Add options for Rekor client, make public key fetcher configurable.

Also makes the default public key fetcher match the behavior of cosign,
which also fixes a bug where verify was getting keys from Rekor
directly.

Signed-off-by: Billy Lynch <billy@chainguard.dev>

pkg/rekor/option.go

@@ -0,0 +1,45 @@
+//
+// Copyright 2023 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 rekor
+
+import (
+	"context"
+
+	"github.com/sigstore/cosign/v2/pkg/cosign"
+	"github.com/sigstore/rekor/pkg/client"
+)
+
+type Option func(*options)
+
+type options struct {
+	rekorPublicKeys CosignRekorKeyProvider
+	clientOpts      []client.Option
+}
+
+// CosignRekorKeyProvider is a function that returns the Rekor public keys in cosign's specialized format.
+type CosignRekorKeyProvider func(ctx context.Context) (*cosign.TrustedTransparencyLogPubKeys, error)
+
+func WithCosignRekorKeyProvider(f CosignRekorKeyProvider) Option {
+	return func(o *options) {
+		o.rekorPublicKeys = f
+	}
+}
+
+func WithClientOption(opts ...client.Option) Option {
+	return func(o *options) {
+		o.clientOpts = opts
+	}
+}

pkg/rekor/rekor.go

@@ -43,7 +43,6 @@ import (
 	hashedrekord_v001 "github.com/sigstore/rekor/pkg/types/hashedrekord/v0.0.1"
 	rekord_v001 "github.com/sigstore/rekor/pkg/types/rekord/v0.0.1"
 	"github.com/sigstore/sigstore/pkg/cryptoutils"
-	"github.com/sigstore/sigstore/pkg/tuf"
 )
 
 // Verifier represents a mechanism to get and verify Rekor entries for the given Git data.
@@ -64,12 +63,26 @@ type Client struct {
 	publicKeys *cosign.TrustedTransparencyLogPubKeys
 }
 
+// Deprecated. Use NewWithOptions instead.
 func New(url string, opts ...rekor.Option) (*Client, error) {
-	c, err := rekor.GetRekorClient(url, opts...)
+	return NewWithOptions(context.TODO(), url, WithClientOption(opts...))
+}
+
+func NewWithOptions(ctx context.Context, url string, opts ...Option) (*Client, error) {
+	// Defaults
+	o := &options{
+		rekorPublicKeys: cosign.GetRekorPubs,
+	}
+	for _, f := range opts {
+		f(o)
+	}
+
+	c, err := rekor.GetRekorClient(url, o.clientOpts...)
 	if err != nil {
 		return nil, err
 	}
-	pubs, err := rekorPubsFromClient(c)
+
+	pubs, err := o.rekorPublicKeys(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -158,21 +171,6 @@ func (c *Client) findTLogEntriesByPayloadAndPK(ctx context.Context, payload, pub
 	return searchIndex.GetPayload(), nil
 }
 
-// rekorPubsFromClient returns a RekorPubKey keyed by the log ID from the Rekor client.
-// NOTE: This **must not** be used in the verification path, but may be used in the
-// sign path to validate return responses are consistent from Rekor.
-func rekorPubsFromClient(rekorClient *client.Rekor) (*cosign.TrustedTransparencyLogPubKeys, error) {
-	publicKeys := cosign.NewTrustedTransparencyLogPubKeys()
-	pubOK, err := rekorClient.Pubkey.GetPublicKey(nil)
-	if err != nil {
-		return nil, fmt.Errorf("unable to fetch rekor public key from rekor: %w", err)
-	}
-	if err := publicKeys.AddTransparencyLogPubKey([]byte(pubOK.Payload), tuf.Active); err != nil {
-		return nil, fmt.Errorf("constructRekorPubKey: %w", err)
-	}
-	return &publicKeys, nil
-}
-
 // Verify verifies a commit using online verification.
 //
 // This is done by: