Fix intoto unmarshal (#366)

* Fix intoto statement marshal/unmarshal (see in-toto/attestation#363)

Signed-off-by: Andrew Gillis <gillis.andrew@gmail.com>

* Apply modified tests from be320de

Co-authored-by: Andrew Gillis <gillis.andrew@gmail.com>
Signed-off-by: Cody Soyland <codysoyland@github.com>

* Simplify custom Marshal, remove Unmarshal

Signed-off-by: Cody Soyland <codysoyland@github.com>

* Add custom UnmarshalJSON to VerificationResult

Signed-off-by: Cody Soyland <codysoyland@github.com>

---------

Signed-off-by: Andrew Gillis <gillis.andrew@gmail.com>
Signed-off-by: Cody Soyland <codysoyland@github.com>
Co-authored-by: Andrew Gillis <gillis.andrew@gmail.com>

pkg/verify/signed_entity.go

@@ -17,6 +17,7 @@ package verify
 import (
 	"crypto/x509"
 	"encoding/asn1"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -26,6 +27,7 @@ import (
 	"github.com/sigstore/sigstore-go/pkg/fulcio/certificate"
 	"github.com/sigstore/sigstore-go/pkg/root"
 	"github.com/sigstore/sigstore/pkg/cryptoutils"
+	"google.golang.org/protobuf/encoding/protojson"
 )
 
 const (
@@ -221,6 +223,40 @@ func NewVerificationResult() *VerificationResult {
 	}
 }
 
+// MarshalJSON deals with protojson needed for the Statement.
+// Can be removed when https://github.com/in-toto/attestation/pull/403 is merged.
+func (b *VerificationResult) MarshalJSON() ([]byte, error) {
+	statement, err := protojson.Marshal(b.Statement)
+	if err != nil {
+		return nil, err
+	}
+	// creating a type alias to avoid infinite recursion, as MarshalJSON is
+	// not copied into the alias.
+	type Alias VerificationResult
+	return json.Marshal(struct {
+		Alias
+		Statement json.RawMessage `json:"statement,omitempty"`
+	}{
+		Alias:     Alias(*b),
+		Statement: statement,
+	})
+}
+
+func (b *VerificationResult) UnmarshalJSON(data []byte) error {
+	b.Statement = &in_toto.Statement{}
+	type Alias VerificationResult
+	aux := &struct {
+		Alias
+		Statement json.RawMessage `json:"statement,omitempty"`
+	}{
+		Alias: Alias(*b),
+	}
+	if err := json.Unmarshal(data, aux); err != nil {
+		return err
+	}
+	return protojson.Unmarshal(aux.Statement, b.Statement)
+}
+
 type PolicyOption func(*PolicyConfig) error
 type ArtifactPolicyOption func(*PolicyConfig) error
 

pkg/verify/signed_entity_test.go

@@ -23,9 +23,11 @@ import (
 	"encoding/hex"
 	"encoding/json"
 
+	in_toto "github.com/in-toto/attestation/go/v1"
 	"github.com/sigstore/sigstore-go/pkg/testing/data"
 	"github.com/sigstore/sigstore-go/pkg/verify"
 	"github.com/stretchr/testify/assert"
+	"google.golang.org/protobuf/types/known/structpb"
 )
 
 func TestSignedEntityVerifierInitialization(t *testing.T) {
@@ -422,3 +424,53 @@ func TestSigstoreBundle2Sig(t *testing.T) {
 	assert.True(t, errors.Is(err, verify.ErrDSSEInvalidSignatureCount))
 	assert.Nil(t, res)
 }
+
+func TestStatementSerializesToValidInTotoStatement(t *testing.T) {
+	statement := in_toto.Statement{}
+	statement.Type = "https://in-toto.io/Statement/v0.1"
+	statement.PredicateType = "https://example.org/predicate"
+	statement.Subject = []*in_toto.ResourceDescriptor{
+		{
+			Name: "artifact-name",
+			Digest: map[string]string{
+				"sha256": "artifact-digest",
+			},
+		},
+	}
+	statement.Predicate = &structpb.Struct{
+		Fields: map[string]*structpb.Value{},
+	}
+
+	result := verify.NewVerificationResult()
+	result.Statement = &statement
+
+	// marshal the statement to JSON
+	resultJSON, err := json.Marshal(result)
+	assert.NoError(t, err)
+	want := `
+	{
+		"mediaType": "application/vnd.dev.sigstore.verificationresult+json;version=0.1",
+		"verifiedTimestamps": null,
+		"statement": {
+			"_type": "https://in-toto.io/Statement/v0.1",
+			"predicateType": "https://example.org/predicate",
+			"subject": [
+				{
+					"name": "artifact-name",
+					"digest": {
+						"sha256": "artifact-digest"
+					}
+				}
+			],
+			"predicate": {}
+		}
+	}`
+	assert.JSONEq(t, want, string(resultJSON))
+
+	// unmarshal the JSON back to a VerificationResult
+	result2 := verify.NewVerificationResult()
+	err = json.Unmarshal(resultJSON, result2)
+	assert.NoError(t, err)
+	assert.Equal(t, result.MediaType, result2.MediaType)
+	assert.Equal(t, result.Statement, result2.Statement)
+}