CrackMe Created By Rayko

Objective

The objective of this (.net) challenge is to crack the algorithm, generate a valid password and make a simple keygen.

De-compiling

Let’s use dnSpy to decompile the program. Now we are looking for btnCheck_Click():

private void btnCheck_Click(object sender, EventArgs e)
{
 string input = Strings.Trim(this.txtName.Text);
 string right = Conversions.ToString(this.Encrypt(input));
 string left = Strings.Trim(this.txtSerial.Text);
 bool flag = Operators.CompareString(left, right, false) == 0;
 if (flag)
 {
    Interaction.MsgBox("You Put In The Right Serial Number", MsgBoxStyle.OkOnly, null);
 }
 else
 {
    Interaction.MsgBox("Try Again", MsgBoxStyle.OkOnly, null);
 }
}

Encryption Function

We found that the name is encrypted using Encrypt(input).

Now let’s dig into this function:

private int Encrypt(string Input)
{
    int num = 0;
    checked {

    int num2 = Input.Length - 1;
    int num3 = num;
    int num6;
    for (;;) {

      int num4 = num3;
      int num5 = num2;
      if (num4 > num5){
        break;
      }
      char @string = Conversions.ToChar(Input.Substring(num3));
      num6 = (int)Math.Round(unchecked(Conversions.ToDouble(Conversion.Oct(Strings.Asc(Conversions.ToString(num6))) + Conversion.Oct(Strings.Asc(@string))) + 666.0));
      num3++;
    }

      return num6;
  } 
}

There are too many conversions. Let’s divide this algorithm into 2 parts:

First part

num6 = 0
Conversion.Oct(Strings.Asc(Conversions.ToString(num6)))
 = Conversion.Oct(Strings.Asc(Conversions.ToString(0)))
 = Conversion.Oct(Strings.Asc("0"))
 = Conversion.Oct(48) = "60"

1) Num6 = 0 in decimal and we will convert it to string so
Note:

Conversions.ToString()      //only converts the first digit

2) Num6 = ‘0’ the ascii code of it is (48)

3) Convert it to octal ==> oct(48) = 60
Note:

Conversion.Oct()    //Returns a string representing the octal value of a number.

4) So, the first part of serial is : “60”

Second part

(i.e:name = “A”)

Conversion.Oct(Strings.Asc(@string)) 
 = Conversion.Oct(Strings.Asc('A')) 
 = Conversion.Oct(65) 
 = "101"

1)@string = ‘A’ and we will convert it to ASCII Ascii code is (65)

2)We convert it to octal oct(65) = “101”

• Finally: (put them together)

Conversions.ToDouble("60"+ "101") + 666 
 = Conversions.ToDouble( "60101") + 666 
 = 60101 + 666 
 = 60767            //Valid password

Boo0M

The password is: 60767

Keygen

Written in Python

name = str(input("Enter name:"))
num6 = '0'

for i in range(0, len(name)):
    #First part
    num6 = str(oct(ord(str(num6[0]))))    #string ==> ascii ==> octal ==>string
    
    #Second part
    strr = str(oct(ord(name[i])))         #ascii ==> octal ==> string
    password = int(num6[2:] + strr[2:]) + 666

    #Generating password
    num6 = str(password)

print(password)