How any attack on a cipher carried out? Are these practical or only theoretical? I have been reading about successful attacks on AES and other ciphers. One such mention is here (see excerpt below):


  AES was published under the name Rijndael in 1998. Refereed
  cryptanalytic papers in the next three years culminated in attacks
  taking time "only" $2^{140}$ to break 7 rounds of 256-bit AES and "only"
  $2^{204}$ (with a huge amount of memory) to break 8 rounds of 256-bit AES.
  Subsequent work made very little progress. (Related-key attacks can
  break more rounds but are not of concern in any properly designed
  cryptographic protocol.)


I want to know:


If any paper mentions attack as $2^{140}$, how the researchers determine this number of operations? 
Are these attacks only on paper or practically proven?
Considering super-computers owned by the government, which researchers don't have access, this figure may vary? Are these attacks taking into account super-computers computing power also?
I'll answer your questions in order:

1. If any paper mentions attack as $2^{140}$, how the researchers determine this number of operations?

By examining the mathematical properties of the algorithm and their attack.

2. Are these attacks only on paper or practically proven?

There is no practical way to perform any operation $2^{140}$ times, so they are only on paper. But note that mathematics is one of the few sciences where we can prove things without resorting to experimentation.

3. Considering super-computers owned by the government, which researchers don't have access, this time-frame may vary? Are these attacks taking into account super-computers computing power also?

$2^{140}$ is the number of elementary operations, usually 1 operation takes a constant amount of operations of the underlying primitive, e.g. 1 AES encryption/decryption operation. The number of operations required does not depend on computer speed.

But yes, faster computers will perform the operations faster. This is however of little importance as $2^{140}$ operations together take multiple universes of time and energy to complete. Only very disruptive technology such as quantum computing or breakthroughs in mathematics could make such attacks practical.

There is always the possibility that the attack itself can be improved upon. Attacks get better, never worse.



The results on 256 bit AES with relayed keys are mainly of importance when the block cipher is used within other constructions such as pseudo random number generators. AES encryption is still considered secure.