[dm-crypt] question regarding Sha1 and 512 bit key xts mode

Arno Wagner arno at wagner.name
Wed Dec 11 19:04:19 CET 2013


On Wed, Dec 11, 2013 at 17:31:13 CET, anderson jackson wrote:
> In the faq it is said that the use of sha1 for the purpose used in Luks is
> valid because it is not the cryptographic feature that is used but instead
> the time delay for retreaving the master key.

No, that is not the statement. The statement is that collision attacks
(the SHA1-weakness) are irrelevant for password hasing.
 
> However is this really the case? The output of Sha1 is a 160 bit string. 
> A password is iterated using PBKDF2(with sha1).  But can't I just use all
> the possible sha1 values to decrypt the master key and validate it with
> the master key checksum?  Does this not effectively reduce the possible
> passwords for an AES 256 bit volume to a password of 160 bit length?

It does. If you can create that table, which you cannot. 

2^160 is about 1.5*10^48. The number of atoms in this planet
is only 1.33*10^50. So if you can convert the whole planet to 
storage space and can store one bit in one atom, you can just 
about do it. Then there is the computing effort: Say, you get 
1M hashes/sec with 1W of power. As PBKDF2 runs with around 
100'000 iterations on average PC hardware, you then get 1
iterated hashe for 0.1 Joule of power. That means for 2^160
of them, you need 150*10^45 Joules. The sun has an energy
output of 3.8*10^26 W. So run the sun for 384*10^18 seconds =
12.8*10^12 years and you have your table.

Sounds pretty unrealistig, right?

AES-256 does not have a 256 bit key to provide a 2^256-sized 
key-space. Key-space-wise, at around 100 bit it becomes reliably 
infeasible for the human race to break things.

AES-256 has a 256 bit key, becuase research could reduce the
effective key-length and the longer the key, the more effective
bits need to be removed by research before brute-forcing becomes 
feasible.

Also note that your password is unlikely to even have 100 bits
of entropy. If you actually use a passwored with more than
160 bits of entropy, moving to SHA-256 as hash function may 
provide an irrelevant security improvement.

Arno
-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
There are two ways of constructing a software design: One way is to make it
so simple that there are obviously no deficiencies, and the other way is to
make it so complicated that there are no obvious deficiencies. The first
method is far more difficult.  --Tony Hoare


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