Merge heads.

diff --cc Doc/library/hmac.rst
index 085c12b8576d05a4244a0739a3a65b2888fcadf0,a8ca3afe44e961a8db0cb48ec62248901d69dd6f..ce93cbc77755f4a3a8941d9e8524b05268030824
--- 1/Doc/library/hmac.rst
--- 2/Doc/library/hmac.rst
+++ b/Doc/library/hmac.rst
@@@ -70,32 -70,16 +70,32 @@@ This module also provides the followin
  
  .. function:: compare_digest(a, b)
  
 -   Return ``a == b``.  This function uses an approach designed to prevent
 -   timing analysis, making it appropriate for cryptography.  *a* and *b*
 +   Return ``a == b``.  This function uses an approach designed to prevent timing
 +   analysis by avoiding content based short circuiting behaviour, making it
 +   appropriate for cryptography.  *a* and *b*
     must both be of the same type: either :class:`str` (ASCII only, as e.g.
     returned by :meth:`HMAC.hexdigest`), or any type that supports the
-    :term:`buffer protocol` (e.g. :class:`bytes`).
+    buffer protocol (e.g. :class:`bytes`).
  
 +   Using a short circuiting comparison (that is, one that terminates as soon as
 +   it finds any difference between the values) to check digests for correctness
 +   can be problematic, as it introduces a potential vulnerability when an
 +   attacker can control both the message to be checked *and* the purported
 +   signature value.  By keeping the plaintext consistent and supplying different
 +   signature values, an attacker may be able to use timing variations to search
 +   the signature space for the expected value in O(n) time rather than the
 +   desired O(2**n).
 +
     .. note::
 -      If *a* and *b* are different lengths, or if an error occurs,
 -      a timing attack may be able to infer information about the types
 -      and lengths of *a* and *b*, but not their values.
 +
 +      While this function reduces the likelihood of leaking the contents of the
 +      expected digest via a timing attack, it still may leak some timing
 +      information when the input values differ in lengths as well as in error
 +      cases like unsupported types or non ASCII strings.  When the inputs have
 +      different length the timing depends solely on the length of ``b``.  It is
 +      assumed that the expected length of the digest is not a secret, as it is
 +      typically published as part of a file format, network protocol or API
 +      definition.
  
     .. versionadded:: 3.3