/*
 *   Elliptic Curve Digital Signature Algorithm (ECDSA)
 *
 *
 *   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=x^3+Ax+B mod p
 *
 *   The file common.ecs is presumed to exist, and to contain the domain
 *   information {p,A,B,q,x,y}, where A and B are curve parameters, (x,y) are
 *   a point of order q, p is the prime modulus, and q is the order of the 
 *   point (x,y). In fact normally q is the prime number of points counted
 *   on the curve. 
 *
 *   Copyright (c) 1997-2005 Shamus Software Ltd.
 */

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

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 void hashing(FILE *fp,big hash)
{ /* compute hash function */
    char h[20];
    int ch;
    sha sh;
    shs_init(&sh);
    while ((ch=fgetc(fp))!=EOF) shs_process(&sh,ch);
    shs_hash(&sh,h);
    bytes_to_big(20,h,hash);
}

int main()
{
    FILE *fp;
    char ifname[50],ofname[50];
    big a,b,p,q,x,y,d,r,s,k,hash;
    epoint *g;
    long seed;
    int bits;
    miracl *mip;
/* get public data */
    fp=fopen("common.ecs","rt");
    if (fp==NULL)
    {
        printf("file common.ecs does not exist\n");
        return 0;
    }
    fscanf(fp,"%d\n",&bits);

    mip=mirsys(bits/4,16);   /* Use Hex internally */
    a=mirvar(0);
    b=mirvar(0);
    p=mirvar(0);
    q=mirvar(0);
    x=mirvar(0);
    y=mirvar(0);
    d=mirvar(0);
    r=mirvar(0);
    s=mirvar(0);
    k=mirvar(0);
    hash=mirvar(0);

    innum(p,fp);     /* modulus        */
    innum(a,fp);     /* curve parameters */
    innum(b,fp);     
    innum(q,fp);     /* order of (x,y) */
    innum(x,fp);     /* (x,y) point on curve of order q */
    innum(y,fp);
    fclose(fp);

/* randomise */
    printf("Enter 9 digit random number seed  = ");
    scanf("%ld",&seed);
    getchar();
    irand(seed);

    ecurve_init(a,b,p,MR_PROJECTIVE);  /* initialise curve */
    g=epoint_init();
    epoint_set(x,y,0,g); /* initialise point of order q */

/* calculate r - this can be done offline, 
   and hence amortized to almost nothing   */
    bigrand(q,k);
    ecurve_mult(k,g,g);      /* see ebrick.c for method to speed this up */
    epoint_get(g,r,r);
    divide(r,q,q);

/* get private key of signer */
    fp=fopen("private.ecs","rt");
    if (fp==NULL)
    {
        printf("file private.ecs does not exist\n");
        return 0;
    }
    innum(d,fp);
    fclose(fp);

/* calculate message digest */
    printf("file to be signed = ");
    gets(ifname);
    strcpy(ofname,ifname);
    strip(ofname);
    strcat(ofname,".ecs");
    if ((fp=fopen(ifname,"rb"))==NULL)
    {
        printf("Unable to open file %s\n",ifname);
        return 0;
    }
    hashing(fp,hash);
    fclose(fp);

/* calculate s */
    xgcd(k,q,k,k,k);

    mad(d,r,hash,q,q,s);
    mad(s,k,k,q,q,s);
    fp=fopen(ofname,"wt");
    otnum(r,fp);
    otnum(s,fp);
    fclose(fp);
    return 0;
}

