Openssl warnings when encrypting

When encrypting with the latest version of openssl you get the following warning
```
*** WARNING : deprecated key derivation used.
Using -iter or -pbkdf2 would be better.
```
This can be fixed by using a hex based key in place of the password
see https://github.com/krzyzanowskim/CryptoSwift/issues/613#issuecomment-387153311