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Keyless signing: short-lived keys from an OIDC identity

Design for oxygenhub#241, written 2026-10-17. Status: accepted (Andy, 2026-10-17, the four decisions below as recommended). Nothing here is built yet. It builds on the hub's OIDC issuer (hub spec §13.5) and on the transparency log (docs/transparency.md, #238/#240).

Why

Every signing key in Oxygen lives long: a runner's runner.key, an agent's key in its home (~/.oxy-agents/claude/keys), a person's key. A delegation can already expire in minutes (oxy identity delegate add --expires 30m), but the key it names stays on disk afterwards, and each grant re-issues the owner's identity by hand. A stolen key works until someone notices and revokes it.

Sigstore's answer (Fulcio) is a key that lives only for one job: the signer proves who it is with an OIDC token, gets a certificate for a fresh key valid for minutes, signs, and throws the key away. Nothing long-lived sits on the runner or in the agent's home, and a policy can name who may sign ("this pipeline on main", "Claude, for andymccall") rather than which key.

What exists already

  • Delegation (spec §14.2, §14.5): an identity lists delegates with a scope (changes, refs, seal, all) and an expiry, checked against the time the signed object claims. The identity issues it itself; no other key can.
  • The hub's OIDC issuer (hub spec §13.5, oidc.rs): a Flow job that declares identity "AUD" gets an RS256 token with sub repo:OWNER/NAME:bookmark:B, the environment and its approval, expiring with the job (at most 6 h). The JWKS is at /.well-known/jwks.json. The hub already uses these tokens for keyless backups to GCS and S3.
  • The hub's key (hub spec §15.5) signs web edits, wiki changes, Diffusion bumps, swarm tickets and SLSA provenance.

What is missing is a delegation issued by someone else (a certificate authority), policy rules that name identities rather than keys, and time a verifier can trust once the certificate has long expired.

The certificate

A new object, certificate, issued and signed by a hub's key (its composite Ed25519 + ML-DSA-65 key, so certificates stay post-quantum):

Certificate {
  version     [0]: 1
  key         [1]: Identity        // the fresh key pair's public identity
  issuer      [2]: KeyId           // the hub key that signs it
  subject     [3]: text            // who proved it: "repo:andymccall/oxygen:bookmark:main", "agent:claude@andymccall"
  claims      [4]: map text->text  // what else the token said: environment, approval, run, job
  scope       [5]: Scope           // changes, refs, seal
  not_before  [6]: u64             // milliseconds
  expires     [7]: u64             // not_before + at most 10 minutes for jobs
}

It travels with what it signed, like an identity: a push carries the certificates of the keys that signed its changes and attestations, and a fetch brings them.

Getting one

POST /api/v1/keyless/certify
  { "token": "<OIDC token>", "identity": "<fresh identity, CBOR, base64>",
    "proof": "<the fresh key's signature over the hub's challenge>", "scope": "refs" }
→ { "certificate": "<CBOR, base64>" }

The hub checks:

  • the token: its own issuer to begin with (others later, see the decisions);
  • that the proof shows the caller holds the fresh key (the challenge signatures of spec §14.3);
  • that the scope asked is one the subject may have.

It then signs a certificate expiring at the earlier of ten minutes and the token's expiry, and logs it (below).

  • A Flow job gets a token as it does today, generates a key in memory, certifies it, signs (a receipt, a tag, an attestation of a release), and the key dies with the job. runner.key remains only to authenticate the runner to the hub, not to sign what jobs made.
  • An agent has no OIDC token of its own. The hub gives it one when its owner says so: oh agent grant claude --scope changes --for 8h on Andy's side, or a hub page with a button, makes an agent token. The agent exchanges that token for certificates as it needs them (each ten minutes). The owner revokes the grant on the hub, and no key is left anywhere. The agent's home holds only the grant token, which signs nothing by itself.
  • People keep their own keys; passkeys (#237) are the separate answer for them.

Policy names identities

A rule today lists keys. It may also list subjects from an issuer:

rule "main" {
  key 4cd9916c…                                   // Andy, as today
  subject "repo:andymccall/oxygen:bookmark:main" issuer HUB scope refs
  subject "agent:claude@andymccall" issuer HUB scope changes
}

HUB is the hub key's KeyId, so the policy says which CA it trusts for which subjects. A pattern may end in *. The policy's roots still sign the policy, as now.

Verifying later

A certificate is valid for ten minutes. A verifier checking a signature a year later must know the signature was made inside those ten minutes, and the signer's own claim of time is not enough (that is #240's problem). Two sources, in order of strength:

  1. The transparency log (docs/transparency.md): the hub logs every certificate it issues and every object signed by a certified key. An object integrated (witnessed) before its certificate's expires was signed in time. This is exactly how Sigstore uses Rekor with Fulcio.
  2. The hub's acceptance, until the log exists: when a push carries a certified signature, the hub countersigns its acceptance time on the object (a small attestation, kind accepted). That trusts the hub's clock alone, which the log later removes.

A signature whose certificate has expired, with neither proof, is invalid.

Revocation

Short certificates mostly replace revocation: a stolen key is useless ten minutes later. What can still be revoked:

  • the grant an agent holds (on the hub, at once);
  • the issuer itself (a policy without the hub's key trusts none of its certificates);
  • one certificate by its hash (a revocation listed in the policy, as keys are now), for the ten minutes it matters.

Order of work

  1. The certificate object in oxy-core: encoding, verification (issuer signature, scope, expiry against a given trusted time), carried in bundles like identities. Spec §14. About three days.
  2. The certify endpoint on the hub: token check, proof of possession, issuing, logging. About two days.
  3. Flow jobs: the runner certifies a fresh key per job and signs receipts with it; the hub checks receipts by certificate as well as by flow_runner_keys. About two days.
  4. Policy subjects: rules naming issuers and subject patterns; oxy policy grant --subject. About two days.
  5. Agent grants: oh agent grant, the hub's grant tokens, and oxy certifying on demand when author.signer is keyless. About three days.
  6. Trusted time: the acceptance attestation now; the log's integrated time when #238 lands.

Decisions (taken 2026-10-17)

Andy chose the recommendation each time: our own composite-signed certificate, built now with the hub's acceptance time (the log's time later), agent grants that retire the agent keys, and the hub's own OIDC issuer first.

  1. Certificate format:
    • Our own CBOR object, composite-signed: post-quantum, and small.
    • X.509, as Fulcio issues: readable by Sigstore's tools, but classical only.
    • Recommendation: our own, with an X.509 export later if a customer needs Sigstore's tools to read them.
  2. Build before or after the transparency log? Certificates are only as good as the proof of when they were used.
    • Recommendation: build steps 1 to 4 now with the hub's acceptance attestation, and move to the log's time when #238 step 1 lands.
  3. Agents: a grant on the hub that the agent exchanges for ten-minute certificates, replacing the long-lived agent keys and the 8 h delegations re-issued by hand.
    • Recommendation: yes, and retire ~/.oxy-agents/*/keys once it works.
  4. Other OIDC issuers (GitHub Actions, GitLab CI, Google service accounts) as subjects, for repositories mirrored or built elsewhere:
    • Recommendation: the hub's own issuer first; others as a site setting listing trusted issuers, later.