Commit message: Timing vulnerability in ECDSA signature generation (CVE-2018-0735)

Preallocate an extra limb for some of the big numbers to avoid a reallocation
that can potentially provide a side channel.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7486)

(cherry picked from commit 99540ec79491f59ed8b46b4edf130e17dc907f52)
CVE: CVE-2018-0735
CWE: CWE-320
Vulnerability description: The OpenSSL ECDSA signature algorithm has been shown to be vulnerable to a timing side channel attack. An attacker could use variations in the signing algorithm to recover the private key. Fixed in OpenSSL 1.1.0j (Affected 1.1.0-1.1.0i). Fixed in OpenSSL 1.1.1a (Affected 1.1.1).

Project: openssl

EC_PRE_COMP *EC_ec_pre_comp_dup(EC_PRE_COMP *pre)
{
    int i;
    if (pre != NULL)
        CRYPTO_UP_REF(&pre->references, &i, pre->lock);
    return pre;
}

Project: openssl

void EC_ec_pre_comp_free(EC_PRE_COMP *pre)
{
    int i;

    if (pre == NULL)
        return;

    CRYPTO_DOWN_REF(&pre->references, &i, pre->lock);
    REF_PRINT_COUNT("EC_ec", pre);
    if (i > 0)
        return;
    REF_ASSERT_ISNT(i < 0);

    if (pre->points != NULL) {
        EC_POINT **pts;

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

Project: openssl

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)
{
    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 (!BN_is_zero(group->order) && !BN_is_zero(group->cofactor)) {
        /*-
         * Handle the common cases where the scalar is secret, enforcing a
         * scalar multiplication implementation based on a Montgomery ladder,
         * with various timing attack defenses.
         */
        if ((scalar != NULL) && (num == 0)) {
            /*-
             * In this case we want to compute scalar * GeneratorPoint: this
             * codepath is reached most prominently by (ephemeral) key
             * generation of EC cryptosystems (i.e. ECDSA keygen and sign setup,
             * ECDH keygen/first half), where the scalar is always secret. This
             * is why we ignore if BN_FLG_CONSTTIME is actually set and we
             * always call the ladder version.
             */
            return ec_scalar_mul_ladder(group, r, scalar, NULL, ctx);
        }
        if ((scalar == NULL) && (num == 1)) {
            /*-
             * In this case we want to compute scalar * VariablePoint: this
             * codepath is reached most prominently by the second half of ECDH,
             * where the secret scalar is multiplied by the peer's public point.
             * To protect the secret scalar, we ignore if BN_FLG_CONSTTIME is
             * actually set and we always call the ladder version.
             */
            return ec_scalar_mul_ladder(group, r, scalars[0], points[0], ctx);
        }
    }

    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 = group->pre_comp.ec;
        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]));
    /* include space for pivot */
    wNAF = OPENSSL_malloc((totalnum + 1) * sizeof(wNAF[0]));
    val_sub = OPENSSL_malloc(totalnum * sizeof(val_sub[0]));

    /* Ensure wNAF is initialised in case we end up going to err */
    if (wNAF != NULL)
        wNAF[0] = NULL;         /* preliminary pivot */

    if (wsize == NULL || wNAF_len == NULL || wNAF == NULL || val_sub == NULL) {
        ECerr(EC_F_EC_WNAF_MUL, ERR_R_MALLOC_FAILURE);
        goto err;
    }

    /*
     * 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] =
            bn_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 = bn_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;
                /*
                 * 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);
                        OPENSSL_free(tmp_wNAF);
                        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);
                            OPENSSL_free(tmp_wNAF);
                            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)) == NULL)
        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 (!EC_POINTs_make_affine(group, num_val, val, ctx))
        goto err;

    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:
    EC_POINT_free(tmp);
    OPENSSL_free(wsize);
    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);
    }
    OPENSSL_free(val_sub);
    return ret;
}