[IMP] auth_crypt: upgrade default kdf to pbkdf2_sha512

Applies to new and yet-to-be-encrypted users, existing already-encrypted will
keep logging in as usual, their password storage will be upgraded on next
password *change*.

addons/auth_crypt/auth_crypt.py

 import logging
 
-from passlib.hash import md5_crypt
+from passlib.context import CryptContext
 
 import openerp
 from openerp.osv import fields, osv
 
 _logger = logging.getLogger(__name__)
 
+default_crypt_context = CryptContext(
+    # kdf which can be verified by the context. The default encryption kdf is
+    # the first of the list
+    ['pbkdf2_sha512', 'md5_crypt'],
+)
 
 class res_users(osv.osv):
     _inherit = "res.users"
 
+    def _crypt_context(self, cr, uid, id, context=None):
+        """ Passlib PasswordHash (or CryptContext) instance used to encrypt
+        and verify passwords. Can be overridden if technical, legal or
+        political matters require different kdfs than the provided default.
+        """
+        return default_crypt_context
+
     def set_pw(self, cr, uid, id, name, value, args, context):
         if value:
-            encrypted = md5_crypt.encrypt(value)
+            encrypted = self._crypt_context(cr, uid, id, context=context)\
+                .encrypt(value)
             cr.execute("update res_users set password='', password_crypt=%s where id=%s", (encrypted, id))
 
     def get_pw( self, cr, uid, ids, name, args, context ):
@@ -38,14 +51,14 @@ class res_users(osv.osv):
         if cr.rowcount:
             stored_password, stored_password_crypt = cr.fetchone()
             if stored_password and not stored_password_crypt:
-                stored_password_crypt = md5_crypt.encrypt(stored_password)
+                stored_password_crypt = self._crypt_context(cr, uid, id).encrypt(stored_password)
                 cr.execute("UPDATE res_users SET password='', password_crypt=%s WHERE id=%s", (stored_password_crypt, uid))
         try:
             return super(res_users, self).check_credentials(cr, uid, password)
         except openerp.exceptions.AccessDenied:
             # check md5crypt
             if stored_password_crypt:
-                if md5_crypt.verify(password, stored_password_crypt):
+                if self._crypt_context(cr, uid, id).verify(password, stored_password_crypt):
                     return
             # Reraise password incorrect
             raise