The Challenge

The binary has a hardcoded encrypted flag and a hardcoded key. The encryption is a custom byte-level operation — not standard XOR but a subtraction: key[i] - flag[i] produces the plaintext character.

Approach

Loading the binary in Ghidra or reading the decompiled output reveals two byte arrays: the ciphertext (flag) and the key (key). The encryption is enc[i] = key[i] - plain[i], so decryption is plain[i] = key[i] - enc[i] — simple arithmetic reversal, no unknown values.

Both arrays are hardcoded, so no network interaction needed. Just read the values directly from the script.

Solution

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from pwn import xor

flag = b"\x57WTEZfXhBBWVTPPhjUeuPCVW"
key = bytes.fromhex("BD C3 B5 AC D5 D9 CD DB B5  B7 C9 E8 B5 BD 81 C7 D6 89 C4 DB BC 77 DD D4".replace(" ",""))
print(len(flag), len(key))
for i in range(len(flag)):
    print(chr(key[i] - flag[i]), end="")

Each character of the flag is key[i] - flag[i] cast back to a printable character. The loop runs over every byte and prints the result directly.

What I Learned

Static analysis challenges live and die on recognizing the operation. Once you see subtraction instead of XOR in the disassembly, the inversion is trivial arithmetic. The tool (pwn.xor) is imported but not used here — the actual operation is manual subtraction, showing it pays to read the decompilation carefully before defaulting to XOR.