How secure is AES-256? The cipher AES-256 is used among other places in SSL/TLS 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 than 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?
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.