Add Crypto.Protocol.KDF.HKDFExpand

Doc/src/protocol/kdf.rst

@@ -125,6 +125,24 @@ Example, for deriving two AES256 keys::
 
 .. autofunction:: Crypto.Protocol.KDF.HKDF
 
+.. _hkdfexpand:
+
+HKDFExpand
++++++
+
+HKDF consists of two stages, extract and expand. This class exposes an
+expand only version of HKDF that is suitable when the key material is
+already cryptographically strong.
+
+Example, for deriving a key with context::
+
+    from Crypto.Protocol.KDF import HKDFExpand
+    from Crypto.Hash import SHA512
+
+    app_key = HKDFExpand(pseudorandom_key, 32, SHA512, context=b"app key")
+
+.. autofunction:: Crypto.Protocol.KDF.HKDFExpand
+
 .. _sp800-180-counter:
 
 SP 800-180 Counter Mode

lib/Crypto/Protocol/KDF.py

@@ -315,14 +315,49 @@ def HKDF(master, key_len, salt, hashmod, num_keys=1, context=None):
         raise ValueError("Too much secret data to derive")
     if not salt:
         salt = b'\x00' * hashmod.digest_size
-    if context is None:
-        context = b""
 
     # Step 1: extract
     hmac = HMAC.new(salt, master, digestmod=hashmod)
     prk = hmac.digest()
 
     # Step 2: expand
+    return HKDFExpand(prk, key_len, hashmod, num_keys, context)
+
+
+def HKDFExpand(prk, key_len, hashmod, num_keys=1, context=None):
+    """HKDF consists of two stages, extract and expand.
+    This class exposes an expand only version of HKDF that
+    is suitable when the key material is already
+    cryptographically strong.
+
+    Args:
+     prk (byte string):
+        Short, cryptographically strong, pseudorandom key material.
+        It must not be a password.
+     key_len (integer):
+        The length in bytes of every derived key.
+     hashmod (module):
+        A cryptographic hash algorithm from :mod:`Crypto.Hash`.
+        :mod:`Crypto.Hash.SHA512` is a good choice.
+     num_keys (integer):
+        The number of keys to derive. Every key is :data:`key_len` bytes long.
+        The maximum cumulative length of all keys is
+        255 times the digest size.
+     context (byte string):
+        Optional identifier describing what the keys are used for.
+
+    Return:
+        A byte string or a tuple of byte strings.
+
+    .. _RFC5869: http://tools.ietf.org/html/rfc5869
+    """
+
+    output_len = key_len * num_keys
+    if output_len > (255 * hashmod.digest_size):
+        raise ValueError("Too much secret data to derive")
+    if context is None:
+        context = b""
+
     t = [ b"" ]
     n = 1
     tlen = 0
@@ -339,7 +374,6 @@ def HKDF(master, key_len, salt, hashmod, num_keys=1, context=None):
     return list(kol[:num_keys])
 
 
-
 def scrypt(password, salt, key_len, N, r, p, num_keys=1):
     """Derive one or more keys from a passphrase.
 

lib/Crypto/Protocol/KDF.pyi

@@ -19,6 +19,7 @@ class _S2V(object):
     def derive(self) -> bytes: ...
 
 def HKDF(master: bytes, key_len: int, salt: bytes, hashmod: ModuleType, num_keys: Optional[int]=1, context: Optional[bytes]=None) -> Union[bytes, Tuple[bytes, ...]]: ...
+def HKDFExpand(prk: bytes, key_len: int, hashmod: ModuleType, num_keys: Optional[int]=1, context: Optional[bytes]=None) -> Union[bytes, Tuple[bytes, ...]]: ...
 
 def scrypt(password: str, salt: str, key_len: int, N: int, r: int, p: int, num_keys: Optional[int]=1) -> Union[bytes, Tuple[bytes, ...]]: ...