server.c

#include<stdio.h>
#include<string.h>    
#include<sys/socket.h>
#include<arpa/inet.h> 
#include<unistd.h>    
#include"sha1.h"
long long int e=0,n=0;
long long int modexp(long long int x, long long int y)
{
	long long int res=1;
         x = x % n;  
    while (y > 0)
    {
        if (y & 1)
            res = (res*x) % n;
        y = y>>1;
        x = (x*x) % n; 
    }
	if(res>0)
		return res;
	else
		return res+n;
}
int main(int argc , char *argv[])
{
    int socket_desc ,i, client_sock , c , read_size ,r2,r3,r4;
	int flag=1;
    struct sockaddr_in server , client;
    unsigned char client_message[2000];
	char sign[2000];
	char hash[2000];
	int length;
	   SHA1_CTX obj;
	long long int m1, m2, ci[50];
    char server_message[2000];
    socket_desc = socket(AF_INET , SOCK_STREAM , 0);
    if (socket_desc == -1)
    {
        printf("Could not create socket");
    }
    puts("Socket created");
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons( 8888 );

    if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
    {
        perror("bind failed. Error");
        return 1;
    }
    puts("bind done");
    listen(socket_desc , 3);
    puts("Waiting for incoming connections...");
    c = sizeof(struct sockaddr_in);
    client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
    if (client_sock < 0)
    {
        perror("accept failed");
        return 1;
    }
    puts("Connection accepted");
     
    while( ((read_size = recv(client_sock , client_message , 2000 , 0)) >0) && (r2=recv(client_sock , ci , sizeof(ci) , 0)>0) && (r3=recv(client_sock , &e , sizeof(e) , 0)>0) && (r4=recv(client_sock , &n , sizeof(n) , 0)>0))
    {
		flag=1;
		printf("CLIENT MESSAGE\n");
		//strcat(client_message,"a");
		puts(client_message);
		length=strlen(client_message);
		sha1_init(&obj);
		sha1_update(&obj,client_message,length);
		sha1_final(&obj,hash);
	
		for(i=0;i<strlen(hash);i++)
		{
			m1=modexp(ci[i],e);
			m2=(long long int)hash[i];
			if(m2<0)
				m2=m2+n;
			if(m1!=m2)
			{
				flag=0; 
				break;
			}
		}
		if(flag)
			strcpy(server_message,"Authentication successful!");
		else
			strcpy(server_message,"Authentication failed!");
		n=0; e=0;
		//strcpy(server_message,"");
        write(client_sock , server_message , strlen(server_message));
	
    }
     
    if(read_size == 0)
    {
        puts("Client disconnected");
        fflush(stdout);
    }
    else if(read_size == -1)
    {
        perror("recv failed");
    }
     
    return 0;
}