cosign: Add a public key for testing

[justaugustus]

What type of PR is this?

/kind feature

What this PR does / why we need it:

(Part of #2227)

cosign: Add a public key for testing

Preliminary steps to sign/verify artifacts via cosign.
The process or needs will evolve over time, so we've opted to generate
a "test" key to start.

Signed-off-by: Stephen Augustus foo@auggie.dev

ref: kubernetes/k8s.io#2614
cc: @kubernetes/sig-release-admins

Which issue(s) this PR fixes:

Special notes for your reviewer:

/hold will add in details about key generation when I'm back at the laptop

Steps (adapted from https://github.com/sigstore/cosign#installation and https://github.com/sigstore/cosign/blob/main/KMS.md):

1. Get cosign:

   go install github.com/sigstore/cosign/cmd/cosign@v1.0.0

2. Check which account you're operating over:

   gcloud config configurations list
   NAME         IS_ACTIVE  ACCOUNT                    PROJECT             COMPUTE_DEFAULT_ZONE  COMPUTE_DEFAULT_REGION
   ...
   releng-prod  True       k8s@auggie.dev             k8s-releng-prod

3. Generate GOOGLE_APPLICATION_CREDENTIALS:

   gcloud auth application-default login
   
   export GOOGLE_APPLICATION_CREDENTIALS="/Users/augustus/.config/gcloud/application_default_credentials.json"

4. Generate a signing key on GCP KMS

   I did this via the console using these instructions.

   Here are the specs on the key:

   • type: generated key
   • name: cosign-test-0
   • protection level: HSM
   • purpose: asymmetric sign
   • key type and algorithm: elliptic curve P-256 / SHA256 digest

5. Generate cosign.pub:

   (Special step)
   Make sure you actually have permissions to sign/verify on GCP KMS: infra/gcp: Grant Release Manager Admins roles/cloudkms.signerVerifier k8s.io#2614

   ~/go/bin/cosign -d generate-key-pair -kms gcpkms://projects/k8s-releng-prod/locations/global/keyRings/release/cryptoKeys/cosign-test-0
   2021/08/25 15:38:40 Key ring projects/k8s-releng-prod/locations/global/keyRings/release already exists in GCP KMS, moving on to creating key.
   Public key written to cosign.pub

6. Check the key:

   cat cosign.pub
   -----BEGIN PUBLIC KEY-----
   MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEwbFSNTX481zrRduH3nu8HQ6hHhSW
   KEPjaz/SlBr+SojMXgYJsSueIRqfLiCJgNcdqEe1gXk70WZRHtAHfVzTNw==
   -----END PUBLIC KEY-----

Does this PR introduce a user-facing change?

- cosign: Add a public key for testing
  Preliminary steps to sign/verify artifacts via `cosign`.
  The process or needs will evolve over time, so we've opted to generate
  a "test" key to start.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justaugustus

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[puerco]

/lgtm

[justaugustus]

/hold cancel
FYI @kubernetes/release-engineering @dlorenc @lukehinds

[cpanato]

thanks!