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Can AES-256 be decrypted if after the encryption process you destroy the key that must be used to decrypt the data, is it possible?

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    If someone who had the key but is now without the key could crack it, then so could someone who never had the key. – Macil Oct 4 '18 at 2:46
  • @Macil that someone who had the key but is now without the key might be able to recover it by remembering it. You could consider it a failure of the key destruction, though. – Ángel Nov 1 at 21:34
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In theory, you could try to brute force the key and recover the data. In practice, that's extraordinarily unlikely to be successful as it would be expected to take a very, very long time.

Per Wikipedia:

AES permits the use of 256-bit keys. Breaking a symmetric 256-bit key by brute force requires 2128 times more computational power than a 128-bit key. Fifty supercomputers that could check a billion billion (1018) AES keys per second (if such a device could ever be made) would, in theory, require about 3×1051 years to exhaust the 256-bit key space.

Keep in mind, the universe has existed for about 1.38×1010 years. Supposedly, a quantum computer could reduce this substantially, but it would still be expected to take longer than the universe has existed.

Of course, you could get really lucky (like win the lottery every week for the next decade lucky) and brute force it in a year or two, but if I were you I wouldn't waste the time or effort.

  • Wow, thanks a lot dude. The statistics used to help the explanation made it really enjoyable to read. Big up! (Y) – zrobot Oct 3 '18 at 21:46
  • this assumes the key is a truly random and uniform sequence of bytes; if it's not the math is far different. – dandavis Oct 4 '18 at 15:58
  • See this answer, summing up: "even if you use all the dollars in the World (including the dollars which do not exist, such as accumulated debts) and fry the whole planet in the process, you can barely do 1/1000th of an exhaustive key search on 128-bit keys. So this will not happen." It is said that a quantum computer with sufficient qubits—if it were to exist—would just reduce a 256 but key to a 128 bit equivalent. – zaph Oct 4 '18 at 21:38
  • @zaph, someone else edited my response to correct the paste issue I had which lost the notion of a number being to a power. – Swashbuckler Oct 5 '18 at 18:50
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You cannot decrypt the data without knowledge of the key. However, that does not guarantee that destroying the key prevents decryption of the data:

  • You may have destroyed one copy of the key, but other copies of the key may exist.
  • The key could have been generated with a weak random number generator, reducing the number of possible keys so much that you can try them all.
  • The key may have been derived from other data, and an attacker may be able to repeat this process.

If the key was securely and randomly generated, and all copies of the key have been destroyed, it is considered impossible to decrypt the data based on what we know. Brute-force attacks on a 256-bit key are impossible (physically impossible, actually).

However, mathematical weaknesses in AES could be discovered in the future (or could already have been discovered and kept secret) that would make it feasible to decrypt AES-encrypted data without the key. That said, symmetric crypto algorithms (like AES) tend to hold up pretty well. DES is now over 40 years old, and no practical attacks faster than brute-force have been publicly discovered so far.

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