add fulcio.DSSEAttestor

Signed-off-by: Jake Sanders <[email protected]>

internal/pkg/cosign/fulcio/attestor.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 fulcio
+
+import (
+	"context"
+	"crypto"
+	"io"
+
+	"github.com/sigstore/cosign/internal/pkg/cosign"
+	"github.com/sigstore/cosign/pkg/oci"
+	"github.com/sigstore/cosign/pkg/oci/mutate"
+)
+
+// fulcioAttestor still needs to actually upload keys to Fulcio and receive
+// the resulting `Cert` and `Chain`, which are added to the returned `oci.Signature`
+type fulcioAttestor struct {
+	inner cosign.DSSEAttestor
+
+	cert, chain []byte
+}
+
+var _ cosign.DSSEAttestor = (*fulcioAttestor)(nil)
+
+// Attest implements `cosign.DSSEAttestor`
+func (fa *fulcioAttestor) DSSEAttest(ctx context.Context, payload io.Reader) (oci.Signature, crypto.PublicKey, error) {
+	att, pub, err := fa.inner.DSSEAttest(ctx, payload)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	newAtt, err := mutate.Signature(att, mutate.WithCertChain(fa.cert, fa.chain))
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return newAtt, pub, nil
+}
+
+// WrapAttestor returns a `cosign.DSSEAttestor` which leverages Fulcio to create
+// a Cert and Chain for the attestation's signature created by the inner `Attestor`
+func WrapAttestor(inner cosign.DSSEAttestor, cert, chain []byte) cosign.DSSEAttestor {
+	return &fulcioAttestor{
+		inner: inner,
+		cert:  cert,
+		chain: chain,
+	}
+}