Atarude — ASIS CTF Quals 2026

Description

No separate organizer description was preserved in the local task files.

We are given a stripped static-PIE x86-64 ELF that reads commands from stdin in the form e <lane> <hex> and s <lane> <hex>. The binary maintains six lane states, emits long fingerprint values after successful e transitions, and only reaches the final flag logic after a later multi-lane verification step.

Analysis

What was fully proven

• e takes 160 input bytes. Internally the binary derives a 160-byte target buffer on the stack and compares the user data against it.
• A dummy e probe plus a hook at the compare site is enough to capture that target buffer. Resubmitting the captured bytes produces a genuine e success and prints the lane fingerprint.
• Two successful e transitions on the same lane satisfy the marker-length gate that enables s.
• s takes 176 input bytes. After the state checks and anti-replay check, the acceptance condition is derive(X, lane)[160:176] == 0.
• We found a valid family of s solutions: for each lane, XORing the same 16-byte delta into blocks 5 and 6 preserves the zero-check, giving a 128-bit affine family of accepted records.
• The later verifier does not use only the visible fingerprint text. The stored lane records are the submitted s inputs themselves.

What remained unresolved

The final outer verifier hashes the six stored 176-byte lane records and compares the resulting 16-byte value against:

10fe0df1471d48b5226d8b3b9e3559f3

fast_5f.py reproduced that outer hash exactly for our reconstructed records and showed that our valid record set lands on a different stable value:

9ff5c6459a95c5f7346e70377b304785

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