check.cpp

/*
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
*/

/* Utility to recommend best choice for BASEBITS 

(MINGW build)

g++ -O2 check.cpp big.cpp miracl.a -o check.exe */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "big.h"

using namespace std;

Miracl precision=20;

int main(int argc, char *argv[])
{
	int p,w,b,n,s,t,e;
	Big lhs,rhs;

	argc--; argv++;

    if (argc!=2)
    {
       printf("Bad arguments\n");
       printf("check wordlength modulus-length\n");
	   printf("Wordlength can be 16, 32 or 64 (or 26 for Javascript)\n");
       printf("Example:\n");
       printf("check 32 256\n");
	   printf("Outputs choices for BASEBITS, number of words per Big, and number of spare bits\n");
	   printf("Normally choose for minimum words per Big, and maximum spare bits\n");
	   printf("(But >= 12 spare bits is enough, and tidier if BASEBITS =0 mod 4) \n");
       exit(0);
    }

    n=atoi(argv[0]);
	p=atoi(argv[1]); // Number of bits in prime modulus

	if (n!=16 && n!=26 && n!=32 && n!=64)
	{
		printf("wordlength must be 16, 32 or 64\n");
		return 0;
	}

	e=3;            // need at least 3 bits to allow adds and subtracts without normalisation
	if (n==26) e=1; // not an issue for Javascript
	for (b=n-e;b>=n-8;b--)
	{
		if (n==64 && b%2!=0) continue; // insist on even values for 64-bit builds 
		w=p/b; if (p%b!=0) w++;
		s=w*b-p;

		lhs=(w+1)*(pow((Big)2,2*b)-pow((Big)2,b+1)+1);
		rhs=pow((Big)2,2*n-1);

		if (lhs>=rhs)    {printf("Stability violation for BASEBITS= %d\n",b); continue;}

// Top bits of Modulus must appear in top word of representation. Also at least 4 bits spare needed for field excess.  
		if (s<4 || s>=b) {printf("Not enough Fp spare for BASEBITS= %d\n",b); continue;}
// At least 2 spare bits needed for FF excess 
		t=b*(1+(p-1)/b) - 8*(1+(p-1)/8);
		if (t<2 || t>=b) {printf("Not enough FF spare for BASEBITS= %d\n",b); continue;}

		printf("Solution for BASEBITS= %d, Words Per Big=%d, Fp spare bits= %d, FF spare bits= %d\n",b,w,s,t);
		//break;
	}
	
	return 0;
}