PlaidCTF 2011 - Fun with Numb3rs (100 marks)

posted Oct 25, 2011, 7:12 AM by Anthony Lai

(From Darkfloyd and AlanH0)
At the beginning, I put it in IDA Pro, and found out there is a if-then case for displaying the message. However, for better understanI simply reverse the .exe file with .NET reflector (http://reflector.red-gate.com/download.aspx?TreatAsUpdate=1) and find out that there is a condition of balancing two formula, it could display two different message boxes. 

We simply write a simply python to bruteforce those variables. The source code is shown as below:

def formula1(a,b,c):
return ((a + (c*b) - c) + (a*a) * c) - b
def formula2(x,y,z):
return ((z * (34*y + 2*x)) + 7488)
#(a + (c*b) - c) + (a*a) * c) == (z * (34y + 2x)) + 7488) && (x > 77))

for a in range(0,256):
for b in range(0,256):
for c in range(0,256):
v1 = formula1(a,b,c)
v2 = formula2(a,b,c)
if (v1 == v2):
   print str(a) + " " + str(b) + " " + str(c)
 

Finally, we have got the values of variables (89,233,144) and, we exchange the value position and got the key. 

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