Use pbkdf2 for backup key derivation

Fixes a warning thrown by OpenSSL during backup runs, and makes it more
secure.

site-cookbooks/backup/templates/default/config.rb.erb

@@ -6,6 +6,18 @@
 # Documentation: http://backup.github.io/backup
 # Issue Tracker: https://github.com/backup/backup/issues
 
+#
+# Monkey patch to not use deprecated key derivation scheme
+# https://github.com/backup/backup/issues/949#issuecomment-589883577
+#
+module OpenSSLFixDeprecatedKeyDerivation
+  def options
+    super + ' -pbkdf2'
+  end
+end
+require 'backup/encryptor/open_ssl'
+Backup::Encryptor::OpenSSL.prepend(OpenSSLFixDeprecatedKeyDerivation)
+
 Storage::S3.defaults do |s3|
   s3.access_key_id     = "<%= @s3_access_key_id %>"
   s3.secret_access_key = "<%= @s3_secret_access_key %>"