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Keyless Signatures

The full design document for this can be found here (join sigstore-dev@googlegroups.com for access).

🚨 🚨 ExPeRiMeNtAl 🚨 🚨

This document explains how the experimental keyless signatures work in cosign. Try it out!

Usage

Keyless signing:

$ COSIGN_EXPERIMENTAL=1 cosign sign gcr.io/dlorenc-vmtest2/demo
Generating ephemeral keys...
Retrieving signed certificate...
Your browser will now be opened to:
https://accounts.google.com/o/oauth2/v2/auth?access_type=offline&client_id=237800849078-rmntmr1b2tcu20kpid66q5dbh1vdt7aj.apps.googleusercontent.com&redirect_uri=http%3A%2F%2F127.0.0.1%3A5556%2Fauth%2Fgoogle%2Fcallback&response_type=code&scope=openid+email&state=8slZXeZhwKQofg%3D%3D
Pushing signature to: gcr.io/dlorenc-vmtest2/demo:sha256-97fc222cee7991b5b061d4d4afdb5f3428fcb0c9054e1690313786befa1e4e36.sig

Keyless verifying:

$ COSIGN_EXPERIMENTAL=1 cosign verify gcr.io/dlorenc-vmtest2/demo
The following checks were performed on all of these signatures:
  - The cosign claims were validated
  - The claims were present in the transparency log
  - The signatures were integrated into the transparency log when the certificate was valid
  - Any certificates were verified against the Fulcio roots.
Certificate subject:  dlorenc@google.com
{"Critical":{"Identity":{"docker-reference":""},"Image":{"Docker-manifest-digest":"sha256:97fc222cee7991b5b061d4d4afdb5f3428fcb0c9054e1690313786befa1e4e36"},"Type":"cosign container image signature"},"Optional":null}

That's it! No keys! The rest of the flags (annotations, claims, tlog, etc.) should all work the same.

Overview

This uses ephemeral keys and certificates, which are signed automatically by the fulcio root CA. Signatures are stored in the rekor transparency log, which automatically provides an attestation as to when the signature was created.

Information on the fulcio root CA can be found in the fulcio repository.

Keys

The root CA keys are hard-coded in cosign today. They can only be changed by recompiling the binary. This will be made more configurable in the future.

OAuth Flows

Cosign supports two OAuth flows today: the standard flow and the device flow.

When there is no terminal attached (non-interactive mode), cosign will automatically use the device flow where a link is printed to stdout. This link must be opened in a browser to complete the flow.

Identity Tokens

In automated environments, cosign also supports directly using OIDC Identity Tokens from specific issuers. These can be supplied on the command line with the --identity-token flag. The audiences field must contain sigstore.

cosign also has support for detecting some of these automated environments and producing an identity token. Currently this supports Google and GitHub.

On GCP

From a GCE VM, you can use the VM's service account identity to sign an image:

$ cosign sign --identity-token=$(
    gcloud auth print-identity-token \
        --audiences=sigstore) \
    gcr.io/dlorenc-vmtest2/demo

From outside a GCE VM, you can impersonate a GCP IAM service account to sign an image:

$ cosign sign --identity-token=$(
    gcloud auth print-identity-token \
        --audiences=sigstore \
        --include-email \
        --impersonate-service-account my-sa@my-project.iam.gserviceaccount.com) \
    gcr.io/dlorenc-vmtest2/demo

In order to impersonate an IAM service account, your account must have the roles/iam.serviceAccountTokenCreator role.

Note: On Google Cloud Build, standard identity tokens are not supported through the GCE metadata server. cosign has a special flow for this case, where you can instruct the Cloud Build service account to impersonate another service account. To configure this flow:

  1. Create a service account to use for signatures (the email address will be present in the certificate subject).
  2. Grant the Cloud Build service account the roles/iam.serviceAccountTokenCreator role for this target account.
  3. Set the GOOGLE_SERVICE_ACCOUNT_NAME environment variable to the name of the target account in your cloudbuild.yaml
  4. Sign images in GCB, without keys!

Timestamps

Signature timestamps are checked in the rekor transparency log. Rekor's IntegratedTime is signed as part of its signedEntryTimestamp. Cosign verifies the signature over the timestamp and checks that the signature was created while the certificate was valid.

Upcoming work

  • Root CA hardening: We should use intermediate certs rather than the root, and support chained verification.
  • Root CA configuration: We should allow users to change the roots and add their own.
  • Other timestamps: We should allow for other timestamp attestations, including attached RFC3161 signatures.
  • Probably a lot more: This is very experimental.
  • More OIDC providers: Obvious.

Custom Infrastructure

If you're running your own sigstore services flags are available to set your own endpoint's, e.g

 COSIGN_EXPERIMENTAL=1 go run cmd/cosign/main.go sign -oidc-issuer "https://oauth2.example.com/auth" \
                        -fulcio-url "https://fulcio.example.com" \
                        -rekor-url "https://rekor.example.com"  \
                        ghcr.io/jdoe/somerepo/testcosign

Custom root Cert

You can override the public good instance root CA using the environment variable SIGSTORE_ROOT_FILE, e.g.

export SIGSTORE_ROOT_FILE="/home/jdoe/myrootCA.pem"