Шифратор текста, работа с файлами.

program cezar;
const
 CSize=1024;
 CExt = '.x';
var
 f,h:file;
 i,r,wr:longint;
 buf:array[1..CSize]of byte;
 s,d:string;
 x:byte;
 enc:boolean;
 
 function ChangeFileExt(oldname,newext:string):string;
 var
  i,n:integer;
  s:string;
 begin
 s:='';
 for i:=1 to length(newext) do
  if not (newext[i] in ['\','/',':','*','?','"','<','>','|']) then s:=s+newext[i];
 newext:=s;
 if newext[1]<>'.' then newext:='.'+newext;
 for i:=length(oldname) downto 1 do
  begin
  if oldname[i]='.' then
   begin
   n:=i-1;
   break;
   end else
    if oldname[i]='\' then
     begin
     n:=-1;
     break;
     end;
  end;
 if n > 0 then
  begin
  oldname:=copy(oldname,1,n);
  ChangeFileExt:=oldname+newext;
  end else
   begin
   if oldname[length(oldname)]='\' then
    begin
    writeln('I/O Error: this is no file!');
    ChangeFileExt:='';
    end else
     begin
     ChangeFileExt:=oldname+newext;
     end;
   end;
 end;
 
begin
writeln;
write('Input number from range 1..255: ');
readln(x);
writeln;
writeln('For encode file - type symbol "1" and press [Enter].');
writeln('For decode file - type symbol "2" and press [Enter].');
writeln('For exit - type other symbol and press [Enter].');
readln(d);
write('Input path to input file: ');
readln(s);
if d[1]='1' then
 begin
 //encode
 d:=ChangeFileExt(s,CExt);
 enc:=true;
 end else
  if d[1]='2' then
   begin
   //decode
   write('Input path to output file: ');
   readln(d);
   enc:=false;
   end else halt;
assign(f,s);
reset(f,1);
assign(h,d);
rewrite(h,1);
while not eof(f) do
 begin
 blockread(f,buf,CSize,r);
 for i:=1 to r do
  begin
  if enc then buf[i]:=buf[i] + x else buf[i]:=buf[i] - x;
  end;
 blockwrite(h,buf,r,wr);
 end;
close(f);
close(h);
writeln;
writeln('Press [Enter] for exit.');
readln;
end.