Add an undocumented (skopeo untrusted-signature-dump-without-verification)

This is a bit better than raw (gpg -d $signature), and it allows testing
of the signature.GetSignatureInformationWithoutVerification function;
but, still, keeping it hidden because relying on this in common
workflows is probably a bad idea and we don’t _neeed_ to expose it right
now.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

cmd/skopeo/main.go

@@ -64,6 +64,7 @@ func createApp() *cli.App {
 		manifestDigestCmd,
 		standaloneSignCmd,
 		standaloneVerifyCmd,
+		untrustedSignatureDumpCmd,
 	}
 	return app
 }

cmd/skopeo/signing.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -88,3 +89,40 @@ var standaloneVerifyCmd = cli.Command{
 	ArgsUsage: "MANIFEST DOCKER-REFERENCE KEY-FINGERPRINT SIGNATURE",
 	Action:    standaloneVerify,
 }
+
+func untrustedSignatureDump(context *cli.Context) error {
+	if len(context.Args()) != 1 {
+		return errors.New("Usage: skopeo untrusted-signature-dump-without-verification signature")
+	}
+	untrustedSignaturePath := context.Args()[0]
+
+	untrustedSignature, err := ioutil.ReadFile(untrustedSignaturePath)
+	if err != nil {
+		return fmt.Errorf("Error reading untrusted signature from %s: %v", untrustedSignaturePath, err)
+	}
+
+	untrustedInfo, err := signature.GetUntrustedSignatureInformationWithoutVerifying(untrustedSignature)
+	if err != nil {
+		return fmt.Errorf("Error decoding untrusted signature: %v", err)
+	}
+	untrustedOut, err := json.MarshalIndent(untrustedInfo, "", "    ")
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(context.App.Writer, string(untrustedOut))
+	return nil
+}
+
+// WARNING: Do not use the contents of this for ANY security decisions,
+// and be VERY CAREFUL about showing this information to humans in any way which suggest that these values “are probably” reliable.
+// There is NO REASON to expect the values to be correct, or not intentionally misleading
+// (including things like “✅ Verified by $authority”)
+//
+// The subcommand is undocumented, and it may be renamed or entirely disappear in the future.
+var untrustedSignatureDumpCmd = cli.Command{
+	Name:      "untrusted-signature-dump-without-verification",
+	Usage:     "Dump contents of a signature WITHOUT VERIFYING IT",
+	ArgsUsage: "SIGNATURE",
+	Hidden:    true,
+	Action:    untrustedSignatureDump,
+}

cmd/skopeo/signing_test.go

@@ -1,9 +1,11 @@
 package main
 
 import (
+	"encoding/json"
 	"io/ioutil"
 	"os"
 	"testing"
+	"time"
 
 	"github.com/containers/image/signature"
 	"github.com/opencontainers/go-digest"
@@ -16,6 +18,8 @@ const (
 	fixturesTestImageManifestDigest = digest.Digest("sha256:20bf21ed457b390829cdbeec8795a7bea1626991fda603e0d01b4e7f60427e55")
 	// fixturesTestKeyFingerprint is the fingerprint of the private key.
 	fixturesTestKeyFingerprint = "1D8230F6CDB6A06716E414C1DB72F2188BB46CC8"
+	// fixturesTestKeyFingerprint is the key ID of the private key.
+	fixturesTestKeyShortID = "DB72F2188BB46CC8"
 )
 
 // Test that results of runSkopeo failed with nothing on stdout, and substring
@@ -125,3 +129,42 @@ func TestStandaloneVerify(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, "Signature verified, digest "+fixturesTestImageManifestDigest.String()+"\n", out)
 }
+
+func TestUntrustedSignatureDump(t *testing.T) {
+	// Invalid command-line arguments
+	for _, args := range [][]string{
+		{},
+		{"a1", "a2"},
+		{"a1", "a2", "a3", "a4"},
+	} {
+		out, err := runSkopeo(append([]string{"untrusted-signature-dump-without-verification"}, args...)...)
+		assertTestFailed(t, out, err, "Usage")
+	}
+
+	// Error reading manifest
+	out, err := runSkopeo("untrusted-signature-dump-without-verification",
+		"/this/doesnt/exist")
+	assertTestFailed(t, out, err, "/this/doesnt/exist")
+
+	// Error reading signature (input is not a signature)
+	out, err = runSkopeo("untrusted-signature-dump-without-verification", "fixtures/image.manifest.json")
+	assertTestFailed(t, out, err, "Error decoding untrusted signature")
+
+	// Success
+	for _, path := range []string{"fixtures/image.signature", "fixtures/corrupt.signature"} {
+		// Success
+		out, err = runSkopeo("untrusted-signature-dump-without-verification", path)
+		require.NoError(t, err)
+
+		var info signature.UntrustedSignatureInformation
+		err := json.Unmarshal([]byte(out), &info)
+		require.NoError(t, err)
+		assert.Equal(t, fixturesTestImageManifestDigest, info.UntrustedDockerManifestDigest)
+		assert.Equal(t, "testing/manifest", info.UntrustedDockerReference)
+		assert.NotNil(t, info.UntrustedCreatorID)
+		assert.Equal(t, "atomic ", *info.UntrustedCreatorID)
+		assert.NotNil(t, info.UntrustedTimestamp)
+		assert.True(t, time.Unix(1458239713, 0).Equal(*info.UntrustedTimestamp))
+		assert.Equal(t, fixturesTestKeyShortID, info.UntrustedShortKeyIdentifier)
+	}
+}