128 bit hex keygen - vb.net

Question

So my professor gave me a challenge to build a decoder that could break his special formula. It was described to be 32 characters in length, alphanumeric numeric when entered but then "it has a system... the first 106 bits must be 50% 1's and the rest 0's, the remaining 22 bits are basically a hash of the previous bits so that the key can be checked..." were his exact words. Sounds to me like a 128 bit encryption with a twist. I found the below but I need VB2010 or VS2010, this says php.
<?php
function string_random($characters, $length)
{
$string = '';
for ($max = mb_strlen($characters) - 1, $i = 0; $i < $length; ++ $i)
{
$string .= mb_substr($characters, mt_rand(0, $max), 1);
}
return $string;
}
// 128 bits is 16 bytes; 2 hex digits to represent each byte
$random_128_bit_hex = string_random('0123456789abcdef', 32);
// $random_128_bit_hex might be: '4374e7bb02ae5d5bc6d0d85af78aa2ce'
Would that work? Or does it need converting? Please help. Oh and thank you :)
I wasn't promised extra credit but either way I would like to surprise him.

Answer 1

So the first 106 bit are 26 character and the first half of the 27.
You have first of all encode somehow the number of 0 and 1, while building the string you need to keep an eye to the number. An idea would be to build a map like this:
0 = 0000 = -4
1 = 0001 = -2
2 = 0010 = -2
3 = 0011 = 0
4 = -2
5 = 0
6 = 0
7 = +2
8 = -2
9 = 0
a = 0
b = +2
c = 0
d = +2
e = +2
f = +4
then everytime you extract a new random number you check the number associated to it and add it to a variable
balanceOfOneAndZero
your objective is have balanceOfOneAndZero = 0 when you hit your 27th character.
to do that you need a control function, that takes current balanceOfOneAndZero, the proposed character proposedChar, and current string lenght currLenght.
Would be better to split the problem into two part. First is reaching the 26th character of the sequence with balanceOfOneAndZero between -2 and 2. Any other value is not acceptable, because your 27th character can have maximum two 1 or two 0 to completely balance the first 106 characters.
so your function should do something like (I'll write in sort of pseudo code since I don't have an IDE right now)
function checkNextLetter(Dim balanceOfOneAndZero As Integer, Dim proposedChar As Char,
Dim currentLenght as Integer) As Boolean
If( ((26 - currentLenght - 1) * 4 + 2) < MOD(Map.ValueOf(proposedChar) + balanceOfOneAndZero) ) Then
Return true
Else
Return false
ENd If
End function
This function basically check if accepting the new character will still make possible to Balance the number of 0 and 1 before the 26th character.
So your main function should have a loop every time it propose a new character, something like
proposedChar = new RandomChar
While (Not checkNextLetter(balanceOfOneAndZero, proposedChar, len(currentString))
proposedChar = new RandomChar
End While
currentString = currentString & proposedChar
this only until you hit the 26th character.
Than you have to check balanceOfOneAndZero, if its 2 you add a character that begin with 00, if it's 0 you can either have 10 or 01, if it's -2 you have to add a character that begin with 11.
After this I can't help you about the rest 22 character, since there are not enough information. You could brute force the rest
EDIT:
so to brute force the rest (il start from when you reach the 26th character):
Dim stringa1, stringa2, stringa3, stringa4 As String
If balanceOfOneAndZero = 2 Then
stringa1 = currentString & '0'
stringa2 = currentString & '1'
stringa3 = currentString & '2'
stringa4 = currentString & '3'
ELse If balanceOfOneAndZero = 0 Then
stringa1 = currentString & '4'
stringa2 = currentString & '5'
stringa3 = currentString & '6'
stringa4 = currentString & '7'
Else
stringa1 = currentString & 'c'
stringa2 = currentString & 'd'
stringa3 = currentString & 'e'
stringa4 = currentString & 'f'
End if
Function GenerateAllCombination(ByVal iLenght As Integer)
Dim arrayLista As New List(Of String)()
Dim arraySubLista As New List(Of String)()
If (iLenght > 1) Then
arraySubLista = GenerateAllCombination(iLenght -1)
for each objString As String in arraySubLista
for each ele As String in arrayValori
arrayLista.add(objString & ele)
loop
loop
Else
for each ele As String in arrayValori
arrayLista.add(ele)
loop
End If
End Function
Now if you use generateAllCombination you will have a List of string with ALL the combination of 5 character.
Now you just create 4 list by concatenating those combination with your string1 to string4 (string1 & combination) etc..
put all those result on a List of string, and you have 100% that at least ONE of the string will break your teacher code
I forgot, arrayValori must be a List with all values from "0" to "f"