Receipt trust and offline signatures

A receipt records a claim about an evaluation. Interpret its evidence at separate levels:

Outcome	What it establishes
hash_valid	The payload matches its attached hash. Someone can forge both; compare against an independently trusted expected hash when tamper detection matters.
signature_valid	A pinned public key verifies the signed bytes. Without a known key this remains unknown.
signer_trusted	The key belongs to the configured signer, is not revoked, and meets local validity dates.
protocol_bound	The valid signature covers a declared protocol ID, version, and content digest.
protocol_recognized	That signed digest matches the verifier's independently supplied expected digest.
execution_verified	Always false here: signatures do not prove execution or protocol conformance.
science_valid	Unknown: signatures do not establish dataset truth, lack of contamination, or metric correctness.

An unsigned receipt can be internally consistent even when its metrics are fabricated. Its evidence level is integrity_only. A trusted signature raises this to authenticated_signer_statement; scientific conclusions still require independent evidence. The compatibility field valid means the requested checks passed, not that every scientific claim is true.

Create a local signing key

Ed25519 is optional; verification needs no account, remote registry, or internet access:

python -m pip install -e '.[signing]'

Run this Python snippet locally once. It writes a private PEM and a public trust store. Keep the private key in the lab's controlled environment.

import base64
from pathlib import Path
import yaml
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
from cryptography.hazmat.primitives.serialization import Encoding, NoEncryption, PrivateFormat, PublicFormat

key = Ed25519PrivateKey.generate()
with Path("lab-private.pem").open("xb") as handle:
    handle.write(key.private_bytes(Encoding.PEM, PrivateFormat.PKCS8, NoEncryption()))
public = base64.b64encode(key.public_key().public_bytes(Encoding.Raw, PublicFormat.Raw)).decode()
store = {
    "schema_version": "receipt-trust-store/v1",
    "keys": {"lab-a-2026": {"signer_id": "lab-a", "public_key": public, "revoked": False}},
}
Path("lab-public-trust.yaml").write_text(yaml.safe_dump(store), encoding="utf-8")

Recipients must authenticate the lab's public key through an independent channel before pinning it. A key supplied alongside an untrusted receipt does not establish the sender's identity. PrivateLabBench never trusts an embedded public key or downloads a key automatically.

Sign an approved aggregate receipt

Create a normal local evaluation first. Save the reviewed config's release mapping as a standalone release.yaml policy, without the outer release key.

plb sign reports/tabular_demo_receipt.json --policy release.yaml --private-key lab-private.pem --signer-id lab-a --key-id lab-a-2026 --out reports/lab-a.signed.shareable.json

Signing passes through the same disclosure boundary as other exports. A failing policy creates no signed shareable file, removes stale output, and preserves the local source. The private-key path and bytes are never included. Existing HMAC-signed local sources can use --signing-secret.

The signed signer-profile/v1 statement records the signer/key ID, signing package/Python version, and optional public model/protocol identities. These are declarations by the signer, bound to the canonical payload including released metrics and the release decision. A signing runtime declaration does not attest which code originally computed imported metrics.

To bind a public protocol, supply all of --protocol-id, --protocol-version, and --protocol-sha256. Compute the digest from the exact public protocol bytes both parties agree on. Optional --model-id and --model-revision bind the public model identity. Local dataset hashes, paths, and identifying commitments remain local.

Verify independently and offline

plb verify reports/lab-a.signed.shareable.json --trust-store lab-public-trust.yaml --require-trusted-signer --json

Add --expected-protocol-sha256 <known-digest> to require a particular signed protocol. A missing/different binding fails that requirement. JSON names each outcome independently and includes limitations. The CLI exits nonzero for failed integrity, bad signatures, revoked/expired/mismatched pinned identities, or unmet trust/protocol requirements.

An Ed25519 receipt without a trust store may still report internally consistent bytes; it is never labeled authenticated. Use --require-trusted-signer in cross-institution workflows. Legacy unsigned v1 receipts remain readable at the limited integrity level.

Rotation and revocation

Give a rotated key a new key_id; keep earlier public keys when historical receipts must remain verifiable. Records may include timezone-aware valid_from and expires_at, such as 2026-01-01T00:00:00Z. Verification checks validity at the current time. Mark compromised keys revoked: true and distribute the updated trust store through the authenticated channel. Offline recipients need the updated store; no central revocation service is assumed.

HMAC remains supported for bilateral local workflows, with evidence level shared_secret_integrity. Anyone with the symmetric secret can create a signature; it provides no independently verifiable public signer identity and cannot satisfy --require-trusted-signer.