RowdyHacks XII: The Gatekeeper

Challenge

Reversing · 200 points. A stripped x86-64 ELF prompts for a 24-byte passphrase and grants or denies access.

Static analysis

Disassembly around 0x40125d shows the check runs per byte. For each index i, the program computes:

((input[i] + 3*i) ^ 0x5a) & 0xff

and compares it to a 24-byte table at virtual address 0x402010 (file offset 0x2010 in this mapping). Because every operation is invertible and position is the only state, the passphrase can be solved one byte at a time:

input[i] = ((table[i] ^ 0x5a) - 3*i) & 0xff

Solution

Inverting the transform over the 24 table bytes reconstructs the passphrase, which the binary then accepts with access granted. nice key.

A strings dump also shows flag{left_over_from_v0_9}, a decoy left over from an earlier build. The reliable path is the byte comparison, not the loose string, which is a good reminder to verify where a candidate actually comes from before trusting it.

Takeaway

A secret checked entirely inside a client is already shipped: every invertible transform, here add, XOR, and compare against a table, runs backwards, so the passphrase falls out byte by byte. The only real control is to move the check server-side against a value the user never receives. A local check raises effort but protects nothing, and a decoy string left in the binary protects less.

Flag

flag{it_compiles_though}