Add docker/utils.ManifestMatchesDigest

docker/utils/manifest.go

@@ -64,3 +64,16 @@ func ManifestDigest(manifest []byte) (string, error) {
 	hash := sha256.Sum256(manifest)
 	return "sha256:" + hex.EncodeToString(hash[:]), nil
 }
+
+// ManifestMatchesDigest returns true iff the manifest matches expectedDigest.
+// Error may be set if this returns false.
+// Note that this is not doing ConstantTimeCompare; by the time we get here, the cryptographic signature must already have been verified,
+// or we are not using a cryptographic channel and the attacker can modify the digest along with the manifest blob.
+func ManifestMatchesDigest(manifest []byte, expectedDigest string) (bool, error) {
+	// This should eventually support various digest types.
+	actualDigest, err := ManifestDigest(manifest)
+	if err != nil {
+		return false, err
+	}
+	return expectedDigest == actualDigest, nil
+}

docker/utils/manifest_test.go

@@ -1,6 +1,8 @@
 package utils
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"io/ioutil"
 	"path/filepath"
 	"testing"
@@ -56,3 +58,43 @@ func TestManifestDigest(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855", digest)
 }
+
+func TestManifestMatchesDigest(t *testing.T) {
+	cases := []struct {
+		path   string
+		digest string
+		result bool
+	}{
+		// Success
+		{"v2s2.manifest.json", TestV2S2ManifestDigest, true},
+		{"v2s1.manifest.json", TestV2S1ManifestDigest, true},
+		// No match (switched s1/s2)
+		{"v2s2.manifest.json", TestV2S1ManifestDigest, false},
+		{"v2s1.manifest.json", TestV2S2ManifestDigest, false},
+		// Unrecognized algorithm
+		{"v2s2.manifest.json", "md5:2872f31c5c1f62a694fbd20c1e85257c", false},
+		// Mangled format
+		{"v2s2.manifest.json", TestV2S2ManifestDigest + "abc", false},
+		{"v2s2.manifest.json", TestV2S2ManifestDigest[:20], false},
+		{"v2s2.manifest.json", "", false},
+	}
+	for _, c := range cases {
+		manifest, err := ioutil.ReadFile(filepath.Join("fixtures", c.path))
+		require.NoError(t, err)
+		res, err := ManifestMatchesDigest(manifest, c.digest)
+		require.NoError(t, err)
+		assert.Equal(t, c.result, res)
+	}
+
+	manifest, err := ioutil.ReadFile("fixtures/v2s1-invalid-signatures.manifest.json")
+	require.NoError(t, err)
+	// Even a correct SHA256 hash is rejected if we can't strip the JSON signature.
+	hash := sha256.Sum256(manifest)
+	res, err := ManifestMatchesDigest(manifest, "sha256:"+hex.EncodeToString(hash[:]))
+	assert.False(t, res)
+	assert.Error(t, err)
+
+	res, err = ManifestMatchesDigest([]byte{}, "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855")
+	assert.True(t, res)
+	assert.NoError(t, err)
+}