Take the 2-minute tour ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

The cipher AES-256 is used among other places in TSL/SSL across the Internet. It's considered among the top ciphers.

In theory it's not crackable since the combinations of keys are massive.

Although NSA has categorized this in Suite B, they have also recommended using higher then 128-bit keys for encryption.

So how secure is this cipher really? Should you assume the worlds top cracker-institutions are near or have cracked it already?

share|improve this question

migrated from stackoverflow.com Apr 1 '12 at 17:03

This question came from our site for professional and enthusiast programmers.

3  
A side note; in general, this question crypto.stackexchange.com/questions/579/… has some good answers on how we can reason about government capabilities. –  Ninefingers Apr 1 '12 at 17:59
1  
By the way, it would be a mistake to assume that AES256 is more secure than AES128. Much more serious attacks are known against AES256 than are known against AES128. –  David Schwartz Apr 1 '12 at 19:30
5  
@DavidSchwartz: The related key attack is mostly of interest when e.g. designing a hash from AES, as far as I understood. It does not really translate to e.g. SSL where there certainly shouldn't be any relation between the session keys. –  owlstead Apr 2 '12 at 1:12
    
"In theory it's not crackable since the combinations of keys are massive." – It is simply not true. A large number of encryption key combinations does not ensure any security level. –  user12107 Feb 20 at 8:28
    
The comment by @user12107 is flat-out incorrect. AES does not have any known "weak key" problems like DES or RSA have. –  Stephen Touset Feb 21 at 23:04

4 Answers 4

We know, from Edward Snowden, that the NSA does, routinely monitor all traffic and can decrypt all standard encryption, such as SSL. So it is likely that they have methods to break AES.

So, if you are concerned about industrial espionage, AES is unlikely to be your algorithm of choice - this is not just because of NSA or GCHQ spying, any third party (retired spook, for example) who has the know-how can also decrypt most things. In fact there are companies for hire that will decrypt pretty much anything for you, using intelligence from the likes of the NSA or GCHQ.

The only known secure algorithm is the one-time pad.

share|improve this answer
    
The released documents do not claim or imply that NSA can break AES, or if you think they do please add a reference. It is IMO more likely that NSA relies on side channels and implementation bugs. –  otus yesterday
    
I think I'm over 50 ciphersuites in my implementation (just an enum, no I'm not going to implement them all). "decrypt all standard encryption such as SSL" is in itself a wrong statement. SSL is a protocol with a large set of possible underlying algorithms. –  owlstead 8 hours ago
    
Good point - to say 'decrypt SSL' is a loose way of saying 'decrypt the usual set of algorithms used in SSL'. –  Peter Brooks 3 hours ago

AES and RSA (based on prime factory) are not information-theoretically (unconditional) secure, because they simply do not have a mathematical proof of being secure. Still today there is not a proof that proves that prime factor is defininively a difficult or simple problem (P=NP? one of the biggest mistery of math).

So in everyday practice they show to be unbreakable because no one has still found a method to break them, but maybe someone knows it and keeps secret( prime factor solving ).

And the fact that NSA recommends to use AES give me some doubts…

  • Do they recommend this because they know how to break it?
  • Why did the communication line (famous red telephone) between the president of the USA and the Russia use a OTP encryption and not AES?

UPDATE 11-11-2014:

  • What about the last news of NSA spying other governments (how did they do this?)
share|improve this answer
    
If I tell you number 5, can you guess how I reached at this number? There is nothing mysterious about it. You do not know and can only speculate the operations taken to reach at 5. Possibly one of those sequence is 4 + 1. –  nvd 22 hours ago
    
Yes, I can speculate only operation I am aware, but maybe there are other ways of reach number 5 both you and I do not know. –  albanx 22 hours ago
    
It is the operation flow that matters and not the number in this situation. ;) –  nvd 22 hours ago

I wouldn't assume that the NSA has cracked AES ciphers. I would assume that most crypto systems that use AES have implementation flaws that the NSA exploits when they feel it is worth it.

In any case, when the only possible way a state can know something is by breaking a cipher, it's difficult for them to use that information; doing so would reveal that the cipher is broken. So in practice, a broken cipher is more likely to be used as a shortcut to find something that could be discovered (albeit with more difficulty) by other methods.


You might be interested in this recent story and commentary about NSA's crypt analytical capabilities.

share|improve this answer
1  
A very good point. –  Gustav Apr 1 '12 at 17:04
    
That is a very interesting way to answer to someone who talks about NSA capabilities. –  Nicolas Thery Mar 19 at 13:01

An interesting thing about some modern standardized ciphers, like AES, is that the government is "eating its own dogfood" by using them internally. (AES 192 and 256 are approved for top-secret data.) Back in the day (up through the 90s), U.S. government internal encryption standards was not closely aligned with public sector cryptography, and we largely had to speculate as to whether public crypto could hold up to the government standards; the NSA had a history of knowing more crypto than they let on. But now that they are willing to stake their own security on them, that seems like a decent endorsement of those algorithms.

The U.S. government has conflicting goals: they want to be able to break crypto, but at the same time, in the interest of protecting the citizen in the digital age, they want us to be protected against the crypto attacks of others. So much of our modern economy relies on crypto that we want a high security margin on it. Since the 90s, crypto knowledge in the public and foreign intelligence domains has sky rocketed, and a vulnerability that the NSA can exploit is possibly a vulnerability that someone else can exploit. So at the drafting of AES, we doubt that they were focused on choosing a candidate that could be broken and kind of suspect they wanted a candidate that could not be.

Since you only break crypto when you don't have the key, to compromise those two goals they could just allow us mathematically secure crypto, then focus on getting the keys instead. If they can recover keys, they don't care how strong our crypto is. Attacking the endpoints that generate the keys is not always as hard as it seems (consider how many user and corporate machines get infected with malware, and think about what sort of key-related backdoors could be planted in popular software), and a simple subpoena might get keys in some situations. As more user data moves toward the cloud, backdoors in public services (voluntarily provided or not) are going to make the job of key recovery even easier.

Summary of these two points:

  • The federal government is allowed to use AES for top-secret information.

  • We don't know that they would actually want AES to be mathematically breakable, so at the AES competition 11 years ago it is possible they would have avoided any algorithm they thought they could break in the near future.

None of that is proof, but we tend to assume that the NSA can't break AES.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.