Fix field names in generated json statement

To keep backwards compatibility with previous struct versions (e.g, github.com/in-toto/in-toto-golang/in_toto) we have to use protojson.Marshal function to transform the struct to its json representation so the tags defined in the intoto.Statement are follow and applied to the resulting string.

pkg/chains/signing.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 
 	"github.com/hashicorp/go-multierror"
+	intoto "github.com/in-toto/attestation/go/v1"
 	"github.com/tektoncd/chains/pkg/artifacts"
 	"github.com/tektoncd/chains/pkg/chains/formats"
 	"github.com/tektoncd/chains/pkg/chains/objects"
@@ -29,6 +30,7 @@ import (
 	"github.com/tektoncd/chains/pkg/chains/storage"
 	"github.com/tektoncd/chains/pkg/config"
 	versioned "github.com/tektoncd/pipeline/pkg/client/clientset/versioned"
+	"google.golang.org/protobuf/encoding/protojson"
 	"k8s.io/apimachinery/pkg/util/sets"
 	"knative.dev/pkg/logging"
 )
@@ -169,7 +171,7 @@ func (o *ObjectSigner) Sign(ctx context.Context, tektonObj objects.TektonObject)
 			}
 
 			logger.Infof("Signing object with %s", signerType)
-			rawPayload, err := json.Marshal(payload)
+			rawPayload, err := getRawPayload(payload)
 			if err != nil {
 				logger.Warnf("Unable to marshal payload: %v", signerType, obj)
 				continue
@@ -248,3 +250,19 @@ func HandleRetry(ctx context.Context, obj objects.TektonObject, ps versioned.Int
 	}
 	return MarkFailed(ctx, obj, ps, annotations)
 }
+
+// getRawPayload returns the payload as a json string. If the given payload is a intoto.Statement type, protojson.Marshal
+// is used to get the proper labels/field names in the resulting json.
+func getRawPayload(payload interface{}) ([]byte, error) {
+	switch payloadObj := payload.(type) {
+	case intoto.Statement:
+		return protojson.Marshal(&payloadObj)
+	case *intoto.Statement:
+		if payloadObj == nil {
+			return json.Marshal(payload)
+		}
+		return protojson.Marshal(payloadObj)
+	default:
+		return json.Marshal(payload)
+	}
+}

pkg/chains/signing_test.go

@@ -14,12 +14,16 @@ limitations under the License.
 package chains
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"reflect"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
+	intoto "github.com/in-toto/attestation/go/v1"
 	"github.com/sigstore/rekor/pkg/generated/models"
 	"github.com/tektoncd/chains/pkg/chains/objects"
 	"github.com/tektoncd/chains/pkg/chains/signing"
@@ -405,6 +409,84 @@ func TestSigningObjects(t *testing.T) {
 	}
 }
 
+func TestGetRawPayload(t *testing.T) {
+	tests := []struct {
+		name     string
+		payload  interface{}
+		expected string
+	}{
+		{
+			name: "intoto.Statement object",
+			payload: intoto.Statement{
+				Type:          "type1",
+				PredicateType: "predicate-type1",
+			},
+			expected: compactJSON(t, []byte(`{"_type":"type1","predicateType":"predicate-type1"}`)),
+		},
+		{
+			name: "*intoto.Statement object",
+			payload: &intoto.Statement{
+				Type:          "type1",
+				PredicateType: "predicate-type1",
+			},
+			expected: compactJSON(t, []byte(`{"_type":"type1","predicateType":"predicate-type1"}`)),
+		},
+		{
+			name:     "*intoto.Statement object - nil",
+			payload:  (func() *intoto.Statement { return nil })(),
+			expected: "null",
+		},
+		{
+			name:     "other object - nil",
+			payload:  nil,
+			expected: "null",
+		},
+		{
+			name: "other object with value",
+			payload: struct {
+				Name  string
+				ID    int
+				Inner any
+			}{
+				Name: "wrapper",
+				ID:   1,
+				Inner: struct {
+					InnerID     int
+					Description string
+					IsArtifact  bool
+				}{
+					InnerID:     2,
+					Description: "some description",
+					IsArtifact:  true,
+				},
+			},
+			expected: compactJSON(t, []byte(`{"Name":"wrapper","ID":1,"Inner": {"InnerID":2,"Description":"some description","IsArtifact":true}}`)),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := getRawPayload(test.payload)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			compactExpected := compactJSON(t, got)
+			if diff := cmp.Diff(test.expected, compactExpected); diff != "" {
+				t.Errorf("getRawPayload(), -want +got, diff = %s", diff)
+			}
+		})
+	}
+}
+
+func compactJSON(t *testing.T, jsonString []byte) string {
+	t.Helper()
+	dst := &bytes.Buffer{}
+	if err := json.Compact(dst, jsonString); err != nil {
+		t.Fatalf("error getting compact JSON: %v", err)
+	}
+	return dst.String()
+}
+
 func fakeAllBackends(backends []*mockBackend) map[string]storage.Backend {
 	newBackends := map[string]storage.Backend{}
 	for _, m := range backends {

test/testdata/slsa/v1/pipeline-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v0.1",
-    "predicate_type": "https://slsa.dev/provenance/v0.2",
+    "_type": "https://in-toto.io/Statement/v0.1",
+    "predicateType": "https://slsa.dev/provenance/v0.2",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v1/task-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v0.1",
-    "predicate_type": "https://slsa.dev/provenance/v0.2",
+    "_type": "https://in-toto.io/Statement/v0.1",
+    "predicateType": "https://slsa.dev/provenance/v0.2",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2/task-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v0.1",
-    "predicate_type": "https://slsa.dev/provenance/v0.2",
+    "_type": "https://in-toto.io/Statement/v0.1",
+    "predicateType": "https://slsa.dev/provenance/v0.2",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2alpha3/pipeline-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2alpha3/task-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2alpha4/pipeline-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2alpha4/pipeline-with-object-type-hinting.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/img1",

test/testdata/slsa/v2alpha4/task-output-image.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/bar",

test/testdata/slsa/v2alpha4/task-with-object-type-hinting.json

@@ -1,6 +1,6 @@
 {
-    "type": "https://in-toto.io/Statement/v1",
-    "predicate_type": "https://slsa.dev/provenance/v1",
+    "_type": "https://in-toto.io/Statement/v1",
+    "predicateType": "https://slsa.dev/provenance/v1",
     "subject": [
         {
             "name": "gcr.io/foo/img2",