Improving --signer-fulcio-token flag to accept both path and raw token string

hack/test.token

@@ -0,0 +1 @@
+eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJmb29iYXIiLCJuYW1lIjoiSm9obiBEb2UiLCJpYXQiOjE1MTYyMzkwMjIsIkVtYWlsIjoidGVzdEBpbi10b3RvLmlvIn0.IswtNc6aJL3zAf-lSGvuz7Okf2tBr-I3ulJ_SRUMt0k

signer/fulcio/fulcio.go

@@ -93,7 +93,7 @@ func init() {
 		),
 		registry.StringConfigOption(
 			"token",
-			"Raw token to use for authentication",
+			"Raw token string to use for authentication to fulcio (cannot be used in conjunction with --fulcio-token-path)",
 			"",
 			func(sp signer.SignerProvider, token string) (signer.SignerProvider, error) {
 				fsp, ok := sp.(FulcioSignerProvider)
@@ -105,6 +105,20 @@ func init() {
 				return fsp, nil
 			},
 		),
+		registry.StringConfigOption(
+			"token-path",
+			"Path to the file containing a raw token to use for authentication to fulcio (cannot be used in conjunction with --fulcio-token)",
+			"",
+			func(sp signer.SignerProvider, tokenPath string) (signer.SignerProvider, error) {
+				fsp, ok := sp.(FulcioSignerProvider)
+				if !ok {
+					return sp, fmt.Errorf("provided signer provider is not a fulcio signer provider")
+				}
+
+				WithTokenPath(tokenPath)(&fsp)
+				return fsp, nil
+			},
+		),
 	)
 }
 
@@ -113,6 +127,7 @@ type FulcioSignerProvider struct {
 	OidcIssuer   string
 	OidcClientID string
 	Token        string
+	TokenPath    string
 }
 
 type Option func(*FulcioSignerProvider)
@@ -141,6 +156,12 @@ func WithToken(tokenOption string) Option {
 	}
 }
 
+func WithTokenPath(tokenPathOption string) Option {
+	return func(fsp *FulcioSignerProvider) {
+		fsp.TokenPath = tokenPathOption
+	}
+}
+
 func New(opts ...Option) FulcioSignerProvider {
 	fsp := FulcioSignerProvider{}
 	for _, opt := range opts {
@@ -210,9 +231,18 @@ func (fsp FulcioSignerProvider) Signer(ctx context.Context) (cryptoutil.Signer,
 			return nil, err
 		}
 
-	case fsp.Token != "":
+		// we want to fail if both flags used (they're mutually exclusive)
+	case fsp.TokenPath != "" && fsp.Token != "":
+		return nil, errors.New("only one of --fulcio-token-path or --fulcio-raw-token can be used")
+	case fsp.Token != "" && fsp.TokenPath == "":
 		raw = fsp.Token
+	case fsp.TokenPath != "" && fsp.Token == "":
+		f, err := os.ReadFile(fsp.TokenPath)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read fulcio token from filepath %s: %w", fsp.TokenPath, err)
+		}
 
+		raw = string(f)
 	case fsp.Token == "" && isatty.IsTerminal(os.Stdin.Fd()):
 		tok, err := oauthflow.OIDConnect(fsp.OidcIssuer, fsp.OidcClientID, "", "", oauthflow.DefaultIDTokenGetter)
 		if err != nil {
@@ -281,7 +311,7 @@ func (fsp FulcioSignerProvider) Signer(ctx context.Context) (cryptoutil.Signer,
 func getCert(ctx context.Context, key *rsa.PrivateKey, fc fulciopb.CAClient, token string) (*fulciopb.SigningCertificate, error) {
 	t, err := jwt.ParseSigned(token)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Failed to parse jwt token for fulcio: %w", err)
 	}
 
 	var claims struct {

signer/fulcio/fulcio_test.go

@@ -24,6 +24,7 @@
 	"fmt"
 	"log"
 	"net"
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -33,6 +34,7 @@
 	fulciopb "github.com/sigstore/fulcio/pkg/generated/protobuf"
 	"github.com/stretchr/testify/require"
 	"go.step.sm/crypto/jose"
+	"path/filepath"
 
 	"google.golang.org/grpc"
 	"gopkg.in/square/go-jose.v2/jwt"
@@ -198,6 +200,25 @@
 	_, err = provider.Signer(ctx)
 	//this should be a tranport err since we cant actually test on 443 which is the default
 	require.ErrorContains(t, err, "lookup test")
+
+	// Test signer with token read from file
+	// NOTE: this function could be refactored to accept a fileSystem or io.Reader so reading the file can be mocked,
+	// but unsure if this is the way we want to go for now
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("failed to get working directory: %v", err)
+	}
+	rootDir := filepath.Dir(filepath.Dir(wd))
+	tp := filepath.Join(rootDir, "hack", "test.token")
+
+	provider = New(WithFulcioURL(fmt.Sprintf("http://%v:%v", hostname, port)), WithTokenPath(tp))
+	signer, err = provider.Signer(ctx)
+	require.NoError(t, err)
+
+	// Test signer with both token read from file and raw token
+	provider = New(WithFulcioURL(fmt.Sprintf("http://%v:%v", hostname, port)), WithTokenPath(tp), WithToken(token))
+	signer, err = provider.Signer(ctx)
+	require.ErrorContains(t, err, "only one of --fulcio-token-path or --fulcio-raw-token can be used")
 }
 
 func generateCertChain(t *testing.T) []string {