Decouple manifest signing and verification

It was probably ill-advised to couple manifest signing and verification to
their respective types. This changeset simply changes them from methods to
functions. These might not even be in this package in the future.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

manifest/verify.go

@@ -0,0 +1,32 @@
+package manifest
+
+import (
+	"crypto/x509"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/docker/libtrust"
+)
+
+// Verify verifies the signature of the signed manifest returning the public
+// keys used during signing.
+func Verify(sm *SignedManifest) ([]libtrust.PublicKey, error) {
+	js, err := libtrust.ParsePrettySignature(sm.Raw, "signatures")
+	if err != nil {
+		logrus.WithField("err", err).Debugf("(*SignedManifest).Verify")
+		return nil, err
+	}
+
+	return js.Verify()
+}
+
+// VerifyChains verifies the signature of the signed manifest against the
+// certificate pool returning the list of verified chains. Signatures without
+// an x509 chain are not checked.
+func VerifyChains(sm *SignedManifest, ca *x509.CertPool) ([][]*x509.Certificate, error) {
+	js, err := libtrust.ParsePrettySignature(sm.Raw, "signatures")
+	if err != nil {
+		return nil, err
+	}
+
+	return js.VerifyChains(ca)
+}