Return VSA and rich errors from verification (#38)

* Start of richer results from verification

* Pull out VSA code from signing

* Expose attestation signing fns

* Add VSA test

* Notes for policy result

* Require separate policy for VSA creation

* Load test signing key from tests

* Return rich object from policy

* Add result object schema and fix tests

* Ensure example test runs

* Remove data.yaml files from mock policies

* Don't run example - TUF policy isn't compatible

* Add attestation to manifests for all subjects

* Ensure adding attestation doesn't touch statements

* Don't export sign function

* Remove attestations from VerificationResult

* Change bool to Outcome enum in result

* Use outputLayout directly

* Make clearer that Outcome strings are for VSA

* Return multiple SLSA levels from policy

* Fix unmarshalling of policy-id (#39)

* Rename function

* Rename policy.VerificationResult -> policy.Result

* Re-add test for canonical input

---------

Co-authored-by: James Carnegie <james.carnegie@docker.com>
Co-authored-by: James Carnegie <kipz@users.noreply.github.com>

pkg/signerverifier/common.go

@@ -6,6 +6,8 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
+	"crypto/x509"
+	"encoding/pem"
 	"fmt"
 
 	"github.com/docker/attest/internal/util"
@@ -45,6 +47,32 @@ func (s *ECDSA256_SignerVerifier) Verify(ctx context.Context, data []byte, sig [
 	return nil
 }
 
+func LoadKeyPair(priv []byte) (dsse.SignerVerifier, error) {
+	privateKey, err := parsePriv(priv)
+	if err != nil {
+		return nil, err
+	}
+	return &ECDSA256_SignerVerifier{
+		Signer: privateKey,
+	}, nil
+}
+
+func parsePriv(privkeyBytes []byte) (*ecdsa.PrivateKey, error) {
+	p, _ := pem.Decode(privkeyBytes)
+	if p == nil {
+		return nil, fmt.Errorf("privkey file does not contain any PEM data")
+	}
+	if p.Type != "EC PRIVATE KEY" {
+		return nil, fmt.Errorf("privkey file does not contain a priavte key")
+	}
+	privKey, err := x509.ParseECPrivateKey(p.Bytes)
+	if err != nil {
+		return nil, fmt.Errorf("error failed to parse public key: %w", err)
+	}
+
+	return privKey, nil
+}
+
 func GenKeyPair() (dsse.SignerVerifier, error) {
 	signer, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
 	if err != nil {