/*

 * Copyright 2014-2026 The OpenSSL Project Authors. All Rights Reserved.

 *

 * Licensed under the Apache License 2.0 (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 "bn_local.h"

#include "internal/constant_time.h"

/*

 * 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.

 */

signed char *bn_compute_wNAF(const BIGNUM *scalar, int w, size_t *ret_len)

{

    int window_val;

    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 == NULL)

            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 */

        ERR_raise(ERR_LIB_BN, 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) {

        ERR_raise(ERR_LIB_BN, 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)

        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)) {

                ERR_raise(ERR_LIB_BN, 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) {

                ERR_raise(ERR_LIB_BN, 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) {

            ERR_raise(ERR_LIB_BN, ERR_R_INTERNAL_ERROR);

            goto err;

        }

    }

    if (j > len + 1) {

        ERR_raise(ERR_LIB_BN, ERR_R_INTERNAL_ERROR);

        goto err;

    }

    *ret_len = j;

    return r;

err:

    OPENSSL_free(r);

    return NULL;

}

int bn_get_top(const BIGNUM *a)

{

    return a->top;

}

int bn_get_dmax(const BIGNUM *a)

{

    return a->dmax;

}

void bn_set_all_zero(BIGNUM *a)

{

    int i;

    for (i = a->top; i < a->dmax; i++)

        a->d[i] = 0;

}

/*

 * Zero-extend |a| so that it occupies exactly |words| words, flag it

 * BN_FLG_FIXED_TOP and leave its numeric value unchanged.

 *

 * This is a companion to bn_correct_top(): where the latter minimises the top

 * of a BIGNUM, this one pins the top to a caller-chosen, value-independent

 * width.  Constant-time code uses it to make the cost of subsequent word-wise

 * operations (e.g. BN_uadd()/BN_add()) independent of the magnitude of a

 * secret value.  |words| must be greater than or equal to the current top.

 *

 * The routine is itself constant time with respect to the current a->top: it

 * always sweeps a fixed |words| iterations and selects value-or-zero per word

 * with an arithmetic mask, rather than looping over the (possibly secret)

 * a->top..words range.  Masking the high words with zero also launders any

 * uninitialised padding, so it is safe for the memory sanitiser.

 */

int bn_set_top_fixed(BIGNUM *a, int words)

{

    size_t i, n = (size_t)words;

    BN_ULONG mask;

    if (words < a->top)

        return 0;

    if (bn_wexpand(a, words) == NULL) {

        ERR_raise(ERR_LIB_BN, ERR_R_BN_LIB);

        return 0;

    }

    for (i = 0; i < n; i++) {

        /* mask = all ones iff i < a->top, else all zeros */

        mask = value_barrier_bn((BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1)));

        a->d[i] &= mask;

    }

    a->top = words;

    a->flags |= BN_FLG_FIXED_TOP;

    return 1;

}

int bn_copy_words(BN_ULONG *out, const BIGNUM *in, int size)

{

    if (in->top > size)

        return 0;

    memset(out, 0, sizeof(*out) * size);

    if (in->d != NULL)

        memcpy(out, in->d, sizeof(*out) * in->top);

    return 1;

}

BN_ULONG *bn_get_words(const BIGNUM *a)

{

    return a->d;

}

void bn_set_static_words(BIGNUM *a, const BN_ULONG *words, int size)

{

    /*

     * |const| qualifier omission is compensated by BN_FLG_STATIC_DATA

     * flag, which effectively means "read-only data".

     */

    a->d = (BN_ULONG *)words;

    a->dmax = a->top = size;

    a->neg = 0;

    a->flags |= BN_FLG_STATIC_DATA;

    bn_correct_top(a);

}

int bn_set_words(BIGNUM *a, const BN_ULONG *words, int num_words)

{

    if (bn_wexpand(a, num_words) == NULL) {

        ERR_raise(ERR_LIB_BN, ERR_R_BN_LIB);

        return 0;

    }

    memcpy(a->d, words, sizeof(BN_ULONG) * num_words);

    a->top = num_words;

    bn_correct_top(a);

    return 1;

}