Elite 2

Elite 2

Author: Marin Radu

Description

Decipher the correct hexadecimal input by understanding the XOR operation performed by the program.

Requirements

• Decompiled source code
• XOR
• Understanding a C program

Solve

Firstly, you should use ghidra or another software to analyze the binary and try to reconstruct the C code.

To reverse engineer the given C program, you need to understand the following steps:

• User Input: The program prompts the user to enter a hexadecimal number.
  The %x format specifier is used with scanf to read the user's input as a hexadecimal integer and store it in the input variable.
• XOR Operation: The program XORs the user's input (stored in input) with the hexadecimal value 0x10101010
  and stores the result in a variable named new.
• Comparison: The program checks if the value stored in new is equal to 0x499602D2.
  If the equality condition is met, it prints a message indicating success.
  Otherwise, it prints a message suggesting that the user should try again.
• Invalid Input: If the user enters invalid input (e.g., non-hexadecimal characters or an invalid format),
  the program prints an "Invalid input" message.

To reverse engineer the program, you would need to determine what input (in hexadecimal format)
produces the result 0x499602D2 when XOR-ed with 0x10101010.
You can calculate this in C by performing the reverse XOR operation:

int correctInput = 0x499602D2 ^ 0x10101010;

You can check it using python as well:

correctInput = 0x499602D2 ^ 0x10101010
print(hex(correct_input))

The value of correctInput is 0x490c23c0.
So, when the user enters 0x490c23c0 as input, the program will XOR it with 0x10101010,
resulting in 0x499602D2, and the success message will be printed.

In summary, you reverse engineer the program by calculating the input that,
when XOR-ed with 0x10101010, produces the expected result 0x499602D2.

Flag: CSCTF{0x490c23c0}