speed up HKDF on longer expands by copying the context (#12994)

Author: reaperhulk

src/cryptography/hazmat/primitives/kdf/hkdf.py

@@ -78,8 +78,9 @@ def _expand(self, key_material: utils.Buffer) -> bytes:
         output = [b""]
         counter = 1
 
+        h_prime = hmac.HMAC(key_material, self._algorithm)
         while self._algorithm.digest_size * (len(output) - 1) < self._length:
-            h = hmac.HMAC(key_material, self._algorithm)
+            h = h_prime.copy()
             h.update(output[-1])
             h.update(self._info)
             h.update(bytes([counter]))

tests/bench/test_hkdf.py

@@ -0,0 +1,19 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives.kdf.hkdf import HKDF
+
+
+def test_hkdf(benchmark):
+    def bench():
+        hkdf = HKDF(
+            hashes.SHA512(),
+            16000,
+            salt=b"salt",
+            info=b"info",
+        )
+        hkdf.derive(b"0" * 64)
+
+    benchmark(bench)