`gcp-iit` attestor

Captures the GCP Instance Identity Token (a signed JWT from the GCE metadata server) and verifies its signature against Google's published JWKS, producing a tamper-evident proof of "this ran on GCE project/instance X."

Name	`gcp-iit`
Predicate type	`https://aflock.ai/attestations/gcp-iit/v0.1`
Lifecycle	`prematerial`
Default binary?	No
Category	ci-context (primary)
Recommended trace	off — no syscall tracing needed
Auto-attaches when	preprobe: gcp_metadata_reachable=true

The facts in this box are generated from the cilock binary's own catalog (cilock tools list). Do not hand-edit — run npm run gen:catalog.

What it captures

The full verified jwt attestor (token, header, claims, JWKS URL) plus the following GCE claims extracted from the google claim of the IIT:

project_id — GCP project (string)
project_number — numeric project ID (string)
zone — GCE zone
instance_id — GCE instance ID
instance_hostname — instance_name from the IIT
instance_creation_timestamp
instance_confidentiality
licence_id — list of GCE license IDs

When the token has no google claim (Workload Identity on GKE), the attestor instead reads the GCE metadata server directly and fills in cluster_name, cluster_uid, cluster_location, plus instance/project fields parsed from the JWT email claim.

Subjects emitted: instanceid:, instancename:, projectid:, projectnumber:, clusteruid:.

When to use

Builds running on GCE VMs, GKE nodes, or Cloud Build workers. The IIT is the GCP analog of the AWS instance identity document — a strong "this ran in our project, this zone, this instance" signal signed by Google.

Flags

None. Configuration is via environment variable:

Env var	Default	What it does
`WITNESS_GCP_JWKS_URL`	`https://www.googleapis.com/oauth2/v3/certs`	Override the JWKS endpoint used to verify the IIT signature.

The identity token is always fetched with audience witness-node-attestor against the default service account.

Output shape

{
  "jwt": { "token": "...", "claims": { ... }, "jwksUrl": "https://www.googleapis.com/oauth2/v3/certs" },
  "project_id": "my-project",
  "project_number": "123456789012",
  "zone": "projects/123/zones/us-central1-a",
  "instance_id": "8675309",
  "instance_hostname": "build-runner-01",
  "instance_creation_timestamp": "1700000000",
  "instance_confidentiality": "0",
  "licence_id": ["1234"],
  "cluster_name": "",
  "cluster_uid": "",
  "cluster_location": ""
}

Gotchas

GCE-only. The identity token is fetched from http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity?audience=witness-node-attestor&format=full with the Metadata-Flavor: Google header. On any non-GCE host the request fails (no DNS, no IMDS) and Attest returns unable to retrieve valid identity token.
Signature verification is delegated to the jwt attestor. The IIT is verified against Google's JWKS at https://www.googleapis.com/oauth2/v3/certs (or WITNESS_GCP_JWKS_URL). If verification fails, the attestor fails — unverified tokens are never recorded.
Workload Identity fallback. GKE pods using Workload Identity get a token without the google claim. The attestor detects this, falls back to direct metadata server lookups (/computeMetadata/v1/instance/... and /computeMetadata/v1/project/...), and parses project ID/number out of the JWT email claim domain. Cluster fields are only populated on this path.
Metadata responses are capped at 1 MB to bound memory if the endpoint is compromised.
The hardcoded audience is witness-node-attestor — verifiers asserting an audience should expect that exact string.

CLI example

See the constraint summary + reproduction recipe at https://github.com/aflock-ai/attestor-compliance-examples/tree/main/26-gcp-iit. This attestor is currently blocked or doc-only — the linked example explains why and shows the recipe to validate once the constraint is removed.

What it captures​

When to use​

Flags​

Output shape​

Gotchas​

CLI example​

See also​