bpo-32433: Optimized HMAC digest (#5023)

The hmac module now has hmac.digest(), which provides an optimized HMAC
digest for short messages. hmac.digest() is up to three times faster
than hmac.HMAC().digest().

Signed-off-by: Christian Heimes <christian@python.org>

Lib/hmac.py

@@ -5,6 +5,13 @@ Implements the HMAC algorithm as described by RFC 2104.
 
 import warnings as _warnings
 from _operator import _compare_digest as compare_digest
+try:
+    import _hashlib as _hashopenssl
+except ImportError:
+    _hashopenssl = None
+    _openssl_md_meths = None
+else:
+    _openssl_md_meths = frozenset(_hashopenssl.openssl_md_meth_names)
 import hashlib as _hashlib
 
 trans_5C = bytes((x ^ 0x5C) for x in range(256))
@@ -142,3 +149,38 @@ def new(key, msg = None, digestmod = None):
     method.
     """
     return HMAC(key, msg, digestmod)
+
+
+def digest(key, msg, digest):
+    """Fast inline implementation of HMAC
+
+    key:    key for the keyed hash object.
+    msg:    input message
+    digest: A hash name suitable for hashlib.new() for best performance. *OR*
+            A hashlib constructor returning a new hash object. *OR*
+            A module supporting PEP 247.
+
+    Note: key and msg must be a bytes or bytearray objects.
+    """
+    if (_hashopenssl is not None and
+            isinstance(digest, str) and digest in _openssl_md_meths):
+        return _hashopenssl.hmac_digest(key, msg, digest)
+
+    if callable(digest):
+        digest_cons = digest
+    elif isinstance(digest, str):
+        digest_cons = lambda d=b'': _hashlib.new(digest, d)
+    else:
+        digest_cons = lambda d=b'': digest.new(d)
+
+    inner = digest_cons()
+    outer = digest_cons()
+    blocksize = getattr(inner, 'block_size', 64)
+    if len(key) > blocksize:
+        key = digest_cons(key).digest()
+    key = key + b'\x00' * (blocksize - len(key))
+    inner.update(key.translate(trans_36))
+    outer.update(key.translate(trans_5C))
+    inner.update(msg)
+    outer.update(inner.digest())
+    return outer.digest()