Expand, extract, verify.
diff --git a/cryptography/hazmat/primitives/kdf/hkdf.py b/cryptography/hazmat/primitives/kdf/hkdf.py
index c7999f1..71c277f 100644
--- a/cryptography/hazmat/primitives/kdf/hkdf.py
+++ b/cryptography/hazmat/primitives/kdf/hkdf.py
@@ -15,6 +15,7 @@
from cryptography import exceptions
from cryptography.hazmat.primitives import hmac
+from cryptography.hazmat.primitives import constant_time
class HKDF(object):
@@ -44,11 +45,27 @@
self._used = False
+ def extract(self, key_material):
+ if self._used:
+ raise exceptions.AlreadyFinalized
+
+ self._used = True
+
+ return self._extract(key_material)
+
def _extract(self, key_material):
h = hmac.HMAC(self._salt, self._algorithm, backend=self._backend)
h.update(key_material)
return h.finalize()
+ def expand(self, key_material):
+ if self._used:
+ raise exceptions.AlreadyFinalized
+
+ self._used = True
+
+ return self._expand(key_material)
+
def _expand(self, key_material):
output = [b'']
counter = 1
@@ -71,7 +88,5 @@
return self._expand(self._extract(key_material))
def verify(self, key_material, expected_key):
- if self._used:
- raise exceptions.AlreadyFinalized
-
- self._used = True
+ if not constant_time.bytes_eq(self.derive(key_material), expected_key):
+ raise exceptions.InvalidKey