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Imagine I have this:
$cdata = AES_256($data, $pass);
AES_256 implements the AES algorithm.
If I know the content of $cdata and the content of $data and also have
the AES_256() code, can I reverse engineer and find $pass?
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AES 256 is virtually impenetrable using brute-force methods. While a 56-bit DES key can be cracked in less than a day, AES would take billions of years to break using current computing technology. Hackers would be foolish to even attempt this type of attack. Nevertheless, no encryption system is entirely secure.
To decrypt in AES, we can simply invert and reverse the cipher's internals and its key-scheduling algorithm. For each decryption round, we use the reverse order of the encryption subkeys.
Encryption can always be reversed. The point of encryption is to take a message and encode it with a secret key so that only another person who has the key can reverse the encryption and read the message.
According to the Snowden documents, the NSA is doing research on whether a cryptographic attack based on tau statistic may help to break AES. At present, there is no known practical attack that would allow someone without knowledge of the key to read data encrypted by AES when correctly implemented.
Another quote, from Wikipedia:
AES permits the use of 256-bit keys. Breaking a symmetric 256-bit key by brute force requires 2^128 times more computational power than a 128-bit key. A device that could check a billion billion (10^18) AES keys per second would require about 3 x 10^51 years to exhaust the 256-bit key space.
Brute forcing when you know the original text might be faster but still, 3 x 10^51 years is a long time. Plus there's the problem of probably not having a device that can check a billion billion (10^18) keys/second.
In short: everything is possible, but this is not feasible in the world we are now living in.
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Simple answer: NO.
This has been tested, and mentioned in the Wiki link.
A related-key attack can break up to 9 rounds of 256-bit AES. A chosen-plaintext attack can break 8 rounds of 192- and 256-bit AES, and 7 rounds of 128-bit AES, although the workload is impractical at 2128 - 2119.
Or put it another way: you have a better chance of being struck by lighting... on the same day you win the Lottery, than breaking it!
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