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 *d, *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_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 -1;
		}

	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)
		{
		ret = BN_one(rr);
		return ret;
		}
	if (a == 0)
		{
		BN_zero(rr);
		ret = 1;
		return ret;
		}

	BN_CTX_start(ctx);
	d = BN_CTX_get(ctx);
	r = BN_CTX_get(ctx);
	t = BN_CTX_get(ctx);
	if (d == NULL || r == NULL || 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) && (mont != NULL)) BN_MONT_CTX_free(mont);
	BN_CTX_end(ctx);
	bn_check_top(rr);
	return(ret);
	}