ec_mult.c

Go to the documentation of this file.

/* crypto/ec/ec_mult.c */
/*
 * Originally written by Bodo Moeller and Nils Larsch for the OpenSSL project.
 */
/* ====================================================================
 * Copyright (c) 1998-2007 The OpenSSL Project.  All rights reserved.
 *
 * 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 above 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 acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please contact
 *    [email protected].
 *
 * 5. Products derived from this software may not be called "OpenSSL"
 *    nor may "OpenSSL" appear in their names without prior written
 *    permission of the OpenSSL Project.
 *
 * 6. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
 *
 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
 * EXPRESSED 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 OpenSSL PROJECT OR
 * ITS 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.
 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * ([email protected]).  This product includes software written by Tim
 * Hudson ([email protected]).
 *
 */
/* ====================================================================
 * Copyright 2002 Sun Microsystems, Inc. ALL RIGHTS RESERVED.
 * Portions of this software developed by SUN MICROSYSTEMS, INC.,
 * and contributed to the OpenSSL project.
 */

#include <string.h>

#include <openssl/err.h>

#include "ec_lcl.h"


/*
 * This file implements the wNAF-based interleaving multi-exponentation method
 * (<URL:http://www.informatik.tu-darmstadt.de/TI/Mitarbeiter/moeller.html#multiexp>);
 * for multiplication with precomputation, we use wNAF splitting
 * (<URL:http://www.informatik.tu-darmstadt.de/TI/Mitarbeiter/moeller.html#fastexp>).
 */




/* structure for precomputed multiples of the generator */
typedef struct ec_pre_comp_st {
    const EC_GROUP *group; /* parent EC_GROUP object */
    size_t blocksize;      /* block size for wNAF splitting */
    size_t numblocks;      /* max. number of blocks for which we have precomputation */
    size_t w;              /* window size */
    EC_POINT **points;     /* array with pre-calculated multiples of generator:
                            * 'num' pointers to EC_POINT objects followed by a NULL */
    size_t num;            /* numblocks * 2^(w-1) */
    int references;
} EC_PRE_COMP;
 
/* functions to manage EC_PRE_COMP within the EC_GROUP extra_data framework */
static void *ec_pre_comp_dup(void *);
static void ec_pre_comp_free(void *);
static void ec_pre_comp_clear_free(void *);

static EC_PRE_COMP *ec_pre_comp_new(const EC_GROUP *group)
    {
    EC_PRE_COMP *ret = NULL;

    if (!group)
        return NULL;

    ret = (EC_PRE_COMP *)OPENSSL_malloc(sizeof(EC_PRE_COMP));
    if (!ret)
        {
        ECerr(EC_F_EC_PRE_COMP_NEW, ERR_R_MALLOC_FAILURE);
        return ret;
        }
    ret->group = group;
    ret->blocksize = 8; /* default */
    ret->numblocks = 0;
    ret->w = 4; /* default */
    ret->points = NULL;
    ret->num = 0;
    ret->references = 1;
    return ret;
    }

static void *ec_pre_comp_dup(void *src_)
    {
    EC_PRE_COMP *src = src_;

    /* no need to actually copy, these objects never change! */

    CRYPTO_add(&src->references, 1, CRYPTO_LOCK_EC_PRE_COMP);

    return src_;
    }

static void ec_pre_comp_free(void *pre_)
    {
    int i;
    EC_PRE_COMP *pre = pre_;

    if (!pre)
        return;

    i = CRYPTO_add(&pre->references, -1, CRYPTO_LOCK_EC_PRE_COMP);
    if (i > 0)
        return;

    if (pre->points)
        {
        EC_POINT **p;

        for (p = pre->points; *p != NULL; p++)
            EC_POINT_free(*p);
        OPENSSL_free(pre->points);
        }
    OPENSSL_free(pre);
    }

static void ec_pre_comp_clear_free(void *pre_)
    {
    int i;
    EC_PRE_COMP *pre = pre_;

    if (!pre)
        return;

    i = CRYPTO_add(&pre->references, -1, CRYPTO_LOCK_EC_PRE_COMP);
    if (i > 0)
        return;

    if (pre->points)
        {
        EC_POINT **p;

        for (p = pre->points; *p != NULL; p++)
            {
            EC_POINT_clear_free(*p);
            OPENSSL_cleanse(p, sizeof *p);
            }
        OPENSSL_free(pre->points);
        }
    OPENSSL_cleanse(pre, sizeof *pre);
    OPENSSL_free(pre);
    }




/* Determine the modified width-(w+1) Non-Adjacent Form (wNAF) of 'scalar'.
 * This is an array  r[]  of values that are either zero or odd with an
 * absolute value less than  2^w  satisfying
 *     scalar = \sum_j r[j]*2^j
 * where at most one of any  w+1  consecutive digits is non-zero
 * with the exception that the most significant digit may be only
 * w-1 zeros away from that next non-zero digit.
 */
static signed char *compute_wNAF(const BIGNUM *scalar, int w, size_t *ret_len)
    {
    int window_val;
    int ok = 0;
    signed char *r = NULL;
    int sign = 1;
    int bit, next_bit, mask;
    size_t len = 0, j;
    
    if (BN_is_zero(scalar))
        {
        r = OPENSSL_malloc(1);
        if (!r)
            {
            ECerr(EC_F_COMPUTE_WNAF, ERR_R_MALLOC_FAILURE);
            goto err;
            }
        r[0] = 0;
        *ret_len = 1;
        return r;
        }
        
    if (w <= 0 || w > 7) /* 'signed char' can represent integers with absolute values less than 2^7 */
        {
        ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
        goto err;
        }
    bit = 1 << w; /* at most 128 */
    next_bit = bit << 1; /* at most 256 */
    mask = next_bit - 1; /* at most 255 */

    if (BN_is_negative(scalar))
        {
        sign = -1;
        }

    if (scalar->d == NULL || scalar->top == 0)
        {
        ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
        goto err;
        }

    len = BN_num_bits(scalar);
    r = OPENSSL_malloc(len + 1); /* modified wNAF may be one digit longer than binary representation
                                  * (*ret_len will be set to the actual length, i.e. at most
                                  * BN_num_bits(scalar) + 1) */
    if (r == NULL)
        {
        ECerr(EC_F_COMPUTE_WNAF, ERR_R_MALLOC_FAILURE);
        goto err;
        }
    window_val = scalar->d[0] & mask;
    j = 0;
    while ((window_val != 0) || (j + w + 1 < len)) /* if j+w+1 >= len, window_val will not increase */
        {
        int digit = 0;

        /* 0 <= window_val <= 2^(w+1) */

        if (window_val & 1)
            {
            /* 0 < window_val < 2^(w+1) */

            if (window_val & bit)
                {
                digit = window_val - next_bit; /* -2^w < digit < 0 */

#if 1 /* modified wNAF */
                if (j + w + 1 >= len)
                    {
                    /* special case for generating modified wNAFs:
                     * no new bits will be added into window_val,
                     * so using a positive digit here will decrease
                     * the total length of the representation */
                    
                    digit = window_val & (mask >> 1); /* 0 < digit < 2^w */
                    }
#endif
                }
            else
                {
                digit = window_val; /* 0 < digit < 2^w */
                }
            
            if (digit <= -bit || digit >= bit || !(digit & 1))
                {
                ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
                goto err;
                }

            window_val -= digit;

            /* now window_val is 0 or 2^(w+1) in standard wNAF generation;
             * for modified window NAFs, it may also be 2^w
             */
            if (window_val != 0 && window_val != next_bit && window_val != bit)
                {
                ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
                goto err;
                }
            }

        r[j++] = sign * digit;

        window_val >>= 1;
        window_val += bit * BN_is_bit_set(scalar, j + w);

        if (window_val > next_bit)
            {
            ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
            goto err;
            }
        }

    if (j > len + 1)
        {
        ECerr(EC_F_COMPUTE_WNAF, ERR_R_INTERNAL_ERROR);
        goto err;
        }
    len = j;
    ok = 1;

 err:
    if (!ok)
        {
        OPENSSL_free(r);
        r = NULL;
        }
    if (ok)
        *ret_len = len;
    return r;
    }


/* TODO: table should be optimised for the wNAF-based implementation,
 *       sometimes smaller windows will give better performance
 *       (thus the boundaries should be increased)
 */
#define EC_window_bits_for_scalar_size(b) \
        ((size_t) \
         ((b) >= 2000 ? 6 : \
          (b) >=  800 ? 5 : \
          (b) >=  300 ? 4 : \
          (b) >=   70 ? 3 : \
          (b) >=   20 ? 2 : \
          1))

/* Compute
 *      \sum scalars[i]*points[i],
 * also including
 *      scalar*generator
 * in the addition if scalar != NULL
 */
int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
    size_t num, const EC_POINT *points[], const BIGNUM *scalars[], BN_CTX *ctx)
    {
    BN_CTX *new_ctx = NULL;
    const EC_POINT *generator = NULL;
    EC_POINT *tmp = NULL;
    size_t totalnum;
    size_t blocksize = 0, numblocks = 0; /* for wNAF splitting */
    size_t pre_points_per_block = 0;
    size_t i, j;
    int k;
    int r_is_inverted = 0;
    int r_is_at_infinity = 1;
    size_t *wsize = NULL; /* individual window sizes */
    signed char **wNAF = NULL; /* individual wNAFs */
    size_t *wNAF_len = NULL;
    size_t max_len = 0;
    size_t num_val;
    EC_POINT **val = NULL; /* precomputation */
    EC_POINT **v;
    EC_POINT ***val_sub = NULL; /* pointers to sub-arrays of 'val' or 'pre_comp->points' */
    const EC_PRE_COMP *pre_comp = NULL;
    int num_scalar = 0; /* flag: will be set to 1 if 'scalar' must be treated like other scalars,
                         * i.e. precomputation is not available */
    int ret = 0;
    
    if (group->meth != r->meth)
        {
        ECerr(EC_F_EC_WNAF_MUL, EC_R_INCOMPATIBLE_OBJECTS);
        return 0;
        }

    if ((scalar == NULL) && (num == 0))
        {
        return EC_POINT_set_to_infinity(group, r);
        }

    for (i = 0; i < num; i++)
        {
        if (group->meth != points[i]->meth)
            {
            ECerr(EC_F_EC_WNAF_MUL, EC_R_INCOMPATIBLE_OBJECTS);
            return 0;
            }
        }

    if (ctx == NULL)
        {
        ctx = new_ctx = BN_CTX_new();
        if (ctx == NULL)
            goto err;
        }

    if (scalar != NULL)
        {
        generator = EC_GROUP_get0_generator(group);
        if (generator == NULL)
            {
            ECerr(EC_F_EC_WNAF_MUL, EC_R_UNDEFINED_GENERATOR);
            goto err;
            }
        
        /* look if we can use precomputed multiples of generator */

        pre_comp = EC_EX_DATA_get_data(group->extra_data, ec_pre_comp_dup, ec_pre_comp_free, ec_pre_comp_clear_free);

        if (pre_comp && pre_comp->numblocks && (EC_POINT_cmp(group, generator, pre_comp->points[0], ctx) == 0))
            {
            blocksize = pre_comp->blocksize;

            /* determine maximum number of blocks that wNAF splitting may yield
             * (NB: maximum wNAF length is bit length plus one) */
            numblocks = (BN_num_bits(scalar) / blocksize) + 1;

            /* we cannot use more blocks than we have precomputation for */
            if (numblocks > pre_comp->numblocks)
                numblocks = pre_comp->numblocks;

            pre_points_per_block = (size_t)1 << (pre_comp->w - 1);

            /* check that pre_comp looks sane */
            if (pre_comp->num != (pre_comp->numblocks * pre_points_per_block))
                {
                ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                goto err;
                }
            }
        else
            {
            /* can't use precomputation */
            pre_comp = NULL;
            numblocks = 1;
            num_scalar = 1; /* treat 'scalar' like 'num'-th element of 'scalars' */
            }
        }
    
    totalnum = num + numblocks;

    wsize    = OPENSSL_malloc(totalnum * sizeof wsize[0]);
    wNAF_len = OPENSSL_malloc(totalnum * sizeof wNAF_len[0]);
    wNAF     = OPENSSL_malloc((totalnum + 1) * sizeof wNAF[0]); /* includes space for pivot */
    val_sub  = OPENSSL_malloc(totalnum * sizeof val_sub[0]);
         
    if (!wsize || !wNAF_len || !wNAF || !val_sub)
        {
        ECerr(EC_F_EC_WNAF_MUL, ERR_R_MALLOC_FAILURE);
        goto err;
        }

    wNAF[0] = NULL; /* preliminary pivot */

    /* num_val will be the total number of temporarily precomputed points */
    num_val = 0;

    for (i = 0; i < num + num_scalar; i++)
        {
        size_t bits;

        bits = i < num ? BN_num_bits(scalars[i]) : BN_num_bits(scalar);
        wsize[i] = EC_window_bits_for_scalar_size(bits);
        num_val += (size_t)1 << (wsize[i] - 1);
        wNAF[i + 1] = NULL; /* make sure we always have a pivot */
        wNAF[i] = compute_wNAF((i < num ? scalars[i] : scalar), wsize[i], &wNAF_len[i]);
        if (wNAF[i] == NULL)
            goto err;
        if (wNAF_len[i] > max_len)
            max_len = wNAF_len[i];
        }

    if (numblocks)
        {
        /* we go here iff scalar != NULL */
        
        if (pre_comp == NULL)
            {
            if (num_scalar != 1)
                {
                ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                goto err;
                }
            /* we have already generated a wNAF for 'scalar' */
            }
        else
            {
            signed char *tmp_wNAF = NULL;
            size_t tmp_len = 0;
            
            if (num_scalar != 0)
                {
                ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                goto err;
                }

            /* use the window size for which we have precomputation */
            wsize[num] = pre_comp->w;
            tmp_wNAF = compute_wNAF(scalar, wsize[num], &tmp_len);
            if (!tmp_wNAF)
                goto err;

            if (tmp_len <= max_len)
                {
                /* One of the other wNAFs is at least as long
                 * as the wNAF belonging to the generator,
                 * so wNAF splitting will not buy us anything. */

                numblocks = 1;
                totalnum = num + 1; /* don't use wNAF splitting */
                wNAF[num] = tmp_wNAF;
                wNAF[num + 1] = NULL;
                wNAF_len[num] = tmp_len;
                if (tmp_len > max_len)
                    max_len = tmp_len;
                /* pre_comp->points starts with the points that we need here: */
                val_sub[num] = pre_comp->points;
                }
            else
                {
                /* don't include tmp_wNAF directly into wNAF array
                 * - use wNAF splitting and include the blocks */

                signed char *pp;
                EC_POINT **tmp_points;
                
                if (tmp_len < numblocks * blocksize)
                    {
                    /* possibly we can do with fewer blocks than estimated */
                    numblocks = (tmp_len + blocksize - 1) / blocksize;
                    if (numblocks > pre_comp->numblocks)
                        {
                        ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                        goto err;
                        }
                    totalnum = num + numblocks;
                    }
                
                /* split wNAF in 'numblocks' parts */
                pp = tmp_wNAF;
                tmp_points = pre_comp->points;

                for (i = num; i < totalnum; i++)
                    {
                    if (i < totalnum - 1)
                        {
                        wNAF_len[i] = blocksize;
                        if (tmp_len < blocksize)
                            {
                            ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                            goto err;
                            }
                        tmp_len -= blocksize;
                        }
                    else
                        /* last block gets whatever is left
                         * (this could be more or less than 'blocksize'!) */
                        wNAF_len[i] = tmp_len;
                    
                    wNAF[i + 1] = NULL;
                    wNAF[i] = OPENSSL_malloc(wNAF_len[i]);
                    if (wNAF[i] == NULL)
                        {
                        ECerr(EC_F_EC_WNAF_MUL, ERR_R_MALLOC_FAILURE);
                        OPENSSL_free(tmp_wNAF);
                        goto err;
                        }
                    memcpy(wNAF[i], pp, wNAF_len[i]);
                    if (wNAF_len[i] > max_len)
                        max_len = wNAF_len[i];

                    if (*tmp_points == NULL)
                        {
                        ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
                        OPENSSL_free(tmp_wNAF);
                        goto err;
                        }
                    val_sub[i] = tmp_points;
                    tmp_points += pre_points_per_block;
                    pp += blocksize;
                    }
                OPENSSL_free(tmp_wNAF);
                }
            }
        }

    /* All points we precompute now go into a single array 'val'.
     * 'val_sub[i]' is a pointer to the subarray for the i-th point,
     * or to a subarray of 'pre_comp->points' if we already have precomputation. */
    val = OPENSSL_malloc((num_val + 1) * sizeof val[0]);
    if (val == NULL)
        {
        ECerr(EC_F_EC_WNAF_MUL, ERR_R_MALLOC_FAILURE);
        goto err;
        }
    val[num_val] = NULL; /* pivot element */

    /* allocate points for precomputation */
    v = val;
    for (i = 0; i < num + num_scalar; i++)
        {
        val_sub[i] = v;
        for (j = 0; j < ((size_t)1 << (wsize[i] - 1)); j++)
            {
            *v = EC_POINT_new(group);
            if (*v == NULL) goto err;
            v++;
            }
        }
    if (!(v == val + num_val))
        {
        ECerr(EC_F_EC_WNAF_MUL, ERR_R_INTERNAL_ERROR);
        goto err;
        }

    if (!(tmp = EC_POINT_new(group)))
        goto err;

    /* prepare precomputed values:
     *    val_sub[i][0] :=     points[i]
     *    val_sub[i][1] := 3 * points[i]
     *    val_sub[i][2] := 5 * points[i]
     *    ...
     */
    for (i = 0; i < num + num_scalar; i++)
        {
        if (i < num)
            {
            if (!EC_POINT_copy(val_sub[i][0], points[i])) goto err;
            }
        else
            {
            if (!EC_POINT_copy(val_sub[i][0], generator)) goto err;
            }

        if (wsize[i] > 1)
            {
            if (!EC_POINT_dbl(group, tmp, val_sub[i][0], ctx)) goto err;
            for (j = 1; j < ((size_t)1 << (wsize[i] - 1)); j++)
                {
                if (!EC_POINT_add(group, val_sub[i][j], val_sub[i][j - 1], tmp, ctx)) goto err;
                }
            }
        }

#if 1 /* optional; EC_window_bits_for_scalar_size assumes we do this step */
    if (!EC_POINTs_make_affine(group, num_val, val, ctx))
        goto err;
#endif

    r_is_at_infinity = 1;

    for (k = max_len - 1; k >= 0; k--)
        {
        if (!r_is_at_infinity)
            {
            if (!EC_POINT_dbl(group, r, r, ctx)) goto err;
            }
        
        for (i = 0; i < totalnum; i++)
            {
            if (wNAF_len[i] > (size_t)k)
                {
                int digit = wNAF[i][k];
                int is_neg;

                if (digit) 
                    {
                    is_neg = digit < 0;

                    if (is_neg)
                        digit = -digit;

                    if (is_neg != r_is_inverted)
                        {
                        if (!r_is_at_infinity)
                            {
                            if (!EC_POINT_invert(group, r, ctx)) goto err;
                            }
                        r_is_inverted = !r_is_inverted;
                        }

                    /* digit > 0 */

                    if (r_is_at_infinity)
                        {
                        if (!EC_POINT_copy(r, val_sub[i][digit >> 1])) goto err;
                        r_is_at_infinity = 0;
                        }
                    else
                        {
                        if (!EC_POINT_add(group, r, r, val_sub[i][digit >> 1], ctx)) goto err;
                        }
                    }
                }
            }
        }

    if (r_is_at_infinity)
        {
        if (!EC_POINT_set_to_infinity(group, r)) goto err;
        }
    else
        {
        if (r_is_inverted)
            if (!EC_POINT_invert(group, r, ctx)) goto err;
        }
    
    ret = 1;

 err:
    if (new_ctx != NULL)
        BN_CTX_free(new_ctx);
    if (tmp != NULL)
        EC_POINT_free(tmp);
    if (wsize != NULL)
        OPENSSL_free(wsize);
    if (wNAF_len != NULL)
        OPENSSL_free(wNAF_len);
    if (wNAF != NULL)
        {
        signed char **w;
        
        for (w = wNAF; *w != NULL; w++)
            OPENSSL_free(*w);
        
        OPENSSL_free(wNAF);
        }
    if (val != NULL)
        {
        for (v = val; *v != NULL; v++)
            EC_POINT_clear_free(*v);

        OPENSSL_free(val);
        }
    if (val_sub != NULL)
        {
        OPENSSL_free(val_sub);
        }
    return ret;
    }


/* ec_wNAF_precompute_mult()
 * creates an EC_PRE_COMP object with preprecomputed multiples of the generator
 * for use with wNAF splitting as implemented in ec_wNAF_mul().
 * 
 * 'pre_comp->points' is an array of multiples of the generator
 * of the following form:
 * points[0] =     generator;
 * points[1] = 3 * generator;
 * ...
 * points[2^(w-1)-1] =     (2^(w-1)-1) * generator;
 * points[2^(w-1)]   =     2^blocksize * generator;
 * points[2^(w-1)+1] = 3 * 2^blocksize * generator;
 * ...
 * points[2^(w-1)*(numblocks-1)-1] = (2^(w-1)) *  2^(blocksize*(numblocks-2)) * generator
 * points[2^(w-1)*(numblocks-1)]   =              2^(blocksize*(numblocks-1)) * generator
 * ...
 * points[2^(w-1)*numblocks-1]     = (2^(w-1)) *  2^(blocksize*(numblocks-1)) * generator
 * points[2^(w-1)*numblocks]       = NULL
 */
int ec_wNAF_precompute_mult(EC_GROUP *group, BN_CTX *ctx)
    {
    const EC_POINT *generator;
    EC_POINT *tmp_point = NULL, *base = NULL, **var;
    BN_CTX *new_ctx = NULL;
    BIGNUM *order;
    size_t i, bits, w, pre_points_per_block, blocksize, numblocks, num;
    EC_POINT **points = NULL;
    EC_PRE_COMP *pre_comp;
    int ret = 0;

    /* if there is an old EC_PRE_COMP object, throw it away */
    EC_EX_DATA_free_data(&group->extra_data, ec_pre_comp_dup, ec_pre_comp_free, ec_pre_comp_clear_free);

    if ((pre_comp = ec_pre_comp_new(group)) == NULL)
        return 0;

    generator = EC_GROUP_get0_generator(group);
    if (generator == NULL)
        {
        ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, EC_R_UNDEFINED_GENERATOR);
        goto err;
        }

    if (ctx == NULL)
        {
        ctx = new_ctx = BN_CTX_new();
        if (ctx == NULL)
            goto err;
        }
    
    BN_CTX_start(ctx);
    order = BN_CTX_get(ctx);
    if (order == NULL) goto err;
    
    if (!EC_GROUP_get_order(group, order, ctx)) goto err;       
    if (BN_is_zero(order))
        {
        ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, EC_R_UNKNOWN_ORDER);
        goto err;
        }

    bits = BN_num_bits(order);
    /* The following parameters mean we precompute (approximately)
     * one point per bit.
     *
     * TBD: The combination  8, 4  is perfect for 160 bits; for other
     * bit lengths, other parameter combinations might provide better
     * efficiency.
     */
    blocksize = 8;
    w = 4;
    if (EC_window_bits_for_scalar_size(bits) > w)
        {
        /* let's not make the window too small ... */
        w = EC_window_bits_for_scalar_size(bits);
        }

    numblocks = (bits + blocksize - 1) / blocksize; /* max. number of blocks to use for wNAF splitting */
    
    pre_points_per_block = (size_t)1 << (w - 1);
    num = pre_points_per_block * numblocks; /* number of points to compute and store */

    points = OPENSSL_malloc(sizeof (EC_POINT*)*(num + 1));
    if (!points)
        {
        ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, ERR_R_MALLOC_FAILURE);
        goto err;
        }

    var = points;
    var[num] = NULL; /* pivot */
    for (i = 0; i < num; i++)
        {
        if ((var[i] = EC_POINT_new(group)) == NULL)
            {
            ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, ERR_R_MALLOC_FAILURE);
            goto err;
            }
        }

    if (!(tmp_point = EC_POINT_new(group)) || !(base = EC_POINT_new(group)))
        {
        ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, ERR_R_MALLOC_FAILURE);
        goto err;
        }   
    
    if (!EC_POINT_copy(base, generator))
        goto err;
    
    /* do the precomputation */
    for (i = 0; i < numblocks; i++)
        {
        size_t j;

        if (!EC_POINT_dbl(group, tmp_point, base, ctx))
            goto err;

        if (!EC_POINT_copy(*var++, base))
            goto err;

        for (j = 1; j < pre_points_per_block; j++, var++)
            {
            /* calculate odd multiples of the current base point */
            if (!EC_POINT_add(group, *var, tmp_point, *(var - 1), ctx))
                goto err;
            }

        if (i < numblocks - 1)
            {
            /* get the next base (multiply current one by 2^blocksize) */
            size_t k;

            if (blocksize <= 2)
                {
                ECerr(EC_F_EC_WNAF_PRECOMPUTE_MULT, ERR_R_INTERNAL_ERROR);
                goto err;
                }               

            if (!EC_POINT_dbl(group, base, tmp_point, ctx))
                goto err;
            for (k = 2; k < blocksize; k++)
                {
                if (!EC_POINT_dbl(group,base,base,ctx))
                    goto err;
                }
            }
        }

    if (!EC_POINTs_make_affine(group, num, points, ctx))
        goto err;
    
    pre_comp->group = group;
    pre_comp->blocksize = blocksize;
    pre_comp->numblocks = numblocks;
    pre_comp->w = w;
    pre_comp->points = points;
    points = NULL;
    pre_comp->num = num;

    if (!EC_EX_DATA_set_data(&group->extra_data, pre_comp,
        ec_pre_comp_dup, ec_pre_comp_free, ec_pre_comp_clear_free))
        goto err;
    pre_comp = NULL;

    ret = 1;
 err:
    if (ctx != NULL)
        BN_CTX_end(ctx);
    if (new_ctx != NULL)
        BN_CTX_free(new_ctx);
    if (pre_comp)
        ec_pre_comp_free(pre_comp);
    if (points)
        {
        EC_POINT **p;

        for (p = points; *p != NULL; p++)
            EC_POINT_free(*p);
        OPENSSL_free(points);
        }
    if (tmp_point)
        EC_POINT_free(tmp_point);
    if (base)
        EC_POINT_free(base);
    return ret;
    }


int ec_wNAF_have_precompute_mult(const EC_GROUP *group)
    {
    if (EC_EX_DATA_get_data(group->extra_data, ec_pre_comp_dup, ec_pre_comp_free, ec_pre_comp_clear_free) != NULL)
        return 1;
    else
        return 0;
    }