/*
 *   Digital Signature Standard (DSS)
 *
 *   Elliptic Curve variation GF(2^m) - See Dr. Dobbs Journal, April 1997
 *
 *   This program asks for the name of a <file>, computes its message digest,
 *   signs it, and outputs the signature to a file <file>.ecs. It is assumed 
 *   that curve parameters are available from a file common.ecs, as well as 
 *   the private key of the signer previously generated by the ecsgen program
 *
 *   The curve is y^2+xy = x^3+Ax^2+B over GF(2^m) using a trinomial or 
 *   pentanomial basis (t^m+t^a+1 or t^m+t^a+t^b+t^c+1). These parameters
 *   can be generated using the findbase.cpp example program, or taken from tables
 *   provided, for example in IEEE-P1363 Annex A
 *
 *   The file common2.ecs is presumed to exist and contain 
 *   {m,A,B,q,x,y,a,b,c} where A and B are parameters of the equation 
 *   above, (x,y) is an initial point on the curve, {m,a,b,c} are the field 
 *   parameters, (b is zero for a trinomial) and q is the order of the 
 *   (x,y) point, itself a large prime. The number of points on the curve is 
 *   cf.q where cf is the "co-factor", normally 2 or 4.
 *
 *   Requires: big.cpp ec2.cpp
 */


#include <iostream>
#include <cstring>
#include <fstream>
#include "ec2.h"

using namespace std;

Miracl precision(200,256);

void strip(char *name)
{ /* strip off filename extension */
    int i;
    for (i=0;name[i]!='\0';i++)
    {
        if (name[i]!='.') continue;
        name[i]='\0';
        break;
    }
}

static Big Hash(ifstream &fp)
{ /* compute hash function */
    char ch,s[20];
    Big h;
    sha sh;
    shs_init(&sh);
    forever 
    { /* read in bytes from message file */
        fp.get(ch);
        if (fp.eof()) break;
        shs_process(&sh,ch);
    }
    shs_hash(&sh,s);
    h=from_binary(20,s);
    return h;
}

int main()
{
    ifstream common("common2.ecs");    /* construct file I/O streams */
    ifstream private_key("private.ecs");
    ifstream message;
    ofstream signature;
    char ifname[50],ofname[50];
    EC2 G;
    Big a2,a6,q,x,y,h,r,s,d,k;
    long seed;
    int m,a,b,c; 
    miracl *mip=&precision;

/* randomise */
    cout << "Enter 9 digit random number seed  = ";
    cin >> seed;
    irand(seed);

/* get common data */

    common >> m;
    mip->IOBASE=16;
    common >> a2 >> a6 >> q >> x >> y;
    mip->IOBASE=10;
    common >> a >> b >> c;

/* calculate r - this can be done off-line,
   and hence amortized to almost nothing    */
    ecurve2(m,a,b,c,a2,a6,FALSE,MR_PROJECTIVE);
    G=EC2(x,y);
    k=rand(q);
    G*=k;            /* see ebrick2.cpp for technique to speed this up */
    G.get(r);
    r%=q;

/* get private key of recipient */
    private_key >> d;

/* get message */
    cout << "file to be signed = " ;
    cin >> ifname;
    strcpy(ofname,ifname);
    strip(ofname);
    strcat(ofname,".ecs");
    message.open(ifname,ios::binary|ios::in); 
    if (!message)
    {
        cout << "Unable to open file " << ifname << "\n";
        return 0;
    }
    h=Hash(message);

/* calculate s */
    k=inverse(k,q);
    s=((h+d*r)*k)%q;
    signature.open(ofname);
    mip->IOBASE=10;
    signature << r << endl;
    signature << s << endl;
    return 0;
}