THE GARDEN

report-archive

Updated 20 Sep 2026

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  Attribution

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    Solved by Muhammad Dhafin Ramadhan during the competition. These are rn’s study notes based on the teammate’s documented solve, not an independent rn solve.

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  What it asked

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    Recover an archived record omitted from the public index and decode its protected body from the supplied artifacts.

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  Approach

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    access_trace.log identified hidden record rid=4109; archive_store.json supplied its role, encoding, and ciphertext.

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    The client code defines key_material = role + ':' + rid + ':journal-rights', hashes it with SHA-256, then repeats the 32-byte digest as an XOR keystream.

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    For ciphertext byte $c_i$ and digest byte $k_{i \bmod 32}$, plaintext is $p_i=c_i\oplus k_{i \bmod 32}$.

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  Solution

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    import hashlib
    
    rid = 4109
    role = "reviewer"
    ct = bytes.fromhex(
        "81b092e81dedcd3bef60630e3924190c5a41f36384e2ea7f"
        "f8cdc5c92f266320b88bbeca0ca6df3ef85c680f2a222d0d4e"
    )
    material = f"{role}:{rid}:journal-rights".encode()
    key = hashlib.sha256(material).digest()
    print(bytes(c ^ key[i % len(key)] for i, c in enumerate(ct)).decode())
    

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  Verification

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    The supplied values decrypt locally to the recorded flag.

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  Concepts

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    Artifact correlation; hidden backend records; repeating-key XOR; SHA-256 key derivation.

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  Source

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    Source: team document KMIPNVIII-unintended.docx, titled “Writeup unintended KMIPN 8 Final”. Page numbers refer to the rendered document. Rendered pp. 8–12.

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    Flag: KMIPN8{archive_rights_require_server_side_checks}.

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