#include<stdio.h>
#include<string.h>
int c;
char i,k,l,nams[20],namd[20],key[20];
FILE *fds,*fdd;
main()
{ printf("\nplease input the key!\n");
  scanf("%s",key);
  l=strlen(key);
  printf("\nplease input the source file name\n");
  scanf("%s",nams);
  printf("\nplease input the dest file name\n");
  scanf("%s",namd);
  fds=fopen(nams,"r");
  fdd=fopen(namd,"w");
  for(c=fgetc(fds);c!=EOF;fputc(c,fdd),c=fgetc(fds))
  { if((c>='a')&&(c<='z'))
    { c+=key[k]-'a';
      if(c>'z') c-=26;
      if(c<'a') c+=26;
      k++;
      k%=l;
    }
    if((c>='A')&&(c<='Z'))
    { c+=key[k]-'a';
      if(c>'Z') c-=26;
      if(c<'A') c+=26;
      k++;
      k%=l;
    }
  }
  fclose(fds);
  fclose(fdd);
}