#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<unistd.h>
#include<netdb.h>
#include<fcntl.h>
#include "feistel.c"

#define SETLOCALINFO bind
#define SETPEERINFO connect

#define LISTENPORT 40004
#define KEYSIZE 128
#define NSIZE 32

void readkey(char *keyname, unsigned char key[KEYSIZE])
{
  int infd, n;
  char filename[NSIZE+4];
  sprintf(filename,"%s.key",keyname);
  infd = open(filename,O_RDONLY,0);
  n = 0;
  while (n<KEYSIZE) n+=read(infd,key,KEYSIZE-n);
  close(infd);
}//readkey

void savenewkey(char* id, unsigned char oldkey[KEYSIZE], unsigned char newkey[KEYSIZE])
    {
      int outfd;
      char filename[NSIZE+8];
      sprintf(filename,"%s.prekey",id);
      outfd = open(filename,O_WRONLY | O_CREAT | O_TRUNC,0755);
      write(outfd,oldkey,KEYSIZE);
      close(outfd);
      sprintf(filename,"%s.key",id);
      outfd = open(filename,O_WRONLY | O_CREAT | O_TRUNC,0755);
      write(outfd,newkey,KEYSIZE);
      close(outfd);
    }//savenew key

int main(int argc, char** argv)  
{
  int s = (int)(long)&main;
  if (argc>1) s = atoi(argv[1]);
  srand(s); // seed random number generator
  printf("RKS (C) server started, using random seed %d\n",s);
  int sockfd, i, x, y,result, mlen;
  unsigned int plen;
  unsigned char Aname[NSIZE];
  unsigned char AK[KEYSIZE];
  unsigned char newAK[KEYSIZE];
  unsigned char outbuf[KEYSIZE+NSIZE];
  unsigned char inbuf[NSIZE];
  unsigned char nbuf[NSIZE];
  struct sockaddr_in peeraddr;  // client addr
  struct sockaddr_in myaddr;    // "sever" addr
  struct sockaddr_in caddr;     // copy of original client addr
  sockfd = socket(AF_INET,SOCK_DGRAM,0); // gets udp over ipv4 socket
  myaddr.sin_family = AF_INET;
  myaddr.sin_addr.s_addr = htonl(INADDR_ANY); // accept from any source ip
  myaddr.sin_port = htons(LISTENPORT);
  plen = sizeof(peeraddr);  mlen = sizeof(myaddr);
  bind(sockfd,(struct sockaddr *)&myaddr,mlen);  // bind still needed.

  // Can't receive packets until other side is ready:
  //printf("press a key to begin: "); getchar();
  result = recvfrom(sockfd,Aname,NSIZE,0,(struct sockaddr*)&peeraddr,&plen);
  printf("RKS (C) request from %s at %s\n",Aname,inet_ntoa(peeraddr.sin_addr));
  memcpy(&caddr,&peeraddr,plen); // save a copy of client address
  readkey(Aname,AK);
  genkey(newAK);
  keycrypt(AK,newAK,KEYSIZE,outbuf,0); // AK(newAK)
  keycrypt(AK,Aname,NSIZE,outbuf+KEYSIZE,0); // AK(Aname);
  result = sendto(sockfd,outbuf,KEYSIZE+NSIZE,0,(struct sockaddr*)&peeraddr,plen); // send back to peeraddr as determined by receive

  do {
    result = recvfrom(sockfd,nbuf,NSIZE,0,(struct sockaddr*)&peeraddr,&plen);
    x = peeraddr.sin_addr.s_addr == caddr.sin_addr.s_addr;
    y = peeraddr.sin_port == caddr.sin_port;
  } while (!x || !y); // why need?
  keycrypt(newAK,nbuf,NSIZE,inbuf,1); // decrypt newAK(Aname);
  result = strcmp(Aname,inbuf);
  if (result==0) // name same
    {
      savenewkey(Aname,AK,newAK);
      printf("new key for %s saved\n",Aname);
    }
  else printf("decrypted client name  %s not authentic\n",inbuf);
  close(sockfd); // only have local meaning - no packets exchanged.
  return 0;
}//main
// goto https://www.gta.ufrj.br/ensino/eel878/sockets/sockaddr_inman.html
// for info on sockaddr_in contents