Merge "Use "multihash" for data download validation"

8 files changed, 394 insertions, 24 deletions

diff --git a/glanceclient/common/utils.py b/glanceclient/common/utils.py
index dee9978..c92836e 100644
--- a/glanceclient/common/utils.py
+++ b/glanceclient/common/utils.py
@@ -449,6 +449,26 @@ def integrity_iter(iter, checksum):
                       (md5sum, checksum))
 
 
+def serious_integrity_iter(iter, hasher, hash_value):
+    """Check image data integrity using the Glance "multihash".
+
+    :param iter: iterable containing the image data
+    :param hasher: a hashlib object
+    :param hash_value: hexdigest of the image data
+    :raises: IOError if the hashdigest of the data is not hash_value
+    """
+    for chunk in iter:
+        yield chunk
+        if isinstance(chunk, six.string_types):
+            chunk = six.b(chunk)
+        hasher.update(chunk)
+    computed = hasher.hexdigest()
+    if computed != hash_value:
+        raise IOError(errno.EPIPE,
+                      'Corrupt image download. Hash was %s expected %s' %
+                      (computed, hash_value))
+
+
 def memoized_property(fn):
     attr_name = '_lazy_once_' + fn.__name__
 
diff --git a/glanceclient/tests/unit/test_shell.py b/glanceclient/tests/unit/test_shell.py
index 0f15007..3027e46 100644
--- a/glanceclient/tests/unit/test_shell.py
+++ b/glanceclient/tests/unit/test_shell.py
@@ -965,7 +965,9 @@ class ShellTestRequests(testutils.TestCase):
             self.requests = self.useFixture(rm_fixture.Fixture())
             self.requests.get('http://example.com/v2/images/%s/file' % id,
                               headers=headers, raw=fake)
-
+            self.requests.get('http://example.com/v2/images/%s' % id,
+                              headers={'Content-type': 'application/json'},
+                              json=image_show_fixture)
             shell = openstack_shell.OpenStackImagesShell()
             argstr = ('--os-image-api-version 2 --os-auth-token faketoken '
                       '--os-image-url http://example.com '
diff --git a/glanceclient/tests/unit/v2/fixtures.py b/glanceclient/tests/unit/v2/fixtures.py
index 5a603c0..22e1ff7 100644
--- a/glanceclient/tests/unit/v2/fixtures.py
+++ b/glanceclient/tests/unit/v2/fixtures.py
@@ -14,6 +14,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import hashlib
+
+
 UUID = "3fc2ba62-9a02-433e-b565-d493ffc69034"
 
 image_list_fixture = {
@@ -65,7 +68,9 @@ image_show_fixture = {
     "tags": [],
     "updated_at": "2015-07-24T12:18:13Z",
     "virtual_size": "null",
-    "visibility": "shared"
+    "visibility": "shared",
+    "os_hash_algo": "sha384",
+    "os_hash_value": hashlib.sha384(b'DATA').hexdigest()
 }
 
 image_create_fixture = {
diff --git a/glanceclient/tests/unit/v2/test_images.py b/glanceclient/tests/unit/v2/test_images.py
index 23cbb43..753f3c4 100644
--- a/glanceclient/tests/unit/v2/test_images.py
+++ b/glanceclient/tests/unit/v2/test_images.py
@@ -14,6 +14,7 @@
 #    under the License.
 
 import errno
+import hashlib
 import mock
 import testtools
 
@@ -193,7 +194,43 @@ data_fixtures = {
             'A',
         ),
     },
-    '/v2/images/66fb18d6-db27-11e1-a1eb-080027cbe205/file': {
+    '/v2/images/5cc4bebc-db27-11e1-a1eb-080027cbe205': {
+        'GET': (
+            {},
+            {},
+        ),
+    },
+    '/v2/images/headeronly-db27-11e1-a1eb-080027cbe205/file': {
+        'GET': (
+            {
+                'content-md5': 'wrong'
+            },
+            'BB',
+        ),
+    },
+    '/v2/images/headeronly-db27-11e1-a1eb-080027cbe205': {
+        'GET': (
+            {},
+            {},
+        ),
+    },
+    '/v2/images/chkonly-db27-11e1-a1eb-080027cbe205/file': {
+        'GET': (
+            {
+                'content-md5': 'wrong'
+            },
+            'BB',
+        ),
+    },
+    '/v2/images/chkonly-db27-11e1-a1eb-080027cbe205': {
+        'GET': (
+            {},
+            {
+                'checksum': 'wrong',
+            },
+        ),
+    },
+    '/v2/images/multihash-db27-11e1-a1eb-080027cbe205/file': {
         'GET': (
             {
                 'content-md5': 'wrong'
@@ -201,7 +238,67 @@ data_fixtures = {
             'BB',
         ),
     },
-    '/v2/images/1b1c6366-dd57-11e1-af0f-02163e68b1d8/file': {
+    '/v2/images/multihash-db27-11e1-a1eb-080027cbe205': {
+        'GET': (
+            {},
+            {
+                'checksum': 'wrong',
+                'os_hash_algo': 'md5',
+                'os_hash_value': 'junk'
+            },
+        ),
+    },
+    '/v2/images/badalgo-db27-11e1-a1eb-080027cbe205/file': {
+        'GET': (
+            {
+                'content-md5': hashlib.md5(b'BB').hexdigest()
+            },
+            'BB',
+        ),
+    },
+    '/v2/images/badalgo-db27-11e1-a1eb-080027cbe205': {
+        'GET': (
+            {},
+            {
+                'checksum': hashlib.md5(b'BB').hexdigest(),
+                'os_hash_algo': 'not_an_algo',
+                'os_hash_value': 'whatever'
+            },
+        ),
+    },
+    '/v2/images/bad-multihash-value-good-checksum/file': {
+        'GET': (
+            {
+                'content-md5': hashlib.md5(b'GOODCHECKSUM').hexdigest()
+            },
+            'GOODCHECKSUM',
+        ),
+    },
+    '/v2/images/bad-multihash-value-good-checksum': {
+        'GET': (
+            {},
+            {
+                'checksum': hashlib.md5(b'GOODCHECKSUM').hexdigest(),
+                'os_hash_algo': 'sha512',
+                'os_hash_value': 'badmultihashvalue'
+            },
+        ),
+    },
+    '/v2/images/headeronly-dd57-11e1-af0f-02163e68b1d8/file': {
+        'GET': (
+            {
+                'content-md5': 'defb99e69a9f1f6e06f15006b1f166ae'
+            },
+            'CCC',
+        ),
+    },
+    '/v2/images/headeronly-dd57-11e1-af0f-02163e68b1d8': {
+        'GET': (
+            {},
+            {},
+        ),
+    },
+    '/v2/images/chkonly-dd57-11e1-af0f-02163e68b1d8/file': {
         'GET': (
             {
                 'content-md5': 'defb99e69a9f1f6e06f15006b1f166ae'
@@ -209,6 +306,32 @@ data_fixtures = {
             'CCC',
         ),
     },
+    '/v2/images/chkonly-dd57-11e1-af0f-02163e68b1d8': {
+        'GET': (
+            {},
+            {
+                'checksum': 'defb99e69a9f1f6e06f15006b1f166ae',
+            },
+        ),
+    },
+    '/v2/images/multihash-dd57-11e1-af0f-02163e68b1d8/file': {
+        'GET': (
+            {
+                'content-md5': 'defb99e69a9f1f6e06f15006b1f166ae'
+            },
+            'CCC',
+        ),
+    },
+    '/v2/images/multihash-dd57-11e1-af0f-02163e68b1d8': {
+        'GET': (
+            {},
+            {
+                'checksum': 'defb99e69a9f1f6e06f15006b1f166ae',
+                'os_hash_algo': 'sha384',
+                'os_hash_value': hashlib.sha384(b'CCC').hexdigest()
+            },
+        ),
+    },
     '/v2/images/87b634c1-f893-33c9-28a9-e5673c99239a/actions/reactivate': {
         'POST': ({}, None)
     },
@@ -846,12 +969,24 @@ class TestController(testtools.TestCase):
         self.assertEqual('A', body)
 
     def test_data_with_wrong_checksum(self):
-        body = self.controller.data('66fb18d6-db27-11e1-a1eb-080027cbe205',
+        body = self.controller.data('headeronly-db27-11e1-a1eb-080027cbe205',
                                     do_checksum=False)
         body = ''.join([b for b in body])
         self.assertEqual('BB', body)
+        body = self.controller.data('headeronly-db27-11e1-a1eb-080027cbe205')
+        try:
+            body = ''.join([b for b in body])
+            self.fail('data did not raise an error.')
+        except IOError as e:
+            self.assertEqual(errno.EPIPE, e.errno)
+            msg = 'was 9d3d9048db16a7eee539e93e3618cbe7 expected wrong'
+            self.assertIn(msg, str(e))
 
-        body = self.controller.data('66fb18d6-db27-11e1-a1eb-080027cbe205')
+        body = self.controller.data('chkonly-db27-11e1-a1eb-080027cbe205',
+                                    do_checksum=False)
+        body = ''.join([b for b in body])
+        self.assertEqual('BB', body)
+        body = self.controller.data('chkonly-db27-11e1-a1eb-080027cbe205')
         try:
             body = ''.join([b for b in body])
             self.fail('data did not raise an error.')
@@ -860,15 +995,103 @@ class TestController(testtools.TestCase):
             msg = 'was 9d3d9048db16a7eee539e93e3618cbe7 expected wrong'
             self.assertIn(msg, str(e))
 
-    def test_data_with_checksum(self):
-        body = self.controller.data('1b1c6366-dd57-11e1-af0f-02163e68b1d8',
+        body = self.controller.data('multihash-db27-11e1-a1eb-080027cbe205',
+                                    do_checksum=False)
+        body = ''.join([b for b in body])
+        self.assertEqual('BB', body)
+        body = self.controller.data('multihash-db27-11e1-a1eb-080027cbe205')
+        try:
+            body = ''.join([b for b in body])
+            self.fail('data did not raise an error.')
+        except IOError as e:
+            self.assertEqual(errno.EPIPE, e.errno)
+            msg = 'was 9d3d9048db16a7eee539e93e3618cbe7 expected junk'
+            self.assertIn(msg, str(e))
+
+        body = self.controller.data('badalgo-db27-11e1-a1eb-080027cbe205',
                                     do_checksum=False)
         body = ''.join([b for b in body])
-        self.assertEqual('CCC', body)
+        self.assertEqual('BB', body)
+        try:
+            body = self.controller.data('badalgo-db27-11e1-a1eb-080027cbe205')
+            self.fail('bad os_hash_algo did not raise an error.')
+        except ValueError as e:
+            msg = 'unsupported hash type not_an_algo'
+            self.assertIn(msg, str(e))
+
+    def test_data_with_checksum(self):
+        for prefix in ['headeronly', 'chkonly', 'multihash']:
+            body = self.controller.data(prefix +
+                                        '-dd57-11e1-af0f-02163e68b1d8',
+                                        do_checksum=False)
+            body = ''.join([b for b in body])
+            self.assertEqual('CCC', body)
+
+            body = self.controller.data(prefix +
+                                        '-dd57-11e1-af0f-02163e68b1d8')
+            body = ''.join([b for b in body])
+            self.assertEqual('CCC', body)
+
+    def test_data_with_checksum_and_fallback(self):
+        # make sure the allow_md5_fallback option does not cause any
+        # incorrect behavior when fallback is not needed
+        for prefix in ['headeronly', 'chkonly', 'multihash']:
+            body = self.controller.data(prefix +
+                                        '-dd57-11e1-af0f-02163e68b1d8',
+                                        do_checksum=False,
+                                        allow_md5_fallback=True)
+            body = ''.join([b for b in body])
+            self.assertEqual('CCC', body)
 
-        body = self.controller.data('1b1c6366-dd57-11e1-af0f-02163e68b1d8')
+            body = self.controller.data(prefix +
+                                        '-dd57-11e1-af0f-02163e68b1d8',
+                                        allow_md5_fallback=True)
+            body = ''.join([b for b in body])
+            self.assertEqual('CCC', body)
+
+    def test_data_with_bad_hash_algo_and_fallback(self):
+        # shouldn't matter when do_checksum is False
+        body = self.controller.data('badalgo-db27-11e1-a1eb-080027cbe205',
+                                    do_checksum=False,
+                                    allow_md5_fallback=True)
+        body = ''.join([b for b in body])
+        self.assertEqual('BB', body)
+
+        # default value for do_checksum is True
+        body = self.controller.data('badalgo-db27-11e1-a1eb-080027cbe205',
+                                    allow_md5_fallback=True)
+        body = ''.join([b for b in body])
+        self.assertEqual('BB', body)
+
+    def test_neg_data_with_bad_hash_value_and_fallback_enabled(self):
+        # make sure download fails when good hash_algo but bad hash_value
+        # even when correct checksum is present regardless of
+        # allow_md5_fallback setting
+        body = self.controller.data('bad-multihash-value-good-checksum',
+                                    allow_md5_fallback=False)
+        try:
+            body = ''.join([b for b in body])
+            self.fail('bad os_hash_value did not raise an error.')
+        except IOError as e:
+            self.assertEqual(errno.EPIPE, e.errno)
+            msg = 'expected badmultihashvalue'
+            self.assertIn(msg, str(e))
+
+        body = self.controller.data('bad-multihash-value-good-checksum',
+                                    allow_md5_fallback=True)
+        try:
+            body = ''.join([b for b in body])
+            self.fail('bad os_hash_value did not raise an error.')
+        except IOError as e:
+            self.assertEqual(errno.EPIPE, e.errno)
+            msg = 'expected badmultihashvalue'
+            self.assertIn(msg, str(e))
+
+        # download should succeed when do_checksum is off, though
+        body = self.controller.data('bad-multihash-value-good-checksum',
+                                    do_checksum=False)
         body = ''.join([b for b in body])
-        self.assertEqual('CCC', body)
+        self.assertEqual('GOODCHECKSUM', body)
 
     def test_image_import(self):
         uri = 'http://example.com/image.qcow'
@@ -883,7 +1106,7 @@ class TestController(testtools.TestCase):
     def test_download_no_data(self):
         resp = utils.FakeResponse(headers={}, status_code=204)
         self.controller.controller.http_client.get = mock.Mock(
-            return_value=(resp, None))
+            return_value=(resp, {}))
         self.controller.data('image_id')
 
     def test_download_forbidden(self):
diff --git a/glanceclient/tests/unit/v2/test_shell_v2.py b/glanceclient/tests/unit/v2/test_shell_v2.py
index 6eeca83..acf93bf 100644
--- a/glanceclient/tests/unit/v2/test_shell_v2.py
+++ b/glanceclient/tests/unit/v2/test_shell_v2.py
@@ -1729,7 +1729,8 @@ class ShellV2Test(testtools.TestCase):
 
     def test_image_download(self):
         args = self._make_args(
-            {'id': 'IMG-01', 'file': 'test', 'progress': True})
+            {'id': 'IMG-01', 'file': 'test', 'progress': True,
+             'allow_md5_fallback': False})
 
         with mock.patch.object(self.gc.images, 'data') as mocked_data, \
                 mock.patch.object(utils, '_extract_request_id'):
@@ -1737,14 +1738,27 @@ class ShellV2Test(testtools.TestCase):
                 [c for c in 'abcdef'])
 
             test_shell.do_image_download(self.gc, args)
-            mocked_data.assert_called_once_with('IMG-01')
+            mocked_data.assert_called_once_with('IMG-01',
+                                                allow_md5_fallback=False)
+
+        # check that non-default value is being passed correctly
+        args.allow_md5_fallback = True
+        with mock.patch.object(self.gc.images, 'data') as mocked_data, \
+                mock.patch.object(utils, '_extract_request_id'):
+            mocked_data.return_value = utils.RequestIdProxy(
+                [c for c in 'abcdef'])
+
+            test_shell.do_image_download(self.gc, args)
+            mocked_data.assert_called_once_with('IMG-01',
+                                                allow_md5_fallback=True)
 
     @mock.patch.object(utils, 'exit')
     @mock.patch('sys.stdout', autospec=True)
     def test_image_download_no_file_arg(self, mocked_stdout,
                                         mocked_utils_exit):
         # Indicate that no file name was given as command line argument
-        args = self._make_args({'id': '1234', 'file': None, 'progress': False})
+        args = self._make_args({'id': '1234', 'file': None, 'progress': False,
+                                'allow_md5_fallback': False})
         # Indicate that no file is specified for output redirection
         mocked_stdout.isatty = lambda: True
         test_shell.do_image_download(self.gc, args)
@@ -1835,7 +1849,8 @@ class ShellV2Test(testtools.TestCase):
     def test_do_image_download_with_forbidden_id(self, mocked_print_err,
                                                  mocked_stdout):
         args = self._make_args({'id': 'IMG-01', 'file': None,
-                                'progress': False})
+                                'progress': False,
+                                'allow_md5_fallback': False})
         mocked_stdout.isatty = lambda: False
         with mock.patch.object(self.gc.images, 'data') as mocked_data:
             mocked_data.side_effect = exc.HTTPForbidden
@@ -1852,7 +1867,8 @@ class ShellV2Test(testtools.TestCase):
     @mock.patch.object(utils, 'print_err')
     def test_do_image_download_with_500(self, mocked_print_err, mocked_stdout):
         args = self._make_args({'id': 'IMG-01', 'file': None,
-                                'progress': False})
+                                'progress': False,
+                                'allow_md5_fallback': False})
         mocked_stdout.isatty = lambda: False
         with mock.patch.object(self.gc.images, 'data') as mocked_data:
             mocked_data.side_effect = exc.HTTPInternalServerError
diff --git a/glanceclient/v2/images.py b/glanceclient/v2/images.py
index be804a2..09d46b9 100644
--- a/glanceclient/v2/images.py
+++ b/glanceclient/v2/images.py
@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import hashlib
 import json
 from oslo_utils import encodeutils
 from requests import codes
@@ -197,13 +198,39 @@ class Controller(object):
         return self._get(image_id)
 
     @utils.add_req_id_to_object()
-    def data(self, image_id, do_checksum=True):
+    def data(self, image_id, do_checksum=True, allow_md5_fallback=False):
         """Retrieve data of an image.
 
-        :param image_id:    ID of the image to download.
-        :param do_checksum: Enable/disable checksum validation.
-        :returns: An iterable body or None
+        When do_checksum is enabled, validation proceeds as follows:
+
+        1. if the image has a 'os_hash_value' property, the algorithm
+           specified in the image's 'os_hash_algo' property will be used
+           to validate against the 'os_hash_value' value.  If the
+           specified hash algorithm is not available AND allow_md5_fallback
+           is True, then continue to step #2
+        2. else if the image has a checksum property, MD5 is used to
+           validate against the 'checksum' value
+        3. else if the download response has a 'content-md5' header, MD5
+           is used to validate against the header value
+        4. if none of 1-3 obtain, the data is **not validated** (this is
+           compatible with legacy behavior)
+
+        :param image_id:    ID of the image to download
+        :param do_checksum: Enable/disable checksum validation
+        :param allow_md5_fallback:
+            Use the MD5 checksum for validation if the algorithm specified by
+            the image's 'os_hash_algo' property is not available
+        :returns: An iterable body or ``None``
         """
+        if do_checksum:
+            # doing this first to prevent race condition if image record
+            # is deleted during the image download
+            url = '/v2/images/%s' % image_id
+            resp, image_meta = self.http_client.get(url)
+            meta_checksum = image_meta.get('checksum', None)
+            meta_hash_value = image_meta.get('os_hash_value', None)
+            meta_hash_algo = image_meta.get('os_hash_algo', None)
+
         url = '/v2/images/%s/file' % image_id
         resp, body = self.http_client.get(url)
         if resp.status_code == codes.no_content:
@@ -212,8 +239,32 @@ class Controller(object):
         checksum = resp.headers.get('content-md5', None)
         content_length = int(resp.headers.get('content-length', 0))
 
-        if do_checksum and checksum is not None:
-            body = utils.integrity_iter(body, checksum)
+        check_md5sum = do_checksum
+        if do_checksum and meta_hash_value is not None:
+            try:
+                hasher = hashlib.new(str(meta_hash_algo))
+                body = utils.serious_integrity_iter(body,
+                                                    hasher,
+                                                    meta_hash_value)
+                check_md5sum = False
+            except ValueError as ve:
+                if (str(ve).startswith('unsupported hash type') and
+                        allow_md5_fallback):
+                    check_md5sum = True
+                else:
+                    raise
+
+        if do_checksum and check_md5sum:
+            if meta_checksum is not None:
+                body = utils.integrity_iter(body, meta_checksum)
+            elif checksum is not None:
+                body = utils.integrity_iter(body, checksum)
+            else:
+                # NOTE(rosmaita): this preserves legacy behavior to return the
+                # image data when checksumming is requested but there's no
+                # 'content-md5' header in the response.  Just want to make it
+                # clear that we're doing this on purpose.
+                pass
 
         return utils.IterableWithLength(body, content_length), resp
 
diff --git a/glanceclient/v2/shell.py b/glanceclient/v2/shell.py
index aaa85bb..54dd789 100644
--- a/glanceclient/v2/shell.py
+++ b/glanceclient/v2/shell.py
@@ -490,6 +490,17 @@ def do_stores_info(gc, args):
         utils.print_dict(stores_info)
 
 
+@utils.arg('--allow-md5-fallback', action='store_true',
+           default=utils.env('OS_IMAGE_ALLOW_MD5_FALLBACK', default=False),
+           help=_('If os_hash_algo and os_hash_value properties are available '
+                  'on the image, they will be used to validate the downloaded '
+                  'image data. If the indicated secure hash algorithm is not '
+                  'available on the client, the download will fail. Use this '
+                  'flag to indicate that in such a case the legacy MD5 image '
+                  'checksum should be used to validate the downloaded data. '
+                  'You can also set the enviroment variable '
+                  'OS_IMAGE_ALLOW_MD5_FALLBACK to any value to activate this '
+                  'option.'))
 @utils.arg('--file', metavar='<FILE>',
            help=_('Local file to save downloaded image data to. '
                   'If this is not specified and there is no redirection '
@@ -506,7 +517,8 @@ def do_image_download(gc, args):
         utils.exit(msg)
 
     try:
-        body = gc.images.data(args.id)
+        body = gc.images.data(args.id,
+                              allow_md5_fallback=args.allow_md5_fallback)
     except (exc.HTTPForbidden, exc.HTTPException) as e:
         msg = "Unable to download image '%s'. (%s)" % (args.id, e)
         utils.exit(msg)
diff --git a/releasenotes/notes/multihash-download-verification-596e91bf7b68e7db.yaml b/releasenotes/notes/multihash-download-verification-596e91bf7b68e7db.yaml
new file mode 100644
index 0000000..f32b4a9
--- /dev/null
+++ b/releasenotes/notes/multihash-download-verification-596e91bf7b68e7db.yaml
@@ -0,0 +1,41 @@
+---
+features:
+  - |
+    This release adds verification of image data downloads using the Glance
+    "multihash" feature introduced in the OpenStack Rocky release.  When
+    the ``os_hash_value`` is populated on an image, the glanceclient will
+    verify this value by computing the hexdigest of the downloaded data
+    using the algorithm specified by the image's ``os_hash_algo`` property.
+
+    Because the secure hash algorithm specified is determined by the cloud
+    provider, it is possible that the ``os_hash_algo`` may identify an
+    algorithm not available in the version of the Python ``hashlib`` library
+    used by the client.  In such a case the download will fail due to an
+    unsupported hash type.  In the event this occurs, a new option,
+    ``--allow-md5-fallback``, is introduced to the ``image-download`` command.
+    When present, this option will allow the glanceclient to use the legacy
+    MD5 checksum to verify the downloaded data if the secure hash algorithm
+    specified by the ``os_hash_algo`` image property is not supported.
+
+    Note that the fallback is *not* used in the case where the algorithm is
+    supported but the hexdigest of the downloaded data does not match the
+    ``os_hash_value``.  In that case the download fails regardless of whether
+    the option is present or not.
+
+    Whether using the ``--allow-md5-fallback`` option is a good idea depends
+    upon the user's expectations for the verification.  MD5 is an insecure
+    hashing algorithm, so if you are interested in making sure that the
+    downloaded image data has not been replaced by a datastream carefully
+    crafted to have the same MD5 checksum, then you should not use the
+    fallback.  If, however, you are using Glance in a trusted environment
+    and your interest is simply to verify that no bits have flipped during
+    the data transfer, the MD5 fallback is sufficient for that purpose.  That
+    being said, it is our recommendation that the multihash should be used
+    whenever possible.
+security:
+  - |
+    This release of the glanceclient uses the Glance "multihash" feature,
+    introduced in Rocky, to use a secure hashing algorithm to verify the
+    integrity of downloaded data.  Legacy images without the "multihash"
+    image properties (``os_hash_algo`` and ``os_hash_value``) are verified
+    using the MD5 ``checksum`` image property.