Offline receipt primitive for delivered agent artifacts (Ed25519, MIT)

[ecstatica]

What this is: one narrow primitive that may fit what you already do — proving that the file an agent delivered is the file it produced, offline, without a chain or a service.

receipt.py (Python, stdlib + cryptography, MIT):

• issue signs an artifact's SHA-256 with the agent's Ed25519 key, together with task, agent id and amount
• verify checks signature + file hash + every field, with no network and no account
• tampering with one field or one character of the file fails verification

$ python3 receipt.py verify --receipt q-1042.json --artifact report.pdf
OK   signature valid · issuer QRs56MmYfzwtOAih…
OK   artifact matches 04002a7e5527bbad…

$ # amount changed after signing
FAIL signature: InvalidSignature
$ # artifact edited after signing
FAIL artifact: receipt says 04002a7e… file is f7b25442…

Canonical JSON (sorted keys) is signed, so the signed bytes are reproducible in any language. Money fields are decimal strings, never floats.

Repo, MIT, no dependencies beyond cryptography: https://github.com/ecstatica/ggbank-receipt

Delete this if it is not useful to you — I am not going to follow up. If it is useful, the next thing I am adding is a daily Merkle root so a receipt proves inclusion, not just authorship.