Set if the use must enter the old passphrase in addition to the new.
oldPassphraseRequired - True, the use must enter the old passphrase in addition to the new.
Prompt the new passphrase and the old passphrase if required from the user. Display the prompt
message to him. Block, until user has finished. If user is allowed to cancel the dialog, and
the user cancels the dialog, then return null.
An arrray of two char arrays. The first array is the old passphrase or null, if this
has not been requested. The second is the new passphrase. This is null, if the user
canceled the input.
(not isCancelAllowed()) implies ((result <> null) and (result.length == 2))