text: /*
   * Copyright 1995-2017 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 "internal/constant_time_locl.h"
  #include "bn_lcl.h"
  
  #include <stdlib.h>
  #ifdef _WIN32
  # include <malloc.h>
  # ifndef alloca
  #  define alloca _alloca
  # endif
  #elif defined(__GNUC__)
  # ifndef alloca
  #  define alloca(s) __builtin_alloca((s))
  # endif
  #elif defined(__sun)
  # include <alloca.h>
  #endif
  
  #include "rsaz_exp.h"
  
  #undef SPARC_T4_MONT
  #if defined(OPENSSL_BN_ASM_MONT) && (defined(__sparc__) || defined(__sparc))
  # include "sparc_arch.h"
  extern unsigned int OPENSSL_sparcv9cap_P[];
  # define SPARC_T4_MONT
  #endif
  
  /* maximum precomputation table size for *variable* sliding windows */
  #define TABLE_SIZE      32
  
  /* this one works - simple but works */
  int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
  {
      int i, bits, ret = 0;
      BIGNUM *v, *rr;
  
      if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(a, BN_FLG_CONSTTIME) != 0) {
          /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
          BNerr(BN_F_BN_EXP, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
          return 0;
      }
  
      BN_CTX_start(ctx);
      rr = ((r == a) || (r == p)) ? BN_CTX_get(ctx) : r;
      v = BN_CTX_get(ctx);
      if (rr == NULL || v == NULL)
          goto err;
  
      if (BN_copy(v, a) == NULL)
          goto err;
      bits = BN_num_bits(p);
  
      if (BN_is_odd(p)) {
          if (BN_copy(rr, a) == NULL)
              goto err;
      } else {
          if (!BN_one(rr))
              goto err;
      }
  
      for (i = 1; i < bits; i++) {
          if (!BN_sqr(v, v, ctx))
              goto err;
          if (BN_is_bit_set(p, i)) {
              if (!BN_mul(rr, rr, v, ctx))
                  goto err;
          }
      }
      if (r != rr && BN_copy(r, rr) == NULL)
          goto err;
  
      ret = 1;
   err:
      BN_CTX_end(ctx);
      bn_check_top(r);
      return ret;
  }
  
  int BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
                 BN_CTX *ctx)
  {
      int ret;
  
      bn_check_top(a);
      bn_check_top(p);
      bn_check_top(m);
  
      /*-
       * For even modulus  m = 2^k*m_odd, it might make sense to compute
       * a^p mod m_odd  and  a^p mod 2^k  separately (with Montgomery
       * exponentiation for the odd part), using appropriate exponent
       * reductions, and combine the results using the CRT.
       *
       * For now, we use Montgomery only if the modulus is odd; otherwise,
       * exponentiation using the reciprocal-based quick remaindering
       * algorithm is used.
       *
       * (Timing obtained with expspeed.c [computations  a^p mod m
       * where  a, p, m  are of the same length: 256, 512, 1024, 2048,
       * 4096, 8192 bits], compared to the running time of the
       * standard algorithm:
       *
       *   BN_mod_exp_mont   33 .. 40 %  [AMD K6-2, Linux, debug configuration]
       *                     55 .. 77 %  [UltraSparc processor, but
       *                                  debug-solaris-sparcv8-gcc conf.]
       *
       *   BN_mod_exp_recp   50 .. 70 %  [AMD K6-2, Linux, debug configuration]
       *                     62 .. 118 % [UltraSparc, debug-solaris-sparcv8-gcc]
       *
       * On the Sparc, BN_mod_exp_recp was faster than BN_mod_exp_mont
       * at 2048 and more bits, but at 512 and 1024 bits, it was
       * slower even than the standard algorithm!
       *
       * "Real" timings [linux-elf, solaris-sparcv9-gcc configurations]
       * should be obtained when the new Montgomery reduction code
       * has been integrated into OpenSSL.)
       */
  
  #define MONT_MUL_MOD
  #define MONT_EXP_WORD
  #define RECP_MUL_MOD
  
  #ifdef MONT_MUL_MOD
      if (BN_is_odd(m)) {
  # ifdef MONT_EXP_WORD
          if (a->top == 1 && !a->neg
              && (BN_get_flags(p, BN_FLG_CONSTTIME) == 0)
              && (BN_get_flags(a, BN_FLG_CONSTTIME) == 0)
              && (BN_get_flags(m, BN_FLG_CONSTTIME) == 0)) {
              BN_ULONG A = a->d[0];
              ret = BN_mod_exp_mont_word(r, A, p, m, ctx, NULL);
          } else
  # endif
              ret = BN_mod_exp_mont(r, a, p, m, ctx, NULL);
      } else
  #endif
  #ifdef RECP_MUL_MOD
      {
          ret = BN_mod_exp_recp(r, a, p, m, ctx);
      }
  #else
      {
          ret = BN_mod_exp_simple(r, a, p, m, ctx);
      }
  #endif
  
      bn_check_top(r);
      return ret;
  }
  
  int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
                      const BIGNUM *m, BN_CTX *ctx)
  {
      int i, j, bits, ret = 0, wstart, wend, window, wvalue;
      int start = 1;
      BIGNUM *aa;
      /* Table of variables obtained from 'ctx' */
      BIGNUM *val[TABLE_SIZE];
      BN_RECP_CTX recp;
  
      if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
          /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
          BNerr(BN_F_BN_MOD_EXP_RECP, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
          return 0;
      }
  
      bits = BN_num_bits(p);
      if (bits == 0) {
          /* x**0 mod 1 is still zero. */
          if (BN_is_one(m)) {
              ret = 1;
              BN_zero(r);
          } else {
              ret = BN_one(r);
          }
          return ret;
      }
  
      BN_CTX_start(ctx);
      aa = BN_CTX_get(ctx);
      val[0] = BN_CTX_get(ctx);
      if (val[0] == NULL)
          goto err;
  
      BN_RECP_CTX_init(&recp);
      if (m->neg) {
          /* ignore sign of 'm' */
          if (!BN_copy(aa, m))
              goto err;
          aa->neg = 0;
          if (BN_RECP_CTX_set(&recp, aa, ctx) <= 0)
              goto err;
      } else {
          if (BN_RECP_CTX_set(&recp, m, ctx) <= 0)
              goto err;
      }
  
      if (!BN_nnmod(val[0], a, m, ctx))
          goto err;               /* 1 */
      if (BN_is_zero(val[0])) {
          BN_zero(r);
          ret = 1;
          goto err;
      }
  
      window = BN_window_bits_for_exponent_size(bits);
      if (window > 1) {
          if (!BN_mod_mul_reciprocal(aa, val[0], val[0], &recp, ctx))
              goto err;           /* 2 */
          j = 1 << (window - 1);
          for (i = 1; i < j; i++) {
              if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
                  !BN_mod_mul_reciprocal(val[i], val[i - 1], aa, &recp, ctx))
                  goto err;
          }
      }
  
      start = 1;                  /* This is used to avoid multiplication etc
                                   * when there is only the value '1' in the
                                   * buffer. */
      wvalue = 0;                 /* The 'value' of the window */
      wstart = bits - 1;          /* The top bit of the window */
      wend = 0;                   /* The bottom bit of the window */
  
      if (!BN_one(r))
          goto err;
  
      for (;;) {
          if (BN_is_bit_set(p, wstart) == 0) {
              if (!start)
                  if (!BN_mod_mul_reciprocal(r, r, r, &recp, ctx))
                      goto err;
              if (wstart == 0)
                  break;
              wstart--;
              continue;
          }
          /*
           * We now have wstart on a 'set' bit, we now need to work out how bit
           * a window to do.  To do this we need to scan forward until the last
           * set bit before the end of the window
           */
          j = wstart;
          wvalue = 1;
          wend = 0;
          for (i = 1; i < window; i++) {
              if (wstart - i < 0)
                  break;
              if (BN_is_bit_set(p, wstart - i)) {
                  wvalue <<= (i - wend);
                  wvalue |= 1;
                  wend = i;
              }
          }
  
          /* wend is the size of the current window */
          j = wend + 1;
          /* add the 'bytes above' */
          if (!start)
              for (i = 0; i < j; i++) {
                  if (!BN_mod_mul_reciprocal(r, r, r, &recp, ctx))
                      goto err;
              }
  
          /* wvalue will be an odd number < 2^window */
          if (!BN_mod_mul_reciprocal(r, r, val[wvalue >> 1], &recp, ctx))
              goto err;
  
          /* move the 'window' down further */
          wstart -= wend + 1;
          wvalue = 0;
          start = 0;
          if (wstart < 0)
              break;
      }
      ret = 1;
   err:
      BN_CTX_end(ctx);
      BN_RECP_CTX_free(&recp);
      bn_check_top(r);
      return ret;
  }
  
  int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                      const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
  {
      int i, j, bits, ret = 0, wstart, wend, window, wvalue;
      int start = 1;
      BIGNUM *d, *r;
      const BIGNUM *aa;
      /* Table of variables obtained from 'ctx' */
      BIGNUM *val[TABLE_SIZE];
      BN_MONT_CTX *mont = NULL;
  
      if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
          return BN_mod_exp_mont_consttime(rr, a, p, m, ctx, in_mont);
      }
  
      bn_check_top(a);
      bn_check_top(p);
      bn_check_top(m);
  
      if (!BN_is_odd(m)) {
          BNerr(BN_F_BN_MOD_EXP_MONT, BN_R_CALLED_WITH_EVEN_MODULUS);
          return 0;
      }
      bits = BN_num_bits(p);
      if (bits == 0) {
          /* x**0 mod 1 is still zero. */
          if (BN_is_one(m)) {
              ret = 1;
              BN_zero(rr);
          } else {
              ret = BN_one(rr);
          }
          return ret;
      }
  
      BN_CTX_start(ctx);
      d = BN_CTX_get(ctx);
      r = BN_CTX_get(ctx);
      val[0] = BN_CTX_get(ctx);
      if (val[0] == NULL)
          goto err;
  
      /*
       * If this is not done, things will break in the montgomery part
       */
  
      if (in_mont != NULL)
          mont = in_mont;
      else {
          if ((mont = BN_MONT_CTX_new()) == NULL)
              goto err;
          if (!BN_MONT_CTX_set(mont, m, ctx))
              goto err;
      }
  
      if (a->neg || BN_ucmp(a, m) >= 0) {
          if (!BN_nnmod(val[0], a, m, ctx))
              goto err;
          aa = val[0];
      } else
          aa = a;
      if (BN_is_zero(aa)) {
          BN_zero(rr);
          ret = 1;
          goto err;
      }
      if (!BN_to_montgomery(val[0], aa, mont, ctx))
          goto err;               /* 1 */
  
      window = BN_window_bits_for_exponent_size(bits);
      if (window > 1) {
          if (!BN_mod_mul_montgomery(d, val[0], val[0], mont, ctx))
              goto err;           /* 2 */
          j = 1 << (window - 1);
          for (i = 1; i < j; i++) {
              if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
                  !BN_mod_mul_montgomery(val[i], val[i - 1], d, mont, ctx))
                  goto err;
          }
      }
  
      start = 1;                  /* This is used to avoid multiplication etc
                                   * when there is only the value '1' in the
                                   * buffer. */
      wvalue = 0;                 /* The 'value' of the window */
      wstart = bits - 1;          /* The top bit of the window */
      wend = 0;                   /* The bottom bit of the window */
  
  #if 1                           /* by Shay Gueron's suggestion */
      j = m->top;                 /* borrow j */
      if (m->d[j - 1] & (((BN_ULONG)1) << (BN_BITS2 - 1))) {
          if (bn_wexpand(r, j) == NULL)
              goto err;
          /* 2^(top*BN_BITS2) - m */
          r->d[0] = (0 - m->d[0]) & BN_MASK2;
          for (i = 1; i < j; i++)
              r->d[i] = (~m->d[i]) & BN_MASK2;
          r->top = j;
          /*
           * Upper words will be zero if the corresponding words of 'm' were
           * 0xfff[...], so decrement r->top accordingly.
           */
          bn_correct_top(r);
      } else
  #endif
      if (!BN_to_montgomery(r, BN_value_one(), mont, ctx))
          goto err;
      for (;;) {
          if (BN_is_bit_set(p, wstart) == 0) {
              if (!start) {
                  if (!BN_mod_mul_montgomery(r, r, r, mont, ctx))
                      goto err;
              }
              if (wstart == 0)
                  break;
              wstart--;
              continue;
          }
          /*
           * We now have wstart on a 'set' bit, we now need to work out how bit
           * a window to do.  To do this we need to scan forward until the last
           * set bit before the end of the window
           */
          j = wstart;
          wvalue = 1;
          wend = 0;
          for (i = 1; i < window; i++) {
              if (wstart - i < 0)
                  break;
              if (BN_is_bit_set(p, wstart - i)) {
                  wvalue <<= (i - wend);
                  wvalue |= 1;
                  wend = i;
              }
          }
  
          /* wend is the size of the current window */
          j = wend + 1;
          /* add the 'bytes above' */
          if (!start)
              for (i = 0; i < j; i++) {
                  if (!BN_mod_mul_montgomery(r, r, r, mont, ctx))
                      goto err;
              }
  
          /* wvalue will be an odd number < 2^window */
          if (!BN_mod_mul_montgomery(r, r, val[wvalue >> 1], mont, ctx))
              goto err;
  
          /* move the 'window' down further */
          wstart -= wend + 1;
          wvalue = 0;
          start = 0;
          if (wstart < 0)
              break;
      }
  #if defined(SPARC_T4_MONT)
      if (OPENSSL_sparcv9cap_P[0] & (SPARCV9_VIS3 | SPARCV9_PREFER_FPU)) {
          j = mont->N.top;        /* borrow j */
          val[0]->d[0] = 1;       /* borrow val[0] */
          for (i = 1; i < j; i++)
              val[0]->d[i] = 0;
          val[0]->top = j;
          if (!BN_mod_mul_montgomery(rr, r, val[0], mont, ctx))
              goto err;
      } else
  #endif
      if (!BN_from_montgomery(rr, r, mont, ctx))
          goto err;
      ret = 1;
   err:
      if (in_mont == NULL)
          BN_MONT_CTX_free(mont);
      BN_CTX_end(ctx);
      bn_check_top(rr);
      return ret;
  }
  
  #if defined(SPARC_T4_MONT)
  static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos)
  {
      BN_ULONG ret = 0;
      int wordpos;
  
      wordpos = bitpos / BN_BITS2;
      bitpos %= BN_BITS2;
      if (wordpos >= 0 && wordpos < a->top) {
          ret = a->d[wordpos] & BN_MASK2;
          if (bitpos) {
              ret >>= bitpos;
              if (++wordpos < a->top)
                  ret |= a->d[wordpos] << (BN_BITS2 - bitpos);
          }
      }
  
      return ret & BN_MASK2;
  }
  #endif
  
  /*
   * BN_mod_exp_mont_consttime() stores the precomputed powers in a specific
   * layout so that accessing any of these table values shows the same access
   * pattern as far as cache lines are concerned.  The following functions are
   * used to transfer a BIGNUM from/to that table.
   */
  
  static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top,
                                          unsigned char *buf, int idx,
                                          int window)
  {
      int i, j;
      int width = 1 << window;
      BN_ULONG *table = (BN_ULONG *)buf;
  
      if (top > b->top)
          top = b->top;           /* this works because 'buf' is explicitly
                                   * zeroed */
      for (i = 0, j = idx; i < top; i++, j += width) {
          table[j] = b->d[i];
      }
  
      return 1;
  }
  
  static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top,
                                            unsigned char *buf, int idx,
                                            int window)
  {
      int i, j;
      int width = 1 << window;
      /*
       * We declare table 'volatile' in order to discourage compiler
       * from reordering loads from the table. Concern is that if
       * reordered in specific manner loads might give away the
       * information we are trying to conceal. Some would argue that
       * compiler can reorder them anyway, but it can as well be
       * argued that doing so would be violation of standard...
       */
      volatile BN_ULONG *table = (volatile BN_ULONG *)buf;
  
      if (bn_wexpand(b, top) == NULL)
          return 0;
  
      if (window <= 3) {
          for (i = 0; i < top; i++, table += width) {
              BN_ULONG acc = 0;
  
              for (j = 0; j < width; j++) {
                  acc |= table[j] &
                         ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
              }
  
              b->d[i] = acc;
          }
      } else {
          int xstride = 1 << (window - 2);
          BN_ULONG y0, y1, y2, y3;
  
          i = idx >> (window - 2);        /* equivalent of idx / xstride */
          idx &= xstride - 1;             /* equivalent of idx % xstride */
  
          y0 = (BN_ULONG)0 - (constant_time_eq_int(i,0)&1);
          y1 = (BN_ULONG)0 - (constant_time_eq_int(i,1)&1);
          y2 = (BN_ULONG)0 - (constant_time_eq_int(i,2)&1);
          y3 = (BN_ULONG)0 - (constant_time_eq_int(i,3)&1);
  
          for (i = 0; i < top; i++, table += width) {
              BN_ULONG acc = 0;
  
              for (j = 0; j < xstride; j++) {
                  acc |= ( (table[j + 0 * xstride] & y0) |
                           (table[j + 1 * xstride] & y1) |
                           (table[j + 2 * xstride] & y2) |
                           (table[j + 3 * xstride] & y3) )
                         & ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
              }
  
              b->d[i] = acc;
          }
      }
  
      b->top = top;
      bn_correct_top(b);
      return 1;
  }
  
  /*
   * Given a pointer value, compute the next address that is a cache line
   * multiple.
   */
  #define MOD_EXP_CTIME_ALIGN(x_) \
          ((unsigned char*)(x_) + (MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH - (((size_t)(x_)) & (MOD_EXP_CTIME_MIN_CACHE_LINE_MASK))))
  
  /*
   * This variant of BN_mod_exp_mont() uses fixed windows and the special
   * precomputation memory layout to limit data-dependency to a minimum to
   * protect secret exponents (cf. the hyper-threading timing attacks pointed
   * out by Colin Percival,
   * http://www.daemonology.net/hyperthreading-considered-harmful/)
   */
  int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                                const BIGNUM *m, BN_CTX *ctx,
                                BN_MONT_CTX *in_mont)
  {
      int i, bits, ret = 0, window, wvalue;
      int top;
      BN_MONT_CTX *mont = NULL;
  
      int numPowers;
      unsigned char *powerbufFree = NULL;
      int powerbufLen = 0;
      unsigned char *powerbuf = NULL;
      BIGNUM tmp, am;
  #if defined(SPARC_T4_MONT)
      unsigned int t4 = 0;
  #endif
  
      bn_check_top(a);
      bn_check_top(p);
      bn_check_top(m);
  
      if (!BN_is_odd(m)) {
          BNerr(BN_F_BN_MOD_EXP_MONT_CONSTTIME, BN_R_CALLED_WITH_EVEN_MODULUS);
          return 0;
      }
  
      top = m->top;
  
      /*
       * Use all bits stored in |p|, rather than |BN_num_bits|, so we do not leak
       * whether the top bits are zero.
       */
      bits = p->top * BN_BITS2;
      if (bits == 0) {
          /* x**0 mod 1 is still zero. */
          if (BN_is_one(m)) {
              ret = 1;
              BN_zero(rr);
          } else {
              ret = BN_one(rr);
          }
          return ret;
      }
  
      BN_CTX_start(ctx);
  
      /*
       * Allocate a montgomery context if it was not supplied by the caller. If
       * this is not done, things will break in the montgomery part.
       */
      if (in_mont != NULL)
          mont = in_mont;
      else {
          if ((mont = BN_MONT_CTX_new()) == NULL)
              goto err;
          if (!BN_MONT_CTX_set(mont, m, ctx))
              goto err;
      }
  
  #ifdef RSAZ_ENABLED
      if (!a->neg) {
          /*
           * If the size of the operands allow it, perform the optimized
           * RSAZ exponentiation. For further information see
           * crypto/bn/rsaz_exp.c and accompanying assembly modules.
           */
          if ((16 == a->top) && (16 == p->top) && (BN_num_bits(m) == 1024)
              && rsaz_avx2_eligible()) {
              if (NULL == bn_wexpand(rr, 16))
                  goto err;
              RSAZ_1024_mod_exp_avx2(rr->d, a->d, p->d, m->d, mont->RR.d,
                                     mont->n0[0]);
              rr->top = 16;
              rr->neg = 0;
              bn_correct_top(rr);
              ret = 1;
              goto err;
          } else if ((8 == a->top) && (8 == p->top) && (BN_num_bits(m) == 512)) {
              if (NULL == bn_wexpand(rr, 8))
                  goto err;
              RSAZ_512_mod_exp(rr->d, a->d, p->d, m->d, mont->n0[0], mont->RR.d);
              rr->top = 8;
              rr->neg = 0;
              bn_correct_top(rr);
              ret = 1;
              goto err;
          }
      }
  #endif
  
      /* Get the window size to use with size of p. */
      window = BN_window_bits_for_ctime_exponent_size(bits);
  #if defined(SPARC_T4_MONT)
      if (window >= 5 && (top & 15) == 0 && top <= 64 &&
          (OPENSSL_sparcv9cap_P[1] & (CFR_MONTMUL | CFR_MONTSQR)) ==
          (CFR_MONTMUL | CFR_MONTSQR) && (t4 = OPENSSL_sparcv9cap_P[0]))
          window = 5;
      else
  #endif
  #if defined(OPENSSL_BN_ASM_MONT5)
      if (window >= 5) {
          window = 5;             /* ~5% improvement for RSA2048 sign, and even
                                   * for RSA4096 */
          /* reserve space for mont->N.d[] copy */
          powerbufLen += top * sizeof(mont->N.d[0]);
      }
  #endif
      (void)0;
  
      /*
       * Allocate a buffer large enough to hold all of the pre-computed powers
       * of am, am itself and tmp.
       */
      numPowers = 1 << window;
      powerbufLen += sizeof(m->d[0]) * (top * numPowers +
                                        ((2 * top) >
                                         numPowers ? (2 * top) : numPowers));
  #ifdef alloca
      if (powerbufLen < 3072)
          powerbufFree =
              alloca(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH);
      else
  #endif
          if ((powerbufFree =
               OPENSSL_malloc(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH))
              == NULL)
          goto err;
  
      powerbuf = MOD_EXP_CTIME_ALIGN(powerbufFree);
      memset(powerbuf, 0, powerbufLen);
  
  #ifdef alloca
      if (powerbufLen < 3072)
          powerbufFree = NULL;
  #endif
  
      /* lay down tmp and am right after powers table */
      tmp.d = (BN_ULONG *)(powerbuf + sizeof(m->d[0]) * top * numPowers);
      am.d = tmp.d + top;
      tmp.top = am.top = 0;
      tmp.dmax = am.dmax = top;
      tmp.neg = am.neg = 0;
      tmp.flags = am.flags = BN_FLG_STATIC_DATA;
  
      /* prepare a^0 in Montgomery domain */
  #if 1                           /* by Shay Gueron's suggestion */
      if (m->d[top - 1] & (((BN_ULONG)1) << (BN_BITS2 - 1))) {
          /* 2^(top*BN_BITS2) - m */
          tmp.d[0] = (0 - m->d[0]) & BN_MASK2;
          for (i = 1; i < top; i++)
              tmp.d[i] = (~m->d[i]) & BN_MASK2;
          tmp.top = top;
      } else
  #endif
      if (!BN_to_montgomery(&tmp, BN_value_one(), mont, ctx))
          goto err;
  
      /* prepare a^1 in Montgomery domain */
      if (a->neg || BN_ucmp(a, m) >= 0) {
          if (!BN_nnmod(&am, a, m, ctx))
              goto err;
          if (!BN_to_montgomery(&am, &am, mont, ctx))
              goto err;
      } else if (!BN_to_montgomery(&am, a, mont, ctx))
          goto err;
  
  #if defined(SPARC_T4_MONT)
      if (t4) {
          typedef int (*bn_pwr5_mont_f) (BN_ULONG *tp, const BN_ULONG *np,
                                         const BN_ULONG *n0, const void *table,
                                         int power, int bits);
          int bn_pwr5_mont_t4_8(BN_ULONG *tp, const BN_ULONG *np,
                                const BN_ULONG *n0, const void *table,
                                int power, int bits);
          int bn_pwr5_mont_t4_16(BN_ULONG *tp, const BN_ULONG *np,
                                 const BN_ULONG *n0, const void *table,
                                 int power, int bits);
          int bn_pwr5_mont_t4_24(BN_ULONG *tp, const BN_ULONG *np,
                                 const BN_ULONG *n0, const void *table,
                                 int power, int bits);
          int bn_pwr5_mont_t4_32(BN_ULONG *tp, const BN_ULONG *np,
                                 const BN_ULONG *n0, const void *table,
                                 int power, int bits);
          static const bn_pwr5_mont_f pwr5_funcs[4] = {
              bn_pwr5_mont_t4_8, bn_pwr5_mont_t4_16,
              bn_pwr5_mont_t4_24, bn_pwr5_mont_t4_32
          };
          bn_pwr5_mont_f pwr5_worker = pwr5_funcs[top / 16 - 1];
  
          typedef int (*bn_mul_mont_f) (BN_ULONG *rp, const BN_ULONG *ap,
                                        const void *bp, const BN_ULONG *np,
                                        const BN_ULONG *n0);
          int bn_mul_mont_t4_8(BN_ULONG *rp, const BN_ULONG *ap, const void *bp,
                               const BN_ULONG *np, const BN_ULONG *n0);
          int bn_mul_mont_t4_16(BN_ULONG *rp, const BN_ULONG *ap,
                                const void *bp, const BN_ULONG *np,
                                const BN_ULONG *n0);
          int bn_mul_mont_t4_24(BN_ULONG *rp, const BN_ULONG *ap,
                                const void *bp, const BN_ULONG *np,
                                const BN_ULONG *n0);
          int bn_mul_mont_t4_32(BN_ULONG *rp, const BN_ULONG *ap,
                                const void *bp, const BN_ULONG *np,
                                const BN_ULONG *n0);
          static const bn_mul_mont_f mul_funcs[4] = {
              bn_mul_mont_t4_8, bn_mul_mont_t4_16,
              bn_mul_mont_t4_24, bn_mul_mont_t4_32
          };
          bn_mul_mont_f mul_worker = mul_funcs[top / 16 - 1];
  
          void bn_mul_mont_vis3(BN_ULONG *rp, const BN_ULONG *ap,
                                const void *bp, const BN_ULONG *np,
                                const BN_ULONG *n0, int num);
          void bn_mul_mont_t4(BN_ULONG *rp, const BN_ULONG *ap,
                              const void *bp, const BN_ULONG *np,
                              const BN_ULONG *n0, int num);
          void bn_mul_mont_gather5_t4(BN_ULONG *rp, const BN_ULONG *ap,
                                      const void *table, const BN_ULONG *np,
                                      const BN_ULONG *n0, int num, int power);
          void bn_flip_n_scatter5_t4(const BN_ULONG *inp, size_t num,
                                     void *table, size_t power);
          void bn_gather5_t4(BN_ULONG *out, size_t num,
                             void *table, size_t power);
          void bn_flip_t4(BN_ULONG *dst, BN_ULONG *src, size_t num);
  
          BN_ULONG *np = mont->N.d, *n0 = mont->n0;
          int stride = 5 * (6 - (top / 16 - 1)); /* multiple of 5, but less
                                                  * than 32 */
  
          /*
           * BN_to_montgomery can contaminate words above .top [in
           * BN_DEBUG[_DEBUG] build]...
           */
          for (i = am.top; i < top; i++)
              am.d[i] = 0;
          for (i = tmp.top; i < top; i++)
              tmp.d[i] = 0;
  
          bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, 0);
          bn_flip_n_scatter5_t4(am.d, top, powerbuf, 1);
          if (!(*mul_worker) (tmp.d, am.d, am.d, np, n0) &&
              !(*mul_worker) (tmp.d, am.d, am.d, np, n0))
              bn_mul_mont_vis3(tmp.d, am.d, am.d, np, n0, top);
          bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, 2);
  
          for (i = 3; i < 32; i++) {
              /* Calculate a^i = a^(i-1) * a */
              if (!(*mul_worker) (tmp.d, tmp.d, am.d, np, n0) &&
                  !(*mul_worker) (tmp.d, tmp.d, am.d, np, n0))
                  bn_mul_mont_vis3(tmp.d, tmp.d, am.d, np, n0, top);
              bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, i);
          }
  
          /* switch to 64-bit domain */
          np = alloca(top * sizeof(BN_ULONG));
          top /= 2;
          bn_flip_t4(np, mont->N.d, top);
  
          bits--;
          for (wvalue = 0, i = bits % 5; i >= 0; i--, bits--)
              wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
          bn_gather5_t4(tmp.d, top, powerbuf, wvalue);
  
          /*
           * Scan the exponent one window at a time starting from the most
           * significant bits.
           */
          while (bits >= 0) {
              if (bits < stride)
                  stride = bits + 1;
              bits -= stride;
              wvalue = bn_get_bits(p, bits + 1);
  
              if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
                  continue;
              /* retry once and fall back */
              if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
                  continue;
  
              bits += stride - 5;
              wvalue >>= stride - 5;
              wvalue &= 31;
              bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_mul_mont_gather5_t4(tmp.d, tmp.d, powerbuf, np, n0, top,
                                     wvalue);
          }
  
          bn_flip_t4(tmp.d, tmp.d, top);
          top *= 2;
          /* back to 32-bit domain */
          tmp.top = top;
          bn_correct_top(&tmp);
          OPENSSL_cleanse(np, top * sizeof(BN_ULONG));
      } else
  #endif
  #if defined(OPENSSL_BN_ASM_MONT5)
      if (window == 5 && top > 1) {
          /*
           * This optimization uses ideas from http://eprint.iacr.org/2011/239,
           * specifically optimization of cache-timing attack countermeasures
           * and pre-computation optimization.
           */
  
          /*
           * Dedicated window==4 case improves 512-bit RSA sign by ~15%, but as
           * 512-bit RSA is hardly relevant, we omit it to spare size...
           */
          void bn_mul_mont_gather5(BN_ULONG *rp, const BN_ULONG *ap,
                                   const void *table, const BN_ULONG *np,
                                   const BN_ULONG *n0, int num, int power);
          void bn_scatter5(const BN_ULONG *inp, size_t num,
                           void *table, size_t power);
          void bn_gather5(BN_ULONG *out, size_t num, void *table, size_t power);
          void bn_power5(BN_ULONG *rp, const BN_ULONG *ap,
                         const void *table, const BN_ULONG *np,
                         const BN_ULONG *n0, int num, int power);
          int bn_get_bits5(const BN_ULONG *ap, int off);
          int bn_from_montgomery(BN_ULONG *rp, const BN_ULONG *ap,
                                 const BN_ULONG *not_used, const BN_ULONG *np,
                                 const BN_ULONG *n0, int num);
  
          BN_ULONG *n0 = mont->n0, *np;
  
          /*
           * BN_to_montgomery can contaminate words above .top [in
           * BN_DEBUG[_DEBUG] build]...
           */
          for (i = am.top; i < top; i++)
              am.d[i] = 0;
          for (i = tmp.top; i < top; i++)
              tmp.d[i] = 0;
  
          /*
           * copy mont->N.d[] to improve cache locality
           */
          for (np = am.d + top, i = 0; i < top; i++)
              np[i] = mont->N.d[i];
  
          bn_scatter5(tmp.d, top, powerbuf, 0);
          bn_scatter5(am.d, am.top, powerbuf, 1);
          bn_mul_mont(tmp.d, am.d, am.d, np, n0, top);
          bn_scatter5(tmp.d, top, powerbuf, 2);
  
  # if 0
          for (i = 3; i < 32; i++) {
              /* Calculate a^i = a^(i-1) * a */
              bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
              bn_scatter5(tmp.d, top, powerbuf, i);
          }
  # else
          /* same as above, but uses squaring for 1/2 of operations */
          for (i = 4; i < 32; i *= 2) {
              bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_scatter5(tmp.d, top, powerbuf, i);
          }
          for (i = 3; i < 8; i += 2) {
              int j;
              bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
              bn_scatter5(tmp.d, top, powerbuf, i);
              for (j = 2 * i; j < 32; j *= 2) {
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_scatter5(tmp.d, top, powerbuf, j);
              }
          }
          for (; i < 16; i += 2) {
              bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
              bn_scatter5(tmp.d, top, powerbuf, i);
              bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
              bn_scatter5(tmp.d, top, powerbuf, 2 * i);
          }
          for (; i < 32; i += 2) {
              bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
              bn_scatter5(tmp.d, top, powerbuf, i);
          }
  # endif
          bits--;
          for (wvalue = 0, i = bits % 5; i >= 0; i--, bits--)
              wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
          bn_gather5(tmp.d, top, powerbuf, wvalue);
  
          /*
           * Scan the exponent one window at a time starting from the most
           * significant bits.
           */
          if (top & 7)
              while (bits >= 0) {
                  for (wvalue = 0, i = 0; i < 5; i++, bits--)
                      wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
  
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
                  bn_mul_mont_gather5(tmp.d, tmp.d, powerbuf, np, n0, top,
                                      wvalue);
          } else {
              while (bits >= 0) {
                  wvalue = bn_get_bits5(p->d, bits - 4);
                  bits -= 5;
                  bn_power5(tmp.d, tmp.d, powerbuf, np, n0, top, wvalue);
              }
          }
  
          ret = bn_from_montgomery(tmp.d, tmp.d, NULL, np, n0, top);
          tmp.top = top;
          bn_correct_top(&tmp);
          if (ret) {
              if (!BN_copy(rr, &tmp))
                  ret = 0;
              goto err;           /* non-zero ret means it's not error */
          }
      } else
  #endif
      {
          if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window))
              goto err;
          if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window))
              goto err;
  
          /*
           * If the window size is greater than 1, then calculate
           * val[i=2..2^winsize-1]. Powers are computed as a*a^(i-1) (even
           * powers could instead be computed as (a^(i/2))^2 to use the slight
           * performance advantage of sqr over mul).
           */
          if (window > 1) {
              if (!BN_mod_mul_montgomery(&tmp, &am, &am, mont, ctx))
                  goto err;
              if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 2,
                                                window))
                  goto err;
              for (i = 3; i < numPowers; i++) {
                  /* Calculate a^i = a^(i-1) * a */
                  if (!BN_mod_mul_montgomery(&tmp, &am, &tmp, mont, ctx))
                      goto err;
                  if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, i,
                                                    window))
                      goto err;
              }
          }
  
          bits--;
          for (wvalue = 0, i = bits % window; i >= 0; i--, bits--)
              wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
          if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue,
                                              window))
              goto err;
  
          /*
           * Scan the exponent one window at a time starting from the most
           * significant bits.
           */
          while (bits >= 0) {
              wvalue = 0;         /* The 'value' of the window */
  
              /* Scan the window, squaring the result as we go */
              for (i = 0; i < window; i++, bits--) {
                  if (!BN_mod_mul_montgomery(&tmp, &tmp, &tmp, mont, ctx))
                      goto err;
                  wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
              }
  
              /*
               * Fetch the appropriate pre-computed value from the pre-buf
               */
              if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue,
                                                  window))
                  goto err;
  
              /* Multiply the result into the intermediate result */
              if (!BN_mod_mul_montgomery(&tmp, &tmp, &am, mont, ctx))
                  goto err;
          }
      }
  
      /* Convert the final result from montgomery to standard format */
  #if defined(SPARC_T4_MONT)
      if (OPENSSL_sparcv9cap_P[0] & (SPARCV9_VIS3 | SPARCV9_PREFER_FPU)) {
          am.d[0] = 1;            /* borrow am */
          for (i = 1; i < top; i++)
              am.d[i] = 0;
          if (!BN_mod_mul_montgomery(rr, &tmp, &am, mont, ctx))
              goto err;
      } else
  #endif
      if (!BN_from_montgomery(rr, &tmp, mont, ctx))
          goto err;
      ret = 1;
   err:
      if (in_mont == NULL)
          BN_MONT_CTX_free(mont);
      if (powerbuf != NULL) {
          OPENSSL_cleanse(powerbuf, powerbufLen);
          OPENSSL_free(powerbufFree);
      }
      BN_CTX_end(ctx);
      return ret;
  }
  
  int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
                           const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
  {
      BN_MONT_CTX *mont = NULL;
      int b, bits, ret = 0;
      int r_is_one;
      BN_ULONG w, next_w;
      BIGNUM *r, *t;
      BIGNUM *swap_tmp;
  #define BN_MOD_MUL_WORD(r, w, m) \
                  (BN_mul_word(r, (w)) && \
                  (/* BN_ucmp(r, (m)) < 0 ? 1 :*/  \
                          (BN_mod(t, r, m, ctx) && (swap_tmp = r, r = t, t = swap_tmp, 1))))
      /*
       * BN_MOD_MUL_WORD is only used with 'w' large, so the BN_ucmp test is
       * probably more overhead than always using BN_mod (which uses BN_copy if
       * a similar test returns true).
       */
      /*
       * We can use BN_mod and do not need BN_nnmod because our accumulator is
       * never negative (the result of BN_mod does not depend on the sign of
       * the modulus).
       */
  #define BN_TO_MONTGOMERY_WORD(r, w, mont) \
                  (BN_set_word(r, (w)) && BN_to_montgomery(r, r, (mont), ctx))
  
      if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
          /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
          BNerr(BN_F_BN_MOD_EXP_MONT_WORD, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
          return 0;
      }
  
      bn_check_top(p);
      bn_check_top(m);
  
      if (!BN_is_odd(m)) {
          BNerr(BN_F_BN_MOD_EXP_MONT_WORD, BN_R_CALLED_WITH_EVEN_MODULUS);
          return 0;
      }
      if (m->top == 1)
          a %= m->d[0];           /* make sure that 'a' is reduced */
  
      bits = BN_num_bits(p);
      if (bits == 0) {
          /* x**0 mod 1 is still zero. */
          if (BN_is_one(m)) {
              ret = 1;
              BN_zero(rr);
          } else {
              ret = BN_one(rr);
          }
          return ret;
      }
      if (a == 0) {
          BN_zero(rr);
          ret = 1;
          return ret;
      }
  
      BN_CTX_start(ctx);
      r = BN_CTX_get(ctx);
      t = BN_CTX_get(ctx);
      if (t == NULL)
          goto err;
  
      if (in_mont != NULL)
          mont = in_mont;
      else {
          if ((mont = BN_MONT_CTX_new()) == NULL)
              goto err;
          if (!BN_MONT_CTX_set(mont, m, ctx))
              goto err;
      }
  
      r_is_one = 1;               /* except for Montgomery factor */
  
      /* bits-1 >= 0 */
  
      /* The result is accumulated in the product r*w. */
      w = a;                      /* bit 'bits-1' of 'p' is always set */
      for (b = bits - 2; b >= 0; b--) {
          /* First, square r*w. */
          next_w = w * w;
          if ((next_w / w) != w) { /* overflow */
              if (r_is_one) {
                  if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
                      goto err;
                  r_is_one = 0;
              } else {
                  if (!BN_MOD_MUL_WORD(r, w, m))
                      goto err;
              }
              next_w = 1;
          }
          w = next_w;
          if (!r_is_one) {
              if (!BN_mod_mul_montgomery(r, r, r, mont, ctx))
                  goto err;
          }
  
          /* Second, multiply r*w by 'a' if exponent bit is set. */
          if (BN_is_bit_set(p, b)) {
              next_w = w * a;
              if ((next_w / a) != w) { /* overflow */
                  if (r_is_one) {
                      if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
                          goto err;
                      r_is_one = 0;
                  } else {
                      if (!BN_MOD_MUL_WORD(r, w, m))
                          goto err;
                  }
                  next_w = a;
              }
              w = next_w;
          }
      }
  
      /* Finally, set r:=r*w. */
      if (w != 1) {
          if (r_is_one) {
              if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
                  goto err;
              r_is_one = 0;
          } else {
              if (!BN_MOD_MUL_WORD(r, w, m))
                  goto err;
          }
      }
  
      if (r_is_one) {             /* can happen only if a == 1 */
          if (!BN_one(rr))
              goto err;
      } else {
          if (!BN_from_montgomery(rr, r, mont, ctx))
              goto err;
      }
      ret = 1;
   err:
      if (in_mont == NULL)
          BN_MONT_CTX_free(mont);
      BN_CTX_end(ctx);
      bn_check_top(rr);
      return ret;
  }
  
  /* The old fallback, simple version :-) */
  int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
                        const BIGNUM *m, BN_CTX *ctx)
  {
      int i, j, bits, ret = 0, wstart, wend, window, wvalue;
      int start = 1;
      BIGNUM *d;
      /* Table of variables obtained from 'ctx' */
      BIGNUM *val[TABLE_SIZE];
  
      if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
              || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
          /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
          BNerr(BN_F_BN_MOD_EXP_SIMPLE, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
          return 0;
      }
  
      bits = BN_num_bits(p);
      if (bits == 0) {
          /* x**0 mod 1 is still zero. */
          if (BN_is_one(m)) {
              ret = 1;
              BN_zero(r);
          } else {
              ret = BN_one(r);
          }
          return ret;
      }
  
      BN_CTX_start(ctx);
      d = BN_CTX_get(ctx);
      val[0] = BN_CTX_get(ctx);
      if (val[0] == NULL)
          goto err;
  
      if (!BN_nnmod(val[0], a, m, ctx))
          goto err;               /* 1 */
      if (BN_is_zero(val[0])) {
          BN_zero(r);
          ret = 1;
          goto err;
      }
  
      window = BN_window_bits_for_exponent_size(bits);
      if (window > 1) {
          if (!BN_mod_mul(d, val[0], val[0], m, ctx))
              goto err;           /* 2 */
          j = 1 << (window - 1);
          for (i = 1; i < j; i++) {
              if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
                  !BN_mod_mul(val[i], val[i - 1], d, m, ctx))
                  goto err;
          }
      }
  
      start = 1;                  /* This is used to avoid multiplication etc
                                   * when there is only the value '1' in the
                                   * buffer. */
      wvalue = 0;                 /* The 'value' of the window */
      wstart = bits - 1;          /* The top bit of the window */
      wend = 0;                   /* The bottom bit of the window */
  
      if (!BN_one(r))
          goto err;
  
      for (;;) {
          if (BN_is_bit_set(p, wstart) == 0) {
              if (!start)
                  if (!BN_mod_mul(r, r, r, m, ctx))
                      goto err;
              if (wstart == 0)
                  break;
              wstart--;
              continue;
          }
          /*
           * We now have wstart on a 'set' bit, we now need to work out how bit
           * a window to do.  To do this we need to scan forward until the last
           * set bit before the end of the window
           */
          j = wstart;
          wvalue = 1;
          wend = 0;
          for (i = 1; i < window; i++) {
              if (wstart - i < 0)
                  break;
              if (BN_is_bit_set(p, wstart - i)) {
                  wvalue <<= (i - wend);
                  wvalue |= 1;
                  wend = i;
              }
          }
  
          /* wend is the size of the current window */
          j = wend + 1;
          /* add the 'bytes above' */
          if (!start)
              for (i = 0; i < j; i++) {
                  if (!BN_mod_mul(r, r, r, m, ctx))
                      goto err;
              }
  
          /* wvalue will be an odd number < 2^window */
          if (!BN_mod_mul(r, r, val[wvalue >> 1], m, ctx))
              goto err;
  
          /* move the 'window' down further */
          wstart -= wend + 1;
          wvalue = 0;
          start = 0;
          if (wstart < 0)
              break;
      }
      ret = 1;
   err:
      BN_CTX_end(ctx);
      bn_check_top(r);
      return ret;
  }
  