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Date: Wed, 2 Apr 2014 20:36:01 -0300 (BRT)
From: mjunior@...c.usp.br
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Catfish and public key hash

Hi there 

I would say that if the attacker needs more than 2x the amount of memory used by the defender to get less than a 2x speed-up, then the attacker is wasting resources: he/she could simply use two cores to get the same throughput... Unless the attacker model considers a limitation in number of cores, which does not seem to be the most common case. 

BR, 

Marcos Simplicio. 

----- Mensagem original -----

> De: "Bo Zhu" <bo.zhu@...terloo.ca>
> Para: discussions@...sword-hashing.net
> Enviadas: Quarta-feira, 2 de Abril de 2014 16:22:24
> Assunto: Re: [PHC] Catfish and public key hash

> Hi Steve,

> Thanks for pointing that out.
> It's a well-known optimization method.

> But [speed-ups only if a large memory is present] isn't one of the
> features that PHC wants in order to thwart the attacks based on
> ASICs and GPUs, right? :)
> And in this case, knowing p and q, the look-up tables can be much
> smaller.

> Best,
> Bo

> On Wed, Apr 2, 2014 at 2:48 PM, Steve Thomas < steve@...tu.com >
> wrote:

> > An attacker with more memory can calculate the public key hash
> > faster
> > than
> 
> > the defender even if the attacker doesn't have p and q.
> 

> > # initial work (done only once)
> 
> > for j = 0 to 2 ** 16
> 
> > short_cut[0][j] = pow_mod(generator, j, N)
> 
> > for i = 1 to BIT_SIZE_N / 16
> 
> > for j = 0 to 2 ** 16
> 
> > short_cut[i][j] = pow_mod(mem[i-1][j], 2 ** 16, N)
> 

> > # calculating the public key hash
> 
> > num = short_cut[i][exponent & 0xffff]
> 
> > exponent = exponent >> 16
> 
> > for i = 1 to BIT_SIZE_N / 16
> 
> > num = mul_mod(num, short_cut[i][exponent & 0xffff], N)
> 
> > exponent = exponent >> 16
> 
> > return num
> 

> > if BIT_SIZE_N is 1024 then with 1024 / 8 * 2 ** 16 * (1024 / 16)
> > bytes of
> 
> > memory (512 MiB) the attacker only needs to do 63 multiplies mod N.
> > Using
> 
> > less than 3.5 GiB you can get it down to 53 multiplies mod N.
> 
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