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;

	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)
		{
		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 (!d || !r || !val[0]) 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 (!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 (!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);
	}