Added internal python-only HKDF

RNS/Cryptography/HKDF.py

@@ -0,0 +1,35 @@
+import hashlib
+from math import ceil
+from RNS.Cryptography import HMAC
+
+def hkdf(length=None, derive_from=None, salt=None, context=None):
+    hash_len = 32
+
+    def hmac_sha256(key, data):
+        return HMAC.new(key, data, hashlib.sha256).digest()
+
+    if length == None or length < 1:
+        raise ValueError("Invalid output key length")
+
+    if derive_from == "None" or derive_from == "":
+        raise ValueError("Cannot derive key from empty input material")
+
+    if salt == None or len(salt) == 0:
+        salt = bytes([0] * hash_len)
+
+    if salt == None:
+        salt = b""
+
+    if context == None:
+        context = b""
+
+    pseudorandom_key = hmac_sha256(salt, derive_from)
+
+    block = b""
+    derived = b""
+
+    for i in range(ceil(length / hash_len)):
+        block = hmac_sha256(pseudorandom_key, block + context + bytes([i + 1]))
+        derived += block
+
+    return derived[:length]