SecureArtisan

My Road to Digital Forensics Excellence

Solution

Posted by Paul Bobby on July 10, 2009

The solution involves basic algebra.

The crackme essentially performs two functions: one against the userid and one against the serial number. If the results of these functions match, then we have a winner.

Function against Userid

  • (c3+c6) * (c3+uc)

Function against Serial Number

  • ((c3*c3)+(c1*c3))+c2

Therefore

  • ((c3*c3)+(c1*c3))+c2 = (c3+c6) * (c3+uc)
  • c3^2 + c1.c3 + c2 = c3^2 + c3.uc +c3.c6 + c6.uc
  • Divide by c3^2
  • c1.c3 + c2 = c3(uc+c6) + c6.uc
  • c1 = uc + c6
  • c2 = c6 * uc

Now that’s something we can handle pretty easily since we know how to calculate the uc and c6 complex numbers using just the Userid.

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