Reject variable-length (SHAKE) checksum algorithms instead of crashing

bagit advertised the SHAKE algorithms (shake_128, shake_256) as usable
manifest checksums because they are in hashlib.algorithms_guaranteed, but
their hexdigest() needs a length argument that bagit never supplies, so
make_bag crashed with a TypeError partway through. Detect variable-length
digests by behavior (not by name) and filter them out of CHECKSUM_ALGOS,
raise a clear BagError if one is requested directly, and skip them in
get_hashers as a safeguard. Closes #158.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

Author: arpitjain099

src/bagit/__init__.py

@@ -120,12 +120,38 @@ def find_locale_dir():
     # Payload-Oxum is autogenerated
 ]
 
+
+def _has_fixed_length_digest(alg):
+    """
+    Return True if the named algorithm produces a fixed-length digest.
+
+    The variable-length algorithms (the SHAKE family) require a ``length``
+    argument to ``hexdigest()`` that bagit never supplies, so they cannot be
+    used for manifests. We detect that by behavior rather than by name so that
+    any future variable-length algorithm hashlib adds is handled too.
+    """
+
+    try:
+        hashlib.new(alg).hexdigest()
+    except TypeError:
+        # Variable-length digest, e.g. shake_128/shake_256
+        return False
+    except ValueError:
+        # Unsupported algorithm; leave it for the existing warning path
+        return True
+    return True
+
+
 try:
-    CHECKSUM_ALGOS = hashlib.algorithms_guaranteed
+    CHECKSUM_ALGOS = {
+        alg for alg in hashlib.algorithms_guaranteed if _has_fixed_length_digest(alg)
+    }
 except AttributeError:
     # FIXME: remove when we drop Python 2 (https://github.com/LibraryOfCongress/bagit-python/issues/102)
     # Python 2.7.0-2.7.8
-    CHECKSUM_ALGOS = set(hashlib.algorithms)
+    CHECKSUM_ALGOS = {
+        alg for alg in hashlib.algorithms if _has_fixed_length_digest(alg)
+    }
 DEFAULT_CHECKSUMS = ["sha256", "sha512"]
 
 #: Block size used when reading files for hashing:
@@ -160,6 +186,13 @@ def make_bag(
     if checksums is None:
         checksums = DEFAULT_CHECKSUMS
 
+    variable_length = [alg for alg in checksums if not _has_fixed_length_digest(alg)]
+    if variable_length:
+        raise BagError(
+            _("Cannot create manifests with variable-length digest algorithm(s): %s")
+            % ", ".join(sorted(variable_length))
+        )
+
     bag_dir = os.path.abspath(bag_dir)
     cwd = os.path.abspath(os.path.curdir)
 
@@ -1129,6 +1162,15 @@ def get_hashers(algorithms):
             )
             continue
 
+        if not _has_fixed_length_digest(alg):
+            LOGGER.warning(
+                _(
+                    "Disabling requested hash algorithm %s: it has a variable-length digest"
+                ),
+                alg,
+            )
+            continue
+
         hashers[alg] = hasher
 
     if not hashers:

test.py

@@ -96,6 +96,31 @@ def test_make_bag_md5_sha1_sha256_manifest(self):
         # check valid with three manifests
         self.assertTrue(self.validate(bag, fast=True))
 
+    def test_make_bag_shake_raises_bagerror(self):
+        # SHAKE algorithms have a variable-length digest and cannot be used for
+        # manifests; make_bag should reject them cleanly instead of crashing
+        # with a TypeError later on. See issue #158.
+        for alg in ("shake_128", "shake_256"):
+            self.assertRaises(
+                bagit.BagError, bagit.make_bag, self.tmpdir, checksums=[alg]
+            )
+
+    def test_make_bag_shake_mixed_with_valid_raises_bagerror(self):
+        # A SHAKE algorithm mixed in with a usable one should still be rejected
+        # rather than crashing in the tagmanifest path.
+        self.assertRaises(
+            bagit.BagError,
+            bagit.make_bag,
+            self.tmpdir,
+            checksums=["sha256", "shake_128"],
+        )
+
+    def test_shake_not_offered_as_checksum_algorithm(self):
+        # Variable-length algorithms must not be advertised, so they are never
+        # auto-discovered as a manifest type or exposed as a CLI flag.
+        self.assertNotIn("shake_128", bagit.CHECKSUM_ALGOS)
+        self.assertNotIn("shake_256", bagit.CHECKSUM_ALGOS)
+
     def test_validate_flipped_bit(self):
         bag = bagit.make_bag(self.tmpdir)
         readme = j(self.tmpdir, "data", "README")