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Date: Sun, 23 Mar 2014 20:38:55 -0700
From: Jeremi Gosney <epixoip@...dshell.nl>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] pufferfish

Hi beloumi,

On 3/23/2014 1:35 PM, beloumi wrote:
> Am 23.03.2014 15:59, schrieb Jeremi Gosney:
>> I've pushed code to a public repository for the candidate I will be
>> submitting, pufferfish. This repository will be frantically updated over
>> the coming week as I prepare my submission, but I wanted to get some
>> initial feedback before the cutoff.
>>
>> https://github.com/epixoip/pufferfish
> I am glad that someone has chosen Bcrypt as a basic for a submission.
> This submission mightbenefit of the proven reliability of a long
> existingalgorithm.

Thank you for your comments!


> What bothers me a bit at first sight is the Base64 encoding. Instead of
> Bcrypts particular Base64 encoding I would prefere the standard Base64
> encoding of MIME. This would be an easement for implementations in
> crypto libraries.

I would like to hear more opinions on this. I prefer itoa64 encoding as
I think the output looks cleaner. But this is purely an asthetic
preference, there is no technical justification for this as far as I'm
concerned.


> Just two unreflective ideas for Pufferfish:
> 1. Wouldn't it be easier to support variable output length by variable
> (length of) ctext in Pufferfishs encryption step instead of an extra
> hash? I can not see any security improvement for the hash.

Sure. Bcrypt uses the output of the encryption step directly, so we
could just as easily do that as well. But on the other hand, there's no
reason not to hash the output, either. Just my preference :)


> 2. Instead of replacing the s-box entries, wouldn't it be easy to expand
> them to a memory-hard vector and use them piece by piece in the last
> encryption step (that means: using a different s-box for each of the 64
> encryptions or - more experimental - for each blowfish round)?
> Please excuse that I write a comment although I have not fully reflect
> the algorithm...

Depending on the m_cost, doing so could break the memory hardness. Since
only one s-box would be needed at a time, the attacker could hold each
s-box in global memory and swap them into local memory when they are
needed. Using each s-box simultaneously prevents this from occuring.


Jeremi

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