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From a cold boot, the only password needed to unlock the primary encrypted volume on my MacBook's SSD is just that of my main admin user. This was not always the case; it used to be that I could specify a fairly long and thorough passphrase to be used in key derivation whenever I had to access the volume in its locked state, then a separate passphrase for the user account after unlocking the partition.

It is very inconvenient for me to type out such a lengthy passphrase for every admin-level action I might need to perform during casual use. It seems unlikely that Apple would roll out such a bold detriment to one of either data security or user experience, so I'm confident that I've missed something. What (if any) safeguards exist to protect against brute force now that the only user input required to decrypt my whole system is a frankly modest password at the boot screen? I understand that it may wipe after N attempts, but of course I am speaking more about the risk of brute force given physical access to the drive/access through an agnostic interface.

Another question: is there any way for me to revert back to the old experience without emulating it with a new user and promoting it to the primary administrator of the system? For what it's worth, I am using OS X 10.10 (Yosemite). I believe that change of experience/apparent ability came during my use of the prior major release of the operating system, but it may have occurred earlier.

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OS X Lion included a major rewrite and redesign of FileVault, that may be the change you are looking at. Legacy FileVault (the old version) continued to work in Lion and even Yosemite if you did an upgrade in place; maybe you did a clean install last time, or got a new machine?

FireVault 2 is a significant upgrade to Legacy FileVault and the general advice is that you should upgrade. You are correct that you will need to have a decent user password, but the thing is that you need to have a decent user password anyway!

FDE is only for protecting the machine when it is turned off; when it is turned on the OS has to protect the machine and it can't do that if you have a weak admin password.

However, you might be over-estimating the strength you need in your password. There was an interesting paper published in 2002 that estimated it would take about 34 years to brute-force a FileVault 2 password with 32 bits of entropy; that is actually a fairly modest password I would say. You only need three Diceware words or a 7 character random alphanumeric to get 32 bits.

You can't revert back to Legacy FileVault. However, you can get a very similar set-up to what you had before by simply keeping all your sensitive data in an encrypted sparsebundle image. This way, you get all the benefits of FileVault 2, but your data is also encrypted with a very long passphrase that you only need to type in once per session.

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