/*
 * Copyright (C) Sergey P. Derevyago, 2005.
 *
 * Permission to copy, use, modify, sell and distribute this software is granted
 * provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied warranty, and
 * with no claim as to its suitability for any purpose.
 *
 */

import java.io.BufferedReader;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.Random;

public class KeyGen
{
 public static void main(String[] args) throws Exception
 {
  if (args.length!=1) {
     System.out.println("Invalid args. key_file is missing");
     return;
  }

  if ((new File(args[0])).exists()) {
     System.out.println(args[0]+" file already exists");
     return;
  }

  FileOutputStream fout=new FileOutputStream(args[0]);
  BufferedReader in=new BufferedReader(new InputStreamReader(System.in));

  int bitKeyLen=readInt(in, "Enter key length in bits", 128, 512);
  int byteKeyLen=(bitKeyLen+7)/8;

  int maxVal=readInt(in, "Enter maximum value", 2, 65536);
  BigInteger biMaxVal=BigInteger.valueOf(maxVal);

  int n=1;
  for (BigInteger a=biMaxVal, end=BigInteger.ONE.shiftLeft(bitKeyLen-1);
       a.compareTo(end)<0; a=a.multiply(biMaxVal))
      n++;

  System.out.println();

  BigInteger keyVal=BigInteger.ZERO;
  for (int i=1; i<=n; i++) {
      int val=readInt(in, "["+i+" of "+n+"] enter value", 1, maxVal);
      keyVal=keyVal.multiply(biMaxVal).add(BigInteger.valueOf(val-1));
  }
  keyVal=keyVal.multiply(BigInteger.valueOf(2)).add(BigInteger.ONE);
  
  System.out.println();

  byte[] bytes=keyVal.toByteArray(), key=new byte[byteKeyLen];
  if (bytes.length<byteKeyLen)
     System.arraycopy(bytes, 0, key, byteKeyLen-bytes.length, bytes.length);
  else System.arraycopy(bytes, bytes.length-byteKeyLen, key, 0, byteKeyLen);

  int msbMask=(bitKeyLen%8!=0) ? 0xff>>(8-bitKeyLen%8) : 0xff;
  key[0]&=msbMask;

  if (key[0]==0) {
     for (Random rnd=new Random(); key[0]==0; )
         key[0]=(byte)(rnd.nextInt(256)&msbMask);

     System.out.println("The most significant byte was changed from 0 to "+
       Integer.toHexString(key[0]&0xff));
  }

  fout.write(key);
  fout.close();

  System.out.println(args[0]+" file created!");
 }

 public static int readInt(BufferedReader in, String msg, int minVal,
   int maxVal) throws Exception
 {
  for (;;) {
      System.out.print(msg+" ("+minVal+"-"+maxVal+"): ");

      try {
          int val=Integer.parseInt(in.readLine());
          if (val>=minVal && val<=maxVal) return val;
      }
      catch (NumberFormatException ignore) {}

      System.out.println("  Error! Reenter value from "+minVal+" to "+maxVal);
  }
 }
}