Add tests to verify attribute certificate signatures

Use an old Intel Transparent Supply Chain certificate to verify the
signatures for the test platform certificates we have that correspond to
it.

attributecert/attributecert_test.go

@@ -15,6 +15,7 @@
 package attributecert
 
 import (
+	"crypto/x509"
 	"encoding/json"
 	"io/ioutil"
 	"reflect"
@@ -22,12 +23,48 @@ import (
 	"testing"
 )
 
+func TestVerifyAttributeCert(t *testing.T) {
+	testfiles := [...]string{"testdata/Intel_nuc_pc2.cer",
+		"testdata/Intel_nuc_pc.cer",
+		"testdata/Intel_pc2.cer",
+		"testdata/Intel_pc3.cer",
+	}
+	data, err := ioutil.ReadFile("testdata/IntelSigningKey_20April2017.cer")
+	if err != nil {
+		t.Fatalf("failed to read Intel intermediate certificate: %v", err)
+	}
+	cert, err := x509.ParseCertificate(data)
+	if err != nil {
+		t.Fatalf("failed to parse Intel intermediate certificate: %v", err)
+	}
+
+	for _, filename := range(testfiles) {
+		data, err = ioutil.ReadFile(filename)
+		if err != nil {
+			t.Fatalf("failed to read %s: %v", filename, err)
+		}
+
+		attributecert, err := ParseAttributeCertificate(data)
+		if err != nil {
+			t.Fatalf("failed to parse %s: %v", filename, err)
+		}
+
+		err = attributecert.CheckSignatureFrom(cert)
+		if err != nil {
+			t.Fatalf("failed to verify signature on %s: %v", filename, err)
+		}
+	}
+}
+
 func TestParseAttributeCerts(t *testing.T) {
 	files, err := ioutil.ReadDir("testdata")
 	if err != nil {
 		t.Fatalf("failed to read test dir: %v", err)
 	}
 	for _, file := range files {
+		if strings.Contains(file.Name(), "Signing") {
+			continue
+		}
 		if strings.HasSuffix(file.Name(), ".json") {
 			continue
 		}