implement pubkey-output flag for sign-blob subcommand

This commit implements the pubkey-output flag for a sign-blob subcommand. With this flag the user is able to store the public key without reading the public key from stdout.

Signed-off-by: Christian Rebischke <chris@shibumi.dev>

cmd/cosign/cli/options/signblob.go

@@ -24,6 +24,7 @@ type SignBlobOptions struct {
 	Key          string
 	Base64Output bool
 	Output       string // TODO: this should be the root output file arg.
+	PubKeyOutput string
 	SecurityKey  SecurityKeyOptions
 	Fulcio       FulcioOptions
 	Rekor        RekorOptions
@@ -49,4 +50,7 @@ func (o *SignBlobOptions) AddFlags(cmd *cobra.Command) {
 
 	cmd.Flags().StringVar(&o.Output, "output", "",
 		"write the signature to FILE")
+
+	cmd.Flags().StringVar(&o.PubKeyOutput, "pubkey-output", "",
+		"write the public key/certificate to FILE")
 }

cmd/cosign/cli/sign/sign_blob.go

@@ -51,7 +51,7 @@ type KeyOpts struct {
 }
 
 // nolint
-func SignBlobCmd(ctx context.Context, ko KeyOpts, regOpts options.RegistryOptions, payloadPath string, b64 bool, output string) ([]byte, error) {
+func SignBlobCmd(ctx context.Context, ko KeyOpts, regOpts options.RegistryOptions, payloadPath string, b64 bool, output string, pubKeyOutput string) ([]byte, error) {
 	var payload []byte
 	var err error
 
@@ -76,9 +76,9 @@ func SignBlobCmd(ctx context.Context, ko KeyOpts, regOpts options.RegistryOption
 		return nil, errors.Wrap(err, "signing blob")
 	}
 
+	var rekorBytes []byte
 	if options.EnableExperimental() {
 		// TODO: Refactor with sign.go
-		var rekorBytes []byte
 		if sv.Cert != nil {
 			fmt.Fprintf(os.Stderr, "signing with ephemeral certificate:\n%s\n", string(sv.Cert))
 			rekorBytes = sv.Cert
@@ -100,6 +100,26 @@ func SignBlobCmd(ctx context.Context, ko KeyOpts, regOpts options.RegistryOption
 		fmt.Fprintln(os.Stderr, "tlog entry created with index:", *entry.LogIndex)
 	}
 
+	if pubKeyOutput != "" {
+		f, err := os.Create(pubKeyOutput)
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+
+		if b64 {
+			_, err = f.Write([]byte(base64.StdEncoding.EncodeToString(rekorBytes)))
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			_, err = f.Write(rekorBytes)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
 	if output != "" {
 		f, err := os.Create(output)
 		if err != nil {

cmd/cosign/cli/signblob.go

@@ -74,7 +74,7 @@ func SignBlob() *cobra.Command {
 				OIDCClientSecret:         o.OIDC.ClientSecret,
 			}
 			for _, blob := range args {
-				if _, err := sign.SignBlobCmd(cmd.Context(), ko, o.Registry, blob, o.Base64Output, o.Output); err != nil {
+				if _, err := sign.SignBlobCmd(cmd.Context(), ko, o.Registry, blob, o.Base64Output, o.Output, o.PubKeyOutput); err != nil {
 					return errors.Wrapf(err, "signing %s", blob)
 				}
 			}