cosigned should support verifying Fulcio signatures

[mattmoor]

Description

Today the cosigned webhook requires folks to setup k8s://cosign-system/verification-key with the public key that should be used for verification. The webhook passes the following CheckOpts:

cosign/pkg/cosign/kubernetes/webhook/validation.go

Lines 61 to 62 in 278ad7d

	RootCerts: fulcioroots.Get(),
	SigVerifier: ecdsaVerifier,

... which includes a SigVerifier based on the verification key, which sends us through this path in Verify:

cosign/pkg/cosign/verify.go

Lines 143 to 150 in 278ad7d

	case co.SigVerifier != nil:
	signature, err := base64.StdEncoding.DecodeString(b64sig)
	if err != nil {
	return err
	}
	if err := co.SigVerifier.VerifySignature(bytes.NewReader(signature), bytes.NewReader(payload), options.WithContext(ctx)); err != nil {
	return err
	}

... but for Fulcio we want things to follow this path:

cosign/pkg/cosign/verify.go

Lines 151 to 184 in 278ad7d

	// If we don't have a public key to check against, we can try a root cert.
	case co.RootCerts != nil:
	// There might be signatures with a public key instead of a cert, though
	if cert == nil {
	return errors.New("no certificate found on signature")
	}
	pub, err := signature.LoadECDSAVerifier(cert.PublicKey.(*ecdsa.PublicKey), crypto.SHA256)
	if err != nil {
	return errors.Wrap(err, "invalid certificate found on signature")
	}
	// Now verify the cert, then the signature.
	if err := TrustedCert(cert, co.RootCerts); err != nil {
	return err
	}
	signature, err := base64.StdEncoding.DecodeString(b64sig)
	if err != nil {
	return err
	}
	if err := pub.VerifySignature(bytes.NewReader(signature), bytes.NewReader(payload), options.WithContext(ctx)); err != nil {
	return err
	}
	if co.CertEmail != "" {
	emailVerified := false
	for _, em := range cert.EmailAddresses {
	if co.CertEmail == em {
	emailVerified = true
	break
	}
	}
	if !emailVerified {
	return errors.New("expected email not found in certificate")
	}
	}

[mattmoor]

Here's sort of what I'm thinking:

1. Switch from using a secret for verification-key to using a ConfigMap (since it's a public key, when specified).
2. When the key isn't specified, verify against the Fulcio roots.

Over time, we can use this configmap to pass additional configuration, including:

• Alternate Fulcio root certs (e.g. if a custom Fulcio root is in use),
• A Rekor URL to verify transparency logs.

... and likely a bunch more (e.g. stuff like CertEmail in CheckOpts).

cc @dlorenc @dekkagaijin @n3wscott for thoughts

[mattmoor]

I guess we could start verifying against Fulcio when no key is specified without the ConfigMap switch too, but I do think a ConfigMap switch is in our not-to-distant future 🤔

[mattmoor]

I'm going to close this, and we can open a new issue for greater configurability.