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Date: Sun, 17 Jan 2016 09:35:48 -0800
From: Henry Corrigan-Gibbs <henrycg@...nford.edu>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: Attack on Argon2i?

Hi Bill,

Thanks for your comments on the paper!

On 1/16/16 10:18 PM, Bill Cox wrote:

> While your algorithm is cool, and showed a weakness in Argon2i, I still
> think your algorithm is too slow.  How fast can you fill memory?
> Memory*time defense goes as the square of this number.  Some good
> algorithms fill 4MiB in 1ms.

I completely agree that filling as much memory as possible per unit time 
is an important design goal.

The Balloon functions (like Argon2 and Catena) make use of a 
cryptographic compression function that maps two k-bit blocks of memory 
into a single k-bit block of memory.

Which compression function you choose, along with a few other security 
parameters, determines how fast the construction runs and how space-hard 
it actually is. A faster compression function is clearly better. 
However, if the cryptographic compression function is weak, then the 
overall construction will not be space-hard, no matter how quickly it runs.


The following are benchmarks from my Intel Xeon E5620 with
   space cost = 4 MiB  and
   time cost = 5 passes (rounds).
Below, "ArgonHash" refers to the compression function G() defined in the 
Argon2 spec and "SHA3" is Keccak with rate=1344. Here are some 
performance numbers, measured in hashes per second, for a few different 
configurations:

Argon2 : 67.0199
Balloon: 51.6794  (ArgonHash + aggressive parameters)
Balloon: 8.97988  (ArgonHash + conservative parameters)
Balloon: 4.53571  (SHA3      + aggressive parameters)
Balloon: 2.84941  (SHA3      + conservative parameters)
Balloon: 0.408044 (SHA3      + very conservative parameters)


As the numbers above indicate, the performance of ArgonHash is amazing. 
We chose not use ArgonHash as our default compression function because 
it does not seem to have the cryptographic properties we wanted. For 
example, if you run ArgonHash in counter mode like this:
   G("00000 ... 00000", "00000 ... 00000")
   G("00000 ... 00000", "00000 ... 00001")
   G("00000 ... 00000", "00000 ... 00002")
   G("00000 ... 00000", "00000 ... 00003")
the bytes that come out seem to have very very strong biases. SHA3 does 
not apparently have these artifacts, though it of course runs much much 
more slowly.

It's not clear whether it would be possible to leverage the strong 
biases in the output of ArgonHash into weaknesses in Argon2 itself (or 
into weaknesses in the Balloon functions when instantiated with 
ArgonHash) -- I have not tried.

To avoid that possibility entirely we just decided to use a standard 
hash function as the default, though again, it would be possible to 
instantiate the Balloon constructions with ArgonHash if you're looking 
for more speed.

Henry



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