THE GARDEN

build-context

Updated 20 Sep 2026

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  Attribution

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    Solved by Fauzi Ismail 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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    Reconstruct release material from CI and DSSE provenance, derive the vault key, and decode the release vault.

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  Approach

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    Decode the DSSE payload, then read the subject digest, builder ID, and release channel. Read the build ID from ci_trace.log.

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    Take the first 20 digest characters and the last path component of the builder ID as the runner name.

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    Serialize build_id:digest_prefix:runner_name:release_channel exactly. Hash the UTF-8 bytes with SHA-256 and repeat that digest as the XOR keystream.

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    The byte equation is $p_i=c_i\oplus\operatorname{SHA256}(M)_{i\bmod32}$.

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  Solution

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    import base64, hashlib, json
    from pathlib import Path
    
    root = Path(".")
    envelope = json.loads((root / "provenance/release_attestation.dsse.json").read_text())
    payload = envelope["payload"] + "=" * (-len(envelope["payload"]) % 4)
    stmt = json.loads(base64.urlsafe_b64decode(payload))
    
    digest = stmt["subject"][0]["digest"]["sha256"]
    predicate = stmt["predicate"]
    builder = predicate["runDetails"]["builder"]["id"]
    channel = predicate["buildDefinition"]["externalParameters"]["release_channel"]
    build_id = next(line.split("build_id=", 1)[1].strip()
                    for line in (root / "ci_trace.log").read_text().splitlines()
                    if "build_id=" in line)
    
    material = f"{build_id}:{digest[:20]}:{builder.rstrip('/').split('/')[-1]}:{channel}"
    key = hashlib.sha256(material.encode()).digest()
    ct = (root / "dist/release.vault").read_bytes()
    print(bytes(c ^ key[i % 32] for i, c in enumerate(ct)).decode())
    

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  Verification

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    The document gives material KMSB-DEVOPS-2026-09:fbe870d85dd28066cc25:devops-release-07:final-onsite. The original vault file was not separately attached for replay.

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  Concepts

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    DSSE payload decoding; provenance metadata; exact serialization; SHA-256 key derivation; repeating-key XOR.

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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. 64–67.

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    Flag: KMIPN8{devops_release_vault_needs_provenance}.

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