I have a hard drive which was encrypted with XTS-AES 128-bit on OS X. I know how long the password is but that's it. How long would it take to decrypt it already knowing this? I have read that in years to come when quantum computers are available it can be broken. Dont comment "never" or "6 billion year" Think outside of the box with advancing technology in years to come. I have memories on here.
The average 12 character password will take 6 billion years to brute-force at 100,000 passwords per second.
Allowing special characters brings this to around 150 billion years.
For more information, see Security.SE:
The block size is 128-bit but the key is 256 bit key."Shown below"
As such, AES-256 is medium term secure against a quantum attack, however AES-128 is broken, and AES-192 isn't looking too good. Source https://crypto.stackexchange.com/questions/6712/is-aes-256-a-post-quantum-secure-cipher-or-not
FileVault uses the user's login password as the encryption pass phrase. It uses the AES-XTS mode of AES with 128 bit blocks and a 256 bit key to encrypt the disk. Source http://training.apple.com/pdf/WP_FileVault2.pdf
This is also should be helpful.
For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.
Source Advanced Encryption Standard on wikipedia.