bn_exp.c

		/* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
		BNerr(BN_F_BN_MOD_EXP_SIMPLE,ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
		return -1;
		}

	bits=BN_num_bits(p);

	if (bits == 0)
		{
		ret = BN_one(r);
		return ret;
		}

	BN_CTX_start(ctx);
	d = BN_CTX_get(ctx);
	val[0] = BN_CTX_get(ctx);
	if(!d || !val[0]) goto err;

	if (!BN_nnmod(val[0],a,m,ctx)) goto err;		/* 1 */
	if (BN_is_zero(val[0]))
		{
		BN_zero(r);
		ret = 1;
		goto err;
		}

	window = BN_window_bits_for_exponent_size(bits);
	if (window > 1)
		{
		if (!BN_mod_mul(d,val[0],val[0],m,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(val[i],val[i-1],d,m,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_one(r)) goto err;

	for (;;)
		{
		if (BN_is_bit_set(p,wstart) == 0)
			{
			if (!start)
				if (!BN_mod_mul(r,r,r,m,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(r,r,r,m,ctx))
					goto err;
				}
		
		/* wvalue will be an odd number < 2^window */
		if (!BN_mod_mul(r,r,val[wvalue>>1],m,ctx))
			goto err;

		/* move the 'window' down further */
		wstart-=wend+1;
		wvalue=0;
		start=0;
		if (wstart < 0) break;
		}
	ret=1;
err:
	BN_CTX_end(ctx);
	bn_check_top(r);
	return(ret);
	}