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How long would it take with current hardware to hack an Disk Image which was encrypted with AES 256?

I ask because i want to backup personal files (contracts, tax-things, code, keys, private documents) and leave them outside of my home (e.g. at the office, at dropbox or AWS).

Is it a good/safe idea or not? Since i am not the president of the united states, i think others are not that interested in my files as i was the president ;) But i want to have still a good sleep while the files are stored as the destinations mentioned above.

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    Secure enough for what? Who do you want to protect against? – schroeder♦ Apr 11 '15 at 18:14
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AES-256 is definitely secure for file storage. The only weakness is the key that you choose. As long as you choose a strong key for it, AES-256 will keep your files safe. According to this Wikipedia page, the best attack on AES was published in 2011 and to break AES-256, it still required 2^254.4 operations. The page further states that:

This is a very small gain, as a 126-bit key (instead of 128-bits) would still take billions of years.

The page then mentions that breaking a 128-bit key would require the storage of 2^81 bits of data (about 38 trillion terabytes).

So, all-in-all, as long as you have a strong, non-brute-forceable key, you should be completely fine to store files online using AES-256 protection. (In fact, you could probably even use AES-128 without worrying, but I don't see any sense in using less protection when you can use more.)

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How fast can you crack AES-256?

Either never (within next two centuries) or within days.

AES-256 is considered incrackable by itself. Even most implementations are considered safe. (no side-channels).

OTR protocol uses AES-128 (weaker than AES-256) as symmetric encryption algorithm and the world's best funded intelligence agency NSA wasn't able to decrypt it.

However it comes down to the question of the context of AES-256.
If you use weak keys (which are easy to guess), it's crackable quite soon. Most implementations use very strong keys, which are as safe as AES.
If you use a weak mode (->ECB) one can recognize patterns and hence "break" AES by breaking the context within it's used. Most implementations use safe modes (CBC/CTR/GCM/CCM/EAX/...)
If your implementation (your overall program) is flawed, it might be possible to extract keys or get the plain text anyhow else. This isn't impossible, but very unlikely if you use the right programs (like TrueCrypt).
If your key-derivation is weak, one could crack that and hence get the keys. This is very unlikely and most implementations use counter-measures.
If your key input is too weak, one could break the system. "weak key input" here means weak passwords, weak keyfiles, weak authentication mechanisms. Most implementations do their very best to protect you even if you choose a somewhat weak password. This should be your greatest concern.

Of course this isn't everything, there are also issues if you consider altered data as a threat.

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  • TrueCrypt seems to have less confidence in itself than you have in it... – KnightOfNi Apr 11 '15 at 19:47
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    @KnightOfNi my confidence comes from here. – SEJPM Apr 11 '15 at 19:54
  • Apparently, I stand corrected :). I wonder why they shut down their project when so many sources seem to like it... – KnightOfNi Apr 11 '15 at 20:01
  • @KnightOfNi, nobody knows. Theories range from conspiracy, national security letter, active attacks, extortion, bad community to not enough money. – SEJPM Apr 11 '15 at 20:04
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    TrueCrypt shouldn't be used anymore (not because the encryption is insecure, but because other bugs allow privilege escalation on any PC with TrueCrypt installed and other bugs). VeraCrypt is the de facto successor. – Josef says Reinstate Monica Jan 23 '17 at 15:56
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AES 256 is a standard of the US Government to protect their own files (FIPS 197). Yes, there are stronger methods and deeper cryptography and NIST is reviewing their standards and will upgrade to something stronger in the future, but for the average person, AES 256 will suffice as a strong encryption.

Now the question will be: how will you handle the keys?

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  • So key storage is the bit that is really difficult to come to terms with. Either you have a secure facility or secure hardware based module. – munchkin Jul 10 '15 at 10:22

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