SIXTY_FOUR_BIT_LONG)
            if (l & 0xffff0000L)
                  {
                  if (l & 0xff000000L)
                        return(bits[(int)(l>>24L)]+24);
                  else  return(bits[(int)(l>>16L)]+16);
                  }
            else
#endif
                  {
#if defined(SIXTEEN_BIT) || defined(THIRTY_TWO_BIT) || defined(SIXTY_FOUR_BIT) || defined(SIXTY_FOUR_BIT_LONG)
                  if (l & 0xff00L)
                        return(bits[(int)(l>>8)]+8);
                  else  
#endif
                        return(bits[(int)(l   )]  );
                  }
            }
      }

int BN_num_bits(const BIGNUM *a)
      {
      int i = a->top - 1;
      bn_check_top(a);

      if (BN_is_zero(a)) return 0;
      return ((i*BN_BITS2) + BN_num_bits_word(a->d[i]));
      }

void BN_clear_free(BIGNUM *a)
      {
      int i;

      if (a == NULL) return;
      bn_check_top(a);
      if (a->d != NULL)
            {
            OPENSSL_cleanse(a->d,a->dmax*sizeof(a->d[0]));
            if (!(BN_get_flags(a,BN_FLG_STATIC_DATA)))
                  OPENSSL_free(a->d);
            }
      i=BN_get_flags(a,BN_FLG_MALLOCED);
      OPENSSL_cleanse(a,sizeof(BIGNUM));
      if (i)
            OPENSSL_free(a);
      }

void BN_free(BIGNUM *a)
      {
      if (a == NULL) return;
      bn_check_top(a);
      if ((a->d != NULL) && !(BN_get_flags(a,BN_FLG_STATIC_DATA)))
            OPENSSL_free(a->d);
      if (a->flags & BN_FLG_MALLOCED)
            OPENSSL_free(a);
      else
            {
#ifndef OPENSSL_NO_DEPRECATED
            a->flags|=BN_FLG_FREE;
#endif
            a->d = NULL;
            }
      }

void BN_init(BIGNUM *a)
      {
      memset(a,0,sizeof(BIGNUM));
      bn_check_top(a);
      }

BIGNUM *BN_new(void)
      {
      BIGNUM *ret;

      if ((ret=(BIGNUM *)OPENSSL_malloc(sizeof(BIGNUM))) == NULL)
            {
            BNerr(BN_F_BN_NEW,ERR_R_MALLOC_FAILURE);
            return(NULL);
            }
      ret->flags=BN_FLG_MALLOCED;
      ret->top=0;
      ret->neg=0;
      ret->dmax=0;
      ret->d=NULL;
      bn_check_top(ret);
      return(ret);
      }

/* This is used both by bn_expand2() and bn_dup_expand() */
/* The caller MUST check that words > b->dmax before calling this */
static BN_ULONG *bn_expand_internal(const BIGNUM *b, int words)
      {
      BN_ULONG *A,*a = NULL;
      const BN_ULONG *B;
      int i;

      bn_check_top(b);

      if (words > (INT_MAX/(4*BN_BITS2)))
            {
            BNerr(BN_F_BN_EXPAND_INTERNAL,BN_R_BIGNUM_TOO_LONG);
            return NULL;
            }
      if (BN_get_flags(b,BN_FLG_STATIC_DATA))
            {
            BNerr(BN_F_BN_EXPAND_INTERNAL,BN_R_EXPAND_ON_STATIC_BIGNUM_DATA);
            return(NULL);
            }
      a=A=(BN_ULONG *)OPENSSL_malloc(sizeof(BN_ULONG)*words);
      if (A == NULL)
            {
            BNerr(BN_F_BN_EXPAND_INTERNAL,ERR_R_MALLOC_FAILURE);
            return(NULL);
            }
#if 1
      B=b->d;
      /* Check if the previous number needs to be copied */
      if (B != NULL)
            {
            for (i=b->top>>2; i>0; i--,A+=4,B+=4)
                  {
                  /*
                   * The fact that the loop is unrolled
                   * 4-wise is a tribute to Intel. It's
                   * the one that doesn't have enough
                   * registers to accomodate more data.
                   * I'd unroll it 8-wise otherwise:-)
                   *
                   *          <appro@fy.chalmers.se>
                   */
                  BN_ULONG a0,a1,a2,a3;
                  a0=B[0]; a1=B[1]; a2=B[2]; a3=B[3];
                  A[0]=a0; A[1]=a1; A[2]=a2; A[3]=a3;
                  }
            switch (b->top&3)
                  {
            case 3:     A[2]=B[2];
            case 2:     A[1]=B[1];
            case 1:     A[0]=B[0];
            case 0: /* workaround for ultrix cc: without 'case 0', the optimizer does
                     * the switch table by doing a=top&3; a--; goto jump_table[a];
                     * which fails for top== 0 */
                  ;
                  }
            }

#else
      memset(A,0,sizeof(BN_ULONG)*words);
      memcpy(A,b->d,sizeof(b->d[0])*b->top);
#endif
            
      return(a);
      }

/* This is an internal function that can be used instead of bn_expand2()
 * when there is a need to copy BIGNUMs instead of only expanding the
 * data part, while still expanding them.
 * Especially useful when needing to expand BIGNUMs that are declared
 * 'const' and should therefore not be changed.
 * The reason to use this instead of a BN_dup() followed by a bn_expand2()
 * is memory allocation overhead.  A BN_dup() followed by a bn_expand2()
 * will allocate new memory for the BIGNUM data twice, and free it once,
 * while bn_dup_expand() makes sure allocation is made only once.
 */

#ifndef OPENSSL_NO_DEPRECATED
BIGNUM *bn_dup_expand(const BIGNUM *b, int words)
      {
      BIGNUM *r = NULL;

      bn_check_top(b);

      /* This function does not work if
       *      words <= b->dmax && top < words
       * because BN_dup() does not preserve 'dmax'!
       * (But bn_dup_expand() is not used anywhere yet.)
       */

      if (words > b->dmax)
            {
            BN_ULONG *a = bn_expand_internal(b, words);

            if (a)
                  {
                  r = BN_new();
                  if (r)
                        {
                        r->top = b->top;
                        r->dmax = words;
                        r->neg = b->neg;
                        r->d = a;
                        }
                  else
                        {
                        /* r == NULL, BN_new failure */
                        OPENSSL_free(a);
                        }
                  }
            /* If a == NULL, there was an error in allocation in
               bn_expand_internal(), and NULL should be returned */
            }
      else
            {
            r = BN_dup(b);
            }

      bn_check_top(r);
      return r;
      }
#endif

/* This is an internal function that should not be used in applications.
 * It ensures that 'b' has enough room for a 'words' word number
 * and initialises any unused part of b->d with leading zeros.
 * It is mostly used by the various BIGNUM routines. If there is an error,
 * NULL is returned. If not, 'b' is returned. */

BIGNUM *bn_expand2(BIGNUM *b, int words)
      {
      bn_check_top(b);

      if (words > b->dmax)
            {
            BN_ULONG *a = bn_expand_internal(b, words);
            if(!a) return NULL;
            if(b->d) OPENSSL_free(b->d);
            b->d=a;
            b->dmax=words;
            }

/* None of this should be necessary because of what b->top means! */
#if 0
      /* NB: bn_wexpand() calls this only if the BIGNUM really has to grow */
      if (b->top < b->dmax)
            {
            int i;
            BN_ULONG *A = &(b->d[b->top]);
            for (i=(b->dmax - b->top)>>3; i>0; i--,A+=8)
                  {
                  A[0]=0; A[1]=0; A[2]=0; A[3]=0;
                  A[4]=0; A[5]=0; A[6]=0; A[7]=0;
                  }
            for (i=(b->dmax - b->top)&7; i>0; i--,A++)
                  A[0]=0;
            assert(A == &(b->d[b->dmax]));
            }
#endif
      bn_check_top(b);
      return b;
      }

BIGNUM *BN_dup(const BIGNUM *a)
      {
      BIGNUM *t;

      if (a == NULL) return NULL;
      bn_check_top(a);

      t = BN_new();
      if (t == NULL) return NULL;
      if(!BN_copy(t, a))
            {
            BN_free(t);
            return NULL;
            }
      bn_check_top(t);
      return t;
      }

BIGNUM *BN_copy(BIGNUM *a, const BIGNUM *b)
      {
      int i;
      BN_ULONG *A;
      const BN_ULONG *B;

      bn_check_top(b);

      if (a == b) return(a);
      if (bn_wexpand(a,b->top) == NULL) return(NULL);

#if 1
      A=a->d;
      B=b->d;
      for (i=b->top>>2; i>0; i--,A+=4,B+=4)
            {
            BN_ULONG a0,a1,a2,a3;
            a0=B[0]; a1=B[1]; a2=B[2]; a3=B[3];
            A[0]=a0; A[1]=a1; A[2]=a2; A[3]=a3;
            }
      switch (b->top&3)
            {
            case 3: A[2]=B[2];
            case 2: A[1]=B[1];
            case 1: A[0]=B[0];
            case 0: ; /* ultrix cc workaround, see comments in bn_expand_internal */
            }
#else
      memcpy(a->d,b->d,sizeof(b->d[0])*b->top);
#endif

      a->top=b->top;
      a->neg=b->neg;
      bn_check_top(a);
      return(a);
      }

void BN_swap(BIGNUM *a, BIGNUM *b)
      {
      int flags_old_a, flags_old_b;
      BN_ULONG *tmp_d;
      int tmp_top, tmp_dmax, tmp_neg;
      
      bn_check_top(a);
      bn_check_top(b);

      flags_old_a = a->flags;
      flags_old_b = b->flags;

      tmp_d = a->d;
      tmp_top = a->top;
      tmp_dmax = a->dmax;
      tmp_neg = a->neg;
      
      a->d = b->d;
      a->top = b->top;
      a->dmax = b->dmax;
      a->neg = b->neg;
      
      b->d = tmp_d;
      b->top = tmp_top;
      b->dmax = tmp_dmax;
      b->neg = tmp_neg;
      
      a->flags = (flags_old_a & BN_FLG_MALLOCED) | (flags_old_b & BN_FLG_STATIC_DATA);
      b->flags = (flags_old_b & BN_FLG_MALLOCED) | (flags_old_a & BN_FLG_STATIC_DATA);
      bn_check_top(a);
      bn_check_top(b);
      }

void BN_clear(BIGNUM *a)
      {
      bn_check_top(a);
      if (a->d != NULL)
            memset(a->d,0,a->dmax*sizeof(a->d[0]));
      a->top=0;
      a->neg=0;
      }

BN_ULONG BN_get_word(const BIGNUM *a)
      {
      if (a->top > 1)
            return BN_MASK2;
      else if (a->top == 1)
            return a->d[0];
      /* a->top == 0 */
      return 0;
      }

int BN_set_word(BIGNUM *a, BN_ULONG w)
      {
      bn_check_top(a);
      if (bn_expand(a,(int)sizeof(BN_ULONG)*8) == NULL) return(0);
      a->neg = 0;
      a->d[0] = w;
      a->top = (w ? 1 : 0);
      bn_check_top(a);
      return(1);
      }

BIGNUM *BN_bin2bn(const unsigned char *s, int len, BIGNUM *ret)
      {
      unsigned int i,m;
      unsigned int n;
      BN_ULONG l;
      BIGNUM  *bn = NULL;

      if (ret == NULL)
            ret = bn = BN_new();
      if (ret == NULL) return(NULL);
      bn_check_top(ret);
      l=0;
      n=len;
      if (n == 0)
            {
            ret->top=0;
            return(ret);
            }
      i=((n-1)/BN_BYTES)+1;
      m=((n-1)%(BN_BYTES));
      if (bn_wexpand(ret, (int)i) == NULL)
            {
            if (bn) BN_free(bn);
            return NULL;
            }
      ret->top=i;
      ret->neg=0;
      while (n--)
            {
            l=(l<<8L)| *(s++);
            if (m-- == 0)
                  {
                  ret->d[--i]=l;
                  l=0;
                  m=BN_BYTES-1;
                  }
            }
      /* need to call this due to clear byte at top if avoiding
       * having the top bit set (-ve number) */
      bn_correct_top(ret);
      return(ret);
      }

/* ignore negative */
int BN_bn2bin(const BIGNUM *a, unsigned char *to)
      {
      int n,i;
      BN_ULONG l;

      bn_check_top(a);
      n=i=BN_num_bytes(a);
      while (i--)
            {
            l=a->d[i/BN_BYTES];
            *(to++)=(unsigned char)(l>>(8*(i%BN_BYTES)))&0xff;
            }
      return(n);
      }

int BN_ucmp(const BIGNUM *a, const BIGNUM *b)
      {
      int i;
      BN_ULONG t1,t2,*ap,*bp;

      bn_check_top(a);
      bn_check_top(b);

      i=a->top-b->top;
      if (i != 0) return(i);
      ap=a->d;
      bp=b->d;
      for (i=a->top-1; i>=0; i--)
            {
            t1= ap[i];
            t2= bp[i];
            if (t1 != t2)
                  return((t1 > t2) ? 1 : -1);
            }
      return(0);
      }

int BN_cmp(const BIGNUM *a, const BIGNUM *b)
      {
      int i;
      int gt,lt;
      BN_ULONG t1,t2;

      if ((a == NULL) || (b == NULL))
            {
            if (a != NULL)
                  return(-1);
            else if (b != NULL)
                  return(1);
            else
                  return(0);
            }

      bn_check_top(a);
      bn_check_top(b);

      if (a->neg != b->neg)
            {
            if (a->neg)
                  return(-1);
            else  return(1);
            }
      if (a->neg == 0)
            { gt=1; lt= -1; }
      else  { gt= -1; lt=1; }

      if (a->top > b->top) return(gt);
      if (a->top < b->top) return(lt);
      for (i=a->top-1; i>=0; i--)
            {
            t1=a->d[i];
            t2=b->d[i];
            if (t1 > t2) return(gt);
            if (t1 < t2) return(lt);
            }
      return(0);
      }

int BN_set_bit(BIGNUM *a, int n)
      {
      int i,j,k;

      if (n < 0)
            return 0;

      i=n/BN_BITS2;
      j=n%BN_BITS2;
      if (a->top <= i)
            {
            if (bn_wexpand(a,i+1) == NULL) return(0);
            for(k=a->top; k<i+1; k++)
                  a->d[k]=0;
            a->top=i+1;
            }

      a->d[i]|=(((BN_ULONG)1)<<j);
      bn_check_top(a);
      return(1);
      }

int BN_clear_bit(BIGNUM *a, int n)
      {
      int i,j;

      bn_check_top(a);
      if (n < 0) return 0;

      i=n/BN_BITS2;
      j=n%BN_BITS2;
      if (a->top <= i) return(0);

      a->d[i]&=(~(((BN_ULONG)1)<<j));
      bn_correct_top(a);
      return(1);
      }

int BN_is_bit_set(const BIGNUM *a, int n)
      {
      int i,j;

      bn_check_top(a);
      if (n < 0) return 0;
      i=n/BN_BITS2;
      j=n%BN_BITS2;
      if (a->top <= i) return 0;
      return(((a->d[i])>>j)&((BN_ULONG)1));
      }

int BN_mask_bits(BIGNUM *a, int n)
      {
      int b,w;

      bn_check_top(a);
      if (n < 0) return 0;

      w=n/BN_BITS2;
      b=n%BN_BITS2;
      if (w >= a->top) return 0;
      if (b == 0)
            a->top=w;
      else
            {
            a->top=w+1;
            a->d[w]&= ~(BN_MASK2<<b);
            }
      bn_correct_top(a);
      return(1);
      }

void BN_set_negative(BIGNUM *a, int b)
      {
      if (b && !BN_is_zero(a))
            a->neg = 1;
      else
            a->neg = 0;
      }

int bn_cmp_words(const BN_ULONG *a, const BN_ULONG *b, int n)
      {
      int i;
      BN_ULONG aa,bb;

      aa=a[n-1];
      bb=b[n-1];
      if (aa != bb) return((aa > bb)?1:-1);
      for (i=n-2; i>=0; i--)
            {
            aa=a[i];
            bb=b[i];
            if (aa != bb) return((aa > bb)?1:-1);
            }
      return(0);
      }

/* Here follows a specialised variants of bn_cmp_words().  It has the
   property of performing the operation on arrays of different sizes.
   The sizes of those arrays is expressed through cl, which is the
   common length ( basicall, min(len(a),len(b)) ), and dl, which is the
   delta between the two lengths, calculated as len(a)-len(b).
   All lengths are the number of BN_ULONGs...  */

int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b,
      int cl, int dl)
      {
      int n,i;
      n = cl-1;

      if (dl < 0)
            {
            for (i=dl; i<0; i++)
                  {
                  if (b[n-i] != 0)
                        return -1; /* a < b */
                  }
            }
      if (dl > 0)
            {
            for (i=dl; i>0; i--)
                  {
                  if (a[n+i] != 0)
                        return 1; /* a > b */
                  }
            }
      return bn_cmp_words(a,b,cl);
      }

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