Feature: Allow cosign to sign/attest digests before they are uploaded.

🎁 This feature allows `cosign` to sign and attest a digest that doesn't exist yet, to support scenarios such as signing an image prior to pushing it.

This adapts the ideas from the two prior approaches, which were closed by stale-bot.

Fixes: #1905

/kind feature

Signed-off-by: Matt Moore <mattmoor@chainguard.dev>

cmd/cosign/cli/attest/attest.go

@@ -214,10 +214,9 @@ func (c *AttestCommand) Exec(ctx context.Context, imageRef string) error {
 		return err
 	}
 
-	se, err := ociremote.SignedEntity(digest, ociremoteOpts...)
-	if err != nil {
-		return err
-	}
+	// We don't actually need to access the remote entity to attach things to it
+	// so we use a placeholder here.
+	se := ociremote.SignedUnknown(digest)
 
 	signOpts := []mutate.SignOption{
 		mutate.WithDupeDetector(dd),

cmd/cosign/cli/sign/sign.go

@@ -179,7 +179,9 @@ func SignCmd(ro *options.RootOptions, ko options.KeyOpts, signOpts options.SignO
 
 		if digest, ok := ref.(name.Digest); ok && !signOpts.Recursive {
 			se, err := ociremote.SignedEntity(ref, opts...)
-			if err != nil {
+			if err == ociremote.ErrEntityNotFound {
+				se = ociremote.SignedUnknown(digest)
+			} else if err != nil {
 				return fmt.Errorf("accessing image: %w", err)
 			}
 			err = signDigest(ctx, digest, staticPayload, ko, signOpts, annotations, dd, sv, se)

pkg/oci/mutate/mutate.go

@@ -135,7 +135,7 @@ func AttachSignatureToEntity(se oci.SignedEntity, sig oci.Signature, opts ...Sig
 	case oci.SignedImageIndex:
 		return AttachSignatureToImageIndex(obj, sig, opts...)
 	default:
-		return nil, fmt.Errorf("unsupported type: %T", se)
+		return AttachSignatureToUnknown(obj, sig, opts...)
 	}
 }
 
@@ -147,7 +147,7 @@ func AttachAttestationToEntity(se oci.SignedEntity, att oci.Signature, opts ...S
 	case oci.SignedImageIndex:
 		return AttachAttestationToImageIndex(obj, att, opts...)
 	default:
-		return nil, fmt.Errorf("unsupported type: %T", se)
+		return AttachAttestationToUnknown(obj, att, opts...)
 	}
 }
 
@@ -159,7 +159,7 @@ func AttachFileToEntity(se oci.SignedEntity, name string, f oci.File, opts ...Si
 	case oci.SignedImageIndex:
 		return AttachFileToImageIndex(obj, name, f, opts...)
 	default:
-		return nil, fmt.Errorf("unsupported type: %T", se)
+		return AttachFileToUnknown(obj, name, f, opts...)
 	}
 }
 
@@ -348,3 +348,108 @@ func (sii *signedImageIndex) Attachment(attName string) (oci.File, error) {
 	}
 	return nil, fmt.Errorf("attachment %q not found", attName)
 }
+
+// AttachSignatureToUnknown attaches the provided signature to the provided image.
+func AttachSignatureToUnknown(se oci.SignedEntity, sig oci.Signature, opts ...SignOption) (oci.SignedEntity, error) {
+	return &signedUnknown{
+		SignedEntity: se,
+		sig:          sig,
+		attachments:  make(map[string]oci.File),
+		so:           makeSignOpts(opts...),
+	}, nil
+}
+
+// AttachAttestationToUnknown attaches the provided attestation to the provided image.
+func AttachAttestationToUnknown(se oci.SignedEntity, att oci.Signature, opts ...SignOption) (oci.SignedEntity, error) {
+	return &signedUnknown{
+		SignedEntity: se,
+		att:          att,
+		attachments:  make(map[string]oci.File),
+		so:           makeSignOpts(opts...),
+	}, nil
+}
+
+// AttachFileToUnknown attaches the provided file to the provided image.
+func AttachFileToUnknown(se oci.SignedEntity, name string, f oci.File, opts ...SignOption) (oci.SignedEntity, error) {
+	return &signedUnknown{
+		SignedEntity: se,
+		attachments: map[string]oci.File{
+			name: f,
+		},
+		so: makeSignOpts(opts...),
+	}, nil
+}
+
+type signedUnknown struct {
+	oci.SignedEntity
+	sig         oci.Signature
+	att         oci.Signature
+	so          *signOpts
+	attachments map[string]oci.File
+}
+
+type digestable interface {
+	Digest() (v1.Hash, error)
+}
+
+// Digest is generally implemented by oci.SignedEntity implementations.
+func (si *signedUnknown) Digest() (v1.Hash, error) {
+	d, ok := si.SignedEntity.(digestable)
+	if !ok {
+		return v1.Hash{}, fmt.Errorf("underlying signed entity not digestable: %T", si.SignedEntity)
+	}
+	return d.Digest()
+}
+
+// Signatures implements oci.SignedEntity
+func (si *signedUnknown) Signatures() (oci.Signatures, error) {
+	base, err := si.SignedEntity.Signatures()
+	if err != nil {
+		return nil, err
+	} else if si.sig == nil {
+		return base, nil
+	}
+	if si.so.dd != nil {
+		if existing, err := si.so.dd.Find(base, si.sig); err != nil {
+			return nil, err
+		} else if existing != nil {
+			// Just return base if the signature is redundant
+			return base, nil
+		}
+	}
+	return AppendSignatures(base, si.sig)
+}
+
+// Attestations implements oci.SignedEntity
+func (si *signedUnknown) Attestations() (oci.Signatures, error) {
+	base, err := si.SignedEntity.Attestations()
+	if err != nil {
+		return nil, err
+	} else if si.att == nil {
+		return base, nil
+	}
+	if si.so.dd != nil {
+		if existing, err := si.so.dd.Find(base, si.att); err != nil {
+			return nil, err
+		} else if existing != nil {
+			// Just return base if the signature is redundant
+			return base, nil
+		}
+	}
+	if si.so.ro != nil {
+		replace, err := si.so.ro.Replace(base, si.att)
+		if err != nil {
+			return nil, err
+		}
+		return ReplaceSignatures(replace)
+	}
+	return AppendSignatures(base, si.att)
+}
+
+// Attachment implements oci.SignedEntity
+func (si *signedUnknown) Attachment(attName string) (oci.File, error) {
+	if f, ok := si.attachments[attName]; ok {
+		return f, nil
+	}
+	return nil, fmt.Errorf("attachment %q not found", attName)
+}

pkg/oci/mutate/mutate_test.go

@@ -163,8 +163,21 @@ func TestSignEntity(t *testing.T) {
 	}
 	sii := signed.ImageIndex(ii)
 
+	// Create an explicitly unknown implementation of oci.SignedEntity, which we
+	// feed through the table tests below.
+	want := make([]byte, 300)
+	rand.Read(want)
+	orig, err := static.NewFile(want)
+	if err != nil {
+		t.Fatalf("static.NewFile() = %v", err)
+	}
+	sunk, err := AttachFileToUnknown(sii, "sbom", orig)
+	if err != nil {
+		t.Fatalf("AttachFileToUnknown() = %v", err)
+	}
+
 	t.Run("attach SBOMs", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			want := make([]byte, 300)
 			rand.Read(want)
 
@@ -197,7 +210,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("without duplicate detector (signature)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewSignature(nil, "")
 			if err != nil {
 				t.Fatalf("static.NewSignature() = %v", err)
@@ -232,7 +245,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("without duplicate detector (attestation)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewAttestation([]byte("payload"))
 			if err != nil {
 				t.Fatalf("static.NewAttestation() = %v", err)
@@ -267,7 +280,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("with duplicate detector (signature)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewSignature(nil, "")
 			if err != nil {
 				t.Fatalf("static.NewSignature() = %v", err)
@@ -306,7 +319,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("with duplicate detector (attestation)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewAttestation([]byte("blah"))
 			if err != nil {
 				t.Fatalf("static.NewAttestation() = %v", err)
@@ -345,7 +358,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("with erroring duplicate detector (signature)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewSignature(nil, "")
 			if err != nil {
 				t.Fatalf("static.NewSignature() = %v", err)
@@ -379,7 +392,7 @@ func TestSignEntity(t *testing.T) {
 	})
 
 	t.Run("with erroring duplicate detector (attestation)", func(t *testing.T) {
-		for _, se := range []oci.SignedEntity{si, sii} {
+		for _, se := range []oci.SignedEntity{si, sii, sunk} {
 			orig, err := static.NewAttestation([]byte("blah"))
 			if err != nil {
 				t.Fatalf("static.NewAttestation() = %v", err)

pkg/oci/remote/remote.go

@@ -36,6 +36,10 @@ var (
 	remoteIndex = remote.Index
 	remoteGet   = remote.Get
 	remoteWrite = remote.Write
+
+	// ErrEntityNotFound is the error that SignedEntity returns when the
+	// provided ref does not exist.
+	ErrEntityNotFound = errors.New("entity not found in registry")
 )
 
 // SignedEntity provides access to a remote reference, and its signatures.
@@ -46,7 +50,7 @@ func SignedEntity(ref name.Reference, options ...Option) (oci.SignedEntity, erro
 	got, err := remoteGet(ref, o.ROpt...)
 	var te *transport.Error
 	if errors.As(err, &te) && te.StatusCode == http.StatusNotFound {
-		return nil, errors.New("entity not found in registry")
+		return nil, ErrEntityNotFound
 	} else if err != nil {
 		return nil, err
 	}

pkg/oci/remote/unknown.go

@@ -0,0 +1,60 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package remote
+
+import (
+	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/sigstore/cosign/v2/pkg/oci"
+)
+
+// SignedUnknown provides access to signed metadata without directly accessing
+// the underlying entity.  This can be used to access signature metadata for
+// digests that have not been published (yet).
+func SignedUnknown(digest name.Digest, options ...Option) oci.SignedEntity {
+	o := makeOptions(digest.Context(), options...)
+	return &unknown{
+		digest: digest,
+		opt:    o,
+	}
+}
+
+type unknown struct {
+	digest name.Digest
+	opt    *options
+}
+
+var _ oci.SignedEntity = (*unknown)(nil)
+
+// Digest implements digestable
+func (i *unknown) Digest() (v1.Hash, error) {
+	return v1.NewHash(i.digest.DigestStr())
+}
+
+// Signatures implements oci.SignedEntity
+func (i *unknown) Signatures() (oci.Signatures, error) {
+	return signatures(i, i.opt)
+}
+
+// Attestations implements oci.SignedEntity
+func (i *unknown) Attestations() (oci.Signatures, error) {
+	return attestations(i, i.opt)
+}
+
+// Attachment implements oci.SignedEntity
+func (i *unknown) Attachment(name string) (oci.File, error) {
+	return attachment(i, name, i.opt)
+}