THE GARDEN

provenance-gatekeeper

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 the deterministic release token expected by a stripped gatekeeper binary from its CI trace and DSSE attestation.

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  Approach

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    The CI trace disclosed both the field order and algorithm. The DSSE statement confirmed builder ID, invocation ID, subject digest, and release branch.

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    Serialize $M=\text{builder}\parallel\texttt{:}\parallel\text{invocation}\parallel\texttt{:}\parallel\text{digest}\parallel\texttt{:}\parallel\text{branch}$, then take the first 32 hexadecimal characters of $\operatorname{SHA256}(M)$.

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    No brute force is needed; the token is deterministic and every input is present in the supplied evidence.

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  Solution

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    import hashlib
    
    fields = [
        "https://github.com/pnup-kamsiber/builder@v4",
        "run-2026-09-06-ctfd-7f31",
        "3e78febe430f86e8081b64a8903015e8ef30985cb9343628f7731dc09f028c93",
        "refs/heads/release/national-final",
    ]
    full_hash = hashlib.sha256(":".join(fields).encode()).hexdigest()
    token = full_hash[:32]
    assert token == "4206b8df935dfd79f6401f280fa8d821"
    print(token)
    

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  Verification

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    The token recomputes locally and matches the successful gatekeeper transcript in the document.

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  Concepts

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    DSSE and in-toto provenance; deterministic token derivation; binary validation paths; supply-chain metadata exposure.

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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. 68–74.

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    Flag: KMIPN8{provenance_token_unlocks_release_gate}.

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