key generation from passphrase is slow

[hmeine]

When trying out different passwords against an encrypted backup, one is interested in maximum speed. Using appscript (accessibility interface), I can remote control iTunes and trying out different passwords takes around 11 seconds each. With iphone-dataprotection's /backup_tool.py, getPasscodekeyFromPasscode() takes around 4 minutes.

I opened this issue not as a complaint, but in order to have a place to discuss reasons and how to mitigate this. (I guess there are enough people interested in fast crypto extensions, so the necessary APIs should be available.)