for i:=0 to razmer do
  begin
    BlockRead(myFile, oneByte, 1);
    oneByte:= oneByte+c;
    seek(myfile,i);
    Blockwrite(cryptfile,oneByte,1);  
    seek(cryptfile,i); 
  end;

for i:=0 to razmer do
  begin
    seek(myfile,i);
    BlockRead(myFile, oneByte, 1);
    oneByte:= oneByte+c;
    seek(cryptfile,i); 
    Blockwrite(cryptfile,oneByte,1);  
  end;