Correctly derive the extra symmetric key
Created by: claucece
As we are currently implementing the revision number 2 of the OTRv4 specification, we need to include a consistent way of using the extra symmetric key.
Please, refer to the "Extra Symmetric Key" section of the OTRv4 spec.
- Check how the sym key functionality is working and if it can work with a client.
- Check and change the KDF being used in the needed functions.
- Allow to derive other keys from it.
- Check that is derived when skipped message keys are stored.