Merge pull request #845 from public/no-memory-wipe-docs

Document our security limitations

docs/index.rst

@@ -85,6 +85,7 @@ The ``cryptography`` open source project
     installation
     development/index
     security
+    limitations
     api-stability
     doing-a-release
     changelog

docs/limitations.rst

@@ -0,0 +1,19 @@
+Known security limitations
+--------------------------
+
+Lack of secure memory wiping
+============================
+
+`Memory wiping`_ is used to protect secret data or key material from attackers
+with access to uninitialized memory. This can be either because the attacker
+has some kind of local user access or because of how other software uses
+uninitialized memory.
+
+Python exposes no API for us to implement this reliably and as such almost all
+software in Python is potentially vulnerable to this attack. However the
+`CERT secure coding guidelines`_ consider this issue as "low severity,
+unlikely, expensive to repair" and we do not consider this a high risk for most
+users.
+
+.. _`Memory wiping`:  http://blogs.msdn.com/b/oldnewthing/archive/2013/05/29/10421912.aspx
+.. _`CERT secure coding guidelines`: https://www.securecoding.cert.org/confluence/display/seccode/MEM03-C.+Clear+sensitive+information+stored+in+reusable+resources