Add tests for the tag construction code and make it a bit safer

Check for sane inputs, reject insane ones

src/allmydata/test/test_hashutil.py

@@ -126,6 +126,42 @@ class HashUtilTests(unittest.TestCase):
             base32.a2b(b"2ckv3dfzh6rgjis6ogfqhyxnzy"),
         )
 
+    def test_convergence_hasher_tag(self):
+        """
+        ``_convergence_hasher_tag`` constructs the convergence hasher tag from a
+        unique prefix, the required, total, and segment size parameters, and a
+        convergence secret.
+        """
+        self.assertEqual(
+            "allmydata_immutable_content_to_key_with_added_secret_v1+"
+            "16:\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42\x42,"
+            "9:3,10,1024,",
+            hashutil._convergence_hasher_tag(
+                k=3,
+                n=10,
+                segsize=1024,
+                convergence=b"\x42" * 16,
+            ),
+        )
+
+    def test_convergence_hasher_out_of_bounds(self):
+        """
+        ``_convergence_hasher_tag`` raises ``ValueError`` if k or n is not between
+        1 and 256 inclusive or if k is greater than n.
+        """
+        segsize = 1024
+        secret = b"\x42" * 16
+        for bad_k in (0, 2, 257):
+            with self.assertRaises(ValueError):
+                hashutil._convergence_hasher_tag(
+                    k=bad_k, n=1, segsize=segsize, convergence=secret,
+                )
+        for bad_n in (0, 1, 257):
+            with self.assertRaises(ValueError):
+                hashutil._convergence_hasher_tag(
+                    k=2, n=bad_n, segsize=segsize, convergence=secret,
+                )
+
     def test_known_answers(self):
         """
         Verify backwards compatibility by comparing hash outputs for some

src/allmydata/util/hashutil.py

@@ -176,10 +176,37 @@ def convergence_hash(k, n, segsize, data, convergence):
     return h.digest()
 
 
-def convergence_hasher(k, n, segsize, convergence):
+def _convergence_hasher_tag(k, n, segsize, convergence):
+    """
+    Create the convergence hashing tag.
+
+    :param int k: Required shares.
+    :param int n: Total shares.
+    :param int segsize: Maximum segment size.
+    :param bytes convergence: The convergence secret.
+
+    :return bytes: The bytestring to use as a tag in the convergence hash.
+    """
     assert isinstance(convergence, bytes)
+    if k > n:
+        raise ValueError(
+            "k > n not allowed; k = {}, n = {}".format(k, n),
+        )
+    if k < 1 or n < 1:
+        raise ValueError(
+            "k, n < 1 not allowed; k = {}, n = {}".format(k, n),
+        )
+    if k > 256 or n > 256:
+        raise ValueError(
+            "k, n > 256 not allowed; k = {}, n = {}".format(k, n),
+        )
     param_tag = netstring(b"%d,%d,%d" % (k, n, segsize))
     tag = CONVERGENT_ENCRYPTION_TAG + netstring(convergence) + param_tag
+    return tag
+
+
+def convergence_hasher(k, n, segsize, convergence):
+    tag = _convergence_hasher_tag(k, n, segsize, convergence)
     return tagged_hasher(tag, KEYLEN)