feat: add attest sign/verify

pkg/attestation/types.go

@@ -1,10 +1,18 @@
 package attestation
 
-import "encoding/base64"
+import (
+	"encoding/base64"
+	"fmt"
+
+	intoto "github.com/in-toto/in-toto-golang/in_toto"
+	v02 "github.com/in-toto/in-toto-golang/in_toto/slsa_provenance/v0.2"
+	ociv1 "github.com/opencontainers/image-spec/specs-go/v1"
+)
 
 const (
-	DockerDsseExtKind = "application/vnd.docker.attestation-verification.v1+json"
-	RekorTlExtKind    = "Rekor"
+	DockerDsseExtKind             = "application/vnd.docker.attestation-verification.v1+json"
+	RekorTlExtKind                = "Rekor"
+	OCIDescriptorDSSEMediaType    = ociv1.MediaTypeDescriptor + "+dsse"
 )
 
 var base64Encoding = base64.StdEncoding.Strict()
@@ -33,3 +41,20 @@ type DockerTlExtension struct {
 	Kind string `json:"kind"`
 	Data any    `json:"data"`
 }
+
+func DSSEMediaType(predicateType string) (string, error) {
+	var predicateName string
+	switch predicateType {
+	case v02.PredicateSLSAProvenance:
+		predicateName = "provenance"
+	case intoto.PredicateSPDX:
+		predicateName = "spdx"
+	case VSAPredicateType:
+		predicateName = "verification_summary"
+
+	default:
+		return "", fmt.Errorf("unknown predicate type %q", predicateType)
+	}
+
+	return fmt.Sprintf("application/vnd.in-toto.%s+dsse", predicateName), nil
+}