backport hmac.compare_digest to partially implement PEP 466 (closes #21306)

Backport from Alex Gaynor.

1 files changed, 33 insertions, 0 deletions

diff --git a/Doc/library/hmac.rst b/Doc/library/hmac.rst
index f88a03e..09e819a 100644
--- a/Doc/library/hmac.rst
+++ b/Doc/library/hmac.rst
@@ -38,6 +38,13 @@ An HMAC object has the following methods:
    This string will be the same length as the *digest_size* of the digest given to
    the constructor.  It may contain non-ASCII characters, including NUL bytes.
 
+   .. warning::
+
+      When comparing the output of :meth:`digest` to an externally-supplied
+      digest during a verification routine, it is recommended to use the
+      :func:`compare_digest` function instead of the ``==`` operator
+      to reduce the vulnerability to timing attacks.
+
 
 .. method:: HMAC.hexdigest()
 
@@ -45,6 +52,13 @@ An HMAC object has the following methods:
    containing only hexadecimal digits.  This may be used to exchange the value
    safely in email or other non-binary environments.
 
+   .. warning::
+
+      When comparing the output of :meth:`hexdigest` to an externally-supplied
+      digest during a verification routine, it is recommended to use the
+      :func:`compare_digest` function instead of the ``==`` operator
+      to reduce the vulnerability to timing attacks.
+
 
 .. method:: HMAC.copy()
 
@@ -52,6 +66,25 @@ An HMAC object has the following methods:
    compute the digests of strings that share a common initial substring.
 
 
+This module also provides the following helper function:
+
+.. function:: compare_digest(a, 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:`unicode` or a :term:`bytes-like object`.
+
+   .. note::
+
+      If *a* and *b* are of different lengths, or if an error occurs,
+      a timing attack could theoretically reveal information about the
+      types and lengths of *a* and *b*--but not their values.
+
+
+   .. versionadded:: 2.7.7
+
+
 .. seealso::
 
    Module :mod:`hashlib`