Verify signatures from a trust store

Add the ability to use an on-disk trust store to verify signatures. Also allow the user to trust any known fingerprint instead of having to specify one.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

cmd/skopeo/signing.go

@@ -67,6 +67,7 @@ func (opts *standaloneSignOptions) run(args []string, stdout io.Writer) error {
 }
 
 type standaloneVerifyOptions struct {
+	truststore string
 }
 
 func standaloneVerifyCmd() *cobra.Command {
@@ -74,8 +75,13 @@ func standaloneVerifyCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "standalone-verify MANIFEST DOCKER-REFERENCE KEY-FINGERPRINT SIGNATURE",
 		Short: "Verify a signature using local files",
+		Long: `Verify a signature using local files
+
+KEY-FINGERPRINT can be an exact fingerprint, or "any" if you trust all the keys in the trust store.`,
 		RunE: commandAction(opts.run),
 	}
+	flags := cmd.Flags()
+	flags.StringVar(&opts.truststore, "truststore", "", "Trust store of public keys. Defaults to using local gpg keys.")
 	adjustUsage(cmd)
 	return cmd
 }
@@ -98,17 +104,34 @@ func (opts *standaloneVerifyOptions) run(args []string, stdout io.Writer) error
 		return fmt.Errorf("Error reading signature from %s: %v", signaturePath, err)
 	}
 
-	mech, err := signature.NewGPGSigningMechanism()
+	var mech signature.SigningMechanism
+	if opts.truststore != "" {
+		truststore, err := os.ReadFile(opts.truststore)
+		if err != nil {
+			return fmt.Errorf("Error reading trust store from %s: %v", opts.truststore, err)
+		}
+		mech, _, err = signature.NewEphemeralGPGSigningMechanism(truststore)
+	} else {
+		mech, err = signature.NewGPGSigningMechanism()
+	}
 	if err != nil {
 		return fmt.Errorf("Error initializing GPG: %v", err)
 	}
 	defer mech.Close()
+
+	if expectedFingerprint == "any" {
+		_, expectedFingerprint, err = mech.Verify(unverifiedSignature)
+		if err != nil {
+			return fmt.Errorf("Could not determine fingerprint from signature: %v", err)
+		}
+	}
+
 	sig, err := signature.VerifyDockerManifestSignature(unverifiedSignature, unverifiedManifest, expectedDockerReference, mech, expectedFingerprint)
 	if err != nil {
 		return fmt.Errorf("Error verifying signature: %v", err)
 	}
 
-	fmt.Fprintf(stdout, "Signature verified, digest %s\n", sig.DockerManifestDigest)
+	fmt.Fprintf(stdout, "Signature verified using fingerprint %s, digest %s\n", expectedFingerprint, sig.DockerManifestDigest)
 	return nil
 }
 

cmd/skopeo/signing_test.go

@@ -131,7 +131,20 @@ func TestStandaloneVerify(t *testing.T) {
 	out, err = runSkopeo("standalone-verify", manifestPath,
 		dockerReference, fixturesTestKeyFingerprint, signaturePath)
 	assert.NoError(t, err)
-	assert.Equal(t, "Signature verified, digest "+fixturesTestImageManifestDigest.String()+"\n", out)
+	assert.Equal(t, "Signature verified using fingerprint 1D8230F6CDB6A06716E414C1DB72F2188BB46CC8, digest "+fixturesTestImageManifestDigest.String()+"\n", out)
+
+	// Using any known fingerprint
+	out, err = runSkopeo("standalone-verify", manifestPath,
+		dockerReference, "any", signaturePath)
+	assert.NoError(t, err)
+	assert.Equal(t, "Signature verified using fingerprint 1D8230F6CDB6A06716E414C1DB72F2188BB46CC8, digest "+fixturesTestImageManifestDigest.String()+"\n", out)
+
+	// Using a trust store
+	t.Setenv("GNUPGHOME", "")
+	out, err = runSkopeo("standalone-verify", "--truststore", "fixtures/pubring.gpg", manifestPath,
+		dockerReference, "any", signaturePath)
+	assert.NoError(t, err)
+	assert.Equal(t, "Signature verified using fingerprint 1D8230F6CDB6A06716E414C1DB72F2188BB46CC8, digest "+fixturesTestImageManifestDigest.String()+"\n", out)
 }
 
 func TestUntrustedSignatureDump(t *testing.T) {

docs/skopeo-standalone-verify.1.md

@@ -16,7 +16,7 @@ as per containers-policy.json(5).
 
   _docker-reference_ A docker reference expected to identify the image in the signature
 
-  _key-fingerprint_ Expected identity of the signing key
+  _key-fingerprint_ Expected identity of the signing key, or "any" to trust any known key
 
   _signature_ Path to signature file
 
@@ -28,6 +28,10 @@ as per containers-policy.json(5).
 
 Print usage statement
 
+**--truststore** _truststore_
+
+Trust store of public keys to use when verifying signatures. If this is not specified, keys from gpg home are used.
+
 ## EXAMPLES
 
 ```console

integration/signing_test.go

@@ -73,7 +73,7 @@ func (s *signingSuite) TestSignVerifySmoke() {
 	assertSkopeoSucceeds(t, "^$", "standalone-sign", "-o", sigOutput.Name(),
 		manifestPath, dockerReference, s.fingerprint)
 
-	expected := fmt.Sprintf("^Signature verified, digest %s\n$", TestImageManifestDigest)
+	expected := fmt.Sprintf("^Signature verified using fingerprint %s, digest %s\n$", s.fingerprint, TestImageManifestDigest)
 	assertSkopeoSucceeds(t, expected, "standalone-verify", manifestPath,
 		dockerReference, s.fingerprint, sigOutput.Name())
 }

systemtest/050-signing.bats

@@ -242,7 +242,7 @@ END_TESTS
                $fingerprint \
                $TESTDIR/busybox.signature
     # manifest digest
-    digest=$(echo "$output" | awk '{print $4;}')
+    digest=$(echo "$output" | awk '{print $NF;}')
     run_skopeo manifest-digest $TESTDIR/busybox/manifest.json
     expect_output $digest
 }