Add bounds checking for Scrypt parameters. (#3130)

* Add bounds checking for Scrypt parameters.

* Pep8.

* More PEP8.

* Change wording.

3 files changed, 30 insertions, 0 deletions

diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index 03260c06..511708d6 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -805,6 +805,9 @@ Different KDFs are suitable for different tasks such as:
         :class:`~cryptography.hazmat.backends.interfaces.ScryptBackend`
 
     :raises TypeError: This exception is raised if ``salt`` is not ``bytes``.
+    :raises ValueError: This exception is raised if ``n`` is less than 2, if
+        ``n`` is not a power of 2, if ``r`` is less than 1 or if ``p`` is less
+        than 1.
 
     .. method:: derive(key_material)
 
diff --git a/src/cryptography/hazmat/primitives/kdf/scrypt.py b/src/cryptography/hazmat/primitives/kdf/scrypt.py
index 09181d97..20935409 100644
--- a/src/cryptography/hazmat/primitives/kdf/scrypt.py
+++ b/src/cryptography/hazmat/primitives/kdf/scrypt.py
@@ -25,6 +25,16 @@ class Scrypt(object):
         self._length = length
         if not isinstance(salt, bytes):
             raise TypeError("salt must be bytes.")
+
+        if n < 2 or (n & (n - 1)) != 0:
+            raise ValueError("n must be greater than 1 and be a power of 2.")
+
+        if r < 1:
+            raise ValueError("r must be greater than or equal to 1.")
+
+        if p < 1:
+            raise ValueError("p must be greater than or equal to 1.")
+
         self._used = False
         self._salt = salt
         self._n = n
diff --git a/tests/hazmat/primitives/test_scrypt.py b/tests/hazmat/primitives/test_scrypt.py
index de4100e3..49b304e0 100644
--- a/tests/hazmat/primitives/test_scrypt.py
+++ b/tests/hazmat/primitives/test_scrypt.py
@@ -117,3 +117,20 @@ class TestScrypt(object):
         scrypt.derive(password)
         with pytest.raises(AlreadyFinalized):
             scrypt.derive(password)
+
+    def test_invalid_n(self, backend):
+        # n is less than 2
+        with pytest.raises(ValueError):
+            Scrypt(b"NaCl", 64, 1, 8, 16, backend)
+
+        # n is not a power of 2
+        with pytest.raises(ValueError):
+            Scrypt(b"NaCl", 64, 3, 8, 16, backend)
+
+    def test_invalid_r(self, backend):
+        with pytest.raises(ValueError):
+            Scrypt(b"NaCl", 64, 2, 0, 16, backend)
+
+    def test_invalid_p(self, backend):
+        with pytest.raises(ValueError):
+            Scrypt(b"NaCl", 64, 2, 8, 0, backend)