bpo-31933: fix blake2 multi-byte params on big endian platforms (#4250)

All Blake2 params have to be encoded in little-endian byte order. For
the two multi-byte integer params, leaf_length and node_offset, that
means that assigning a native-endian integer to them appears to work on
little-endian platforms, but gives the wrong result on big-endian. The
current libb2 API doesn't make that very clear, and @sneves is working
on new API functions in the GH issue above. In the meantime, we can work
around the problem by explicitly assigning little-endian values to the
parameter block.

See https://github.com/BLAKE2/libb2/issues/12.

Lib/test/test_hashlib.py

@@ -615,6 +615,24 @@ class HashLibTestCase(unittest.TestCase):
           "ba80a53f981c4d0d6a2797b69f12f6e94c212f14685ac4b74b12bb6fdbffa2d1"+
           "7d87c5392aab792dc252d5de4533cc9518d38aa8dbf1925ab92386edd4009923")
 
+    @requires_blake2
+    def test_case_blake2b_all_parameters(self):
+        # This checks that all the parameters work in general, and also that
+        # parameter byte order doesn't get confused on big endian platforms.
+        self.check('blake2b', b"foo",
+          "920568b0c5873b2f0ab67bedb6cf1b2b",
+          digest_size=16,
+          key=b"bar",
+          salt=b"baz",
+          person=b"bing",
+          fanout=2,
+          depth=3,
+          leaf_size=4,
+          node_offset=5,
+          node_depth=6,
+          inner_size=7,
+          last_node=True)
+
     @requires_blake2
     def test_blake2b_vectors(self):
         for msg, key, md in read_vectors('blake2b'):
@@ -640,6 +658,24 @@ class HashLibTestCase(unittest.TestCase):
         self.check('blake2s', b"abc",
           "508c5e8c327c14e2e1a72ba34eeb452f37458b209ed63a294d999b4c86675982")
 
+    @requires_blake2
+    def test_case_blake2s_all_parameters(self):
+        # This checks that all the parameters work in general, and also that
+        # parameter byte order doesn't get confused on big endian platforms.
+        self.check('blake2s', b"foo",
+          "bf2a8f7fe3c555012a6f8046e646bc75",
+          digest_size=16,
+          key=b"bar",
+          salt=b"baz",
+          person=b"bing",
+          fanout=2,
+          depth=3,
+          leaf_size=4,
+          node_offset=5,
+          node_depth=6,
+          inner_size=7,
+          last_node=True)
+
     @requires_blake2
     def test_blake2s_vectors(self):
         for msg, key, md in read_vectors('blake2s'):