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Challenge 33: (70 Points) Reverse engineer challen...

Created on: October 9, 2026

Answered using GPT-5.6 Thinking by Chat01

Question

challenge3.exe

Challenge 33: (70 Points)
Reverse engineer challenge3.exe on the target machine with the IP address 172.25.65.103 and identify the standard algorithm implemented by the membership validation function.(Answer Format: Xxxx Xxxxxxxxx)
Answer Text
Challenge 34: (30 Points)
Analyze the fiile challenge3.exe and determine the hexadecimal value used as the divisor in its final checksum validation on the machine with the IP address 172.25.65.103.(Answer Format: NxNX)

Answer

I’ll analyze the uploaded challenge3.exe statically first, focusing on the membership-validation routine and the final checksum comparison. I’ll avoid executing the binary unless static inspection leaves something unresolved.

Thought

Inspected executable metadata, strings, UPX, and binary analysis tools

Answer

The executable is a 64-bit Windows PE packed with UPX, so its meaningful code is compressed. I’ve confirmed the packing signature; next I’m unpacking or reconstructing the code section, then I’ll trace the validation math.

Thought

Decoding bit logic

I’m interpreting getbit as returning the buffer’s top bit while maintaining a sentinel. The main loop appears to build a value from one bit, then read a second bit as a continuation flag; a set flag ends that phase.

Thought

Analyzed UPX-packed binary structure, decompression, strings, and validation logic

Worked for 1m 46s

Answer

Challenge 33: Luhn Algorithm
Challenge 34: 0x0A

The validation routine doubles alternating digits from the right, subtracts 9 when the result exceeds 9, sums the digits, then checks whether the checksum is divisible by 10—the standard Luhn Algorithm.

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