
Date: Thu, 22 Dec 2011 22:50:20 +0530
From: piyush mittal <piyush.cse29@...il.com>
To: johndev@...ts.openwall.com
Subject: Re: Bit slice implementation of DES based hashes
>
> "each 64 bit"?
>
That is application dependent like whether we want to use all bits or not.
Where does bit 0 of the block go? And where does bit 0 of the key go?
> Ditto, say, for bit 11 of each.
>
According to the paper what I understood is as follows but it can be wrong
as well..
0th bit of block will go to the 0th bit of word.
0th bit of key will go to 0th bit of word.
11th bit of block will go to the 0th bit of 11th word
11th bit of key will go to the 0th bit of 11th word.
Then, supposed we have two DES blocks to encrypt with two different keys.
> Where does bit 0 of the second DES block to encrypt go? Where does bit
> 0 of the second DES key go? Ditto, say, for bit 11 of each.
>
0th bit of 2nd block will go to the 2nd bit of first word
0th bit of 2nd key will go to the 2nd bit of first word.
11th bit of 2nd block will go to the 2nd bit of 11th word
11th bit of 2nd key will go to the 2nd bit of 11th word
On Thu, Dec 22, 2011 at 8:10 PM, Solar Designer <solar@...nwall.com> wrote:
> On Thu, Dec 22, 2011 at 05:21:51PM +0530, piyush mittal wrote:
> > > Each bit of what?
> >
> > Each bit of processor is what I am talking of,like in 64 bit processor
> each
> > 64 bit will behave as different processor.
>
> "each 64 bit"?
>
> > According to Eli Biham's paper,In Des Bitslice implementation he
> considered
> > processor as a SIMD computer,ie as 64 parallel one bit processor
> computing
> > the same instruction.
>
> Right.
>
> > > "To further increase the speed"? Would there be any speed increase at
> > > all (compared to a nonbitslice implementation) without this? And what
> > > alternative(s) did we have?
> >
> > Yes.Speed has been increased,like in his paper he told:
>
> Sure, but that's not what I asked you above. You used the word
> "further", implying that there would be some speed increase (just a
> smaller increase) even without representation of the Sboxes as
> Boolean expressions. In turn, this implies that such bitslice
> implementations are possible. This made me ask (indirectly): are they
> possible? How? And would they provide any speed increase at all?
> If you don't know the answers to these, and you clearly don't, then your
> statement about a "further" speed increase was not justified.
>
> Then, it might in fact help you understand this stuff better if you try
> to come up with answers to these related questions. They are actually
> pretty difficult.
>
> > In DES, the input of each S box depends only on the output
> > of only six S boxes in the previous round. Thus, the code can be
> optimised
> > to start computing the next round while still computing the preceding
> one.
> > This can speed up implementations on pipelined processors, where we can
> > compute several instances in parallel.
>
> Yes, but this is a minor and secondary effect, it is by far not the
> primary reason for speedup. Moreover, when there are enough registers
> (or when cached memory accesses are fast enough), similar extra
> instruction level parallelism can be achieved without bitslicing by
> mixing instructions from two nonbitslice implementations of DES (for
> two different keys or/and input blocks). (But no one bothered to do
> that, as far as I'm aware, because bitslicing is so much faster.)
>
> Where does the major speed increase from bitslicing DES come from?
>
> > Truly speaking I am not getting how B and K is being initialised in JTR
>
> For a while let's speak of bitslice DES in general, not referring to JtR
> specifically. B[] is a 64element array, and K[] is a 56element array.
> Each element is an Nbit machine word. Let's say we have only one DES
> block to encrypt with one key. It is nonoptimal to use a bitslice
> implementation (unless on a true 1bit machine) for that purpose, but
> let's say we're going to do that anyway. Where does bit 0 of the block
> go? And where does bit 0 of the key go? Ditto, say, for bit 11 of each.
> Please try to answer these questions. I tried to make them simpler by
> introducing these extra specifics and by moving away from JtR.
>
> Then, supposed we have two DES blocks to encrypt with two different keys.
> Where does bit 0 of the second DES block to encrypt go? Where does bit
> 0 of the second DES key go? Ditto, say, for bit 11 of each.
>
> > code according to the architecture of processor specified.And if I will
> be
> > able to get it then i can also complete my conversions.However I read
> its
> > corresponding header file but then also I am not getting.Even I tried to
> > take some idea from DES_bs_get_hash() function where we are fetching hash
> > from B but I am not getting what "B[0] DEPTH" represents and why we have
> > taken b[0] to b[19].
>
> Let's revisit this later, or maybe it'll become obvious to you after you
> answer the nonJtR questions above.
>
> Alexander
>

Piyush Mittal
Department of Computer Science and Engineering
National Institute of Technology,Rourkela
INDIA
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