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Date: Sat, 30 Aug 2014 05:46:14 -0400
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] A review per day - RIG

On 08/30/2014 12:11 AM, Solar Designer wrote:
> On Fri, Aug 29, 2014 at 06:01:07PM -0400, Bill Cox wrote:
>> The XOR-ing over memory is an idea from Gambit that we talked
>> about quite a bit.
>
> I think this idea pre-dates Gambit, or do I miss something?  Here
> it is in a comment by Anthony Ferrara (@ircmaxell) made 3 years
> ago:

I also said, "Multiple entries use the XOR-ing thing, so I have less
trouble with that, but it follows a pattern..."  I didn't mean to say
Gambit invented the idea.  However, it was discussed mostly while
discussing Gambit, I think.

> https://www.drupal.org/node/1201444#comment-4675994
>
> | This "issue" could be eliminated by simply writing to V_j in the
> | second loop, for example: | | For i = 0 ... N - 1 |     j <--
> Integerify(X) mod N |     V_j <-- X xor V_j |     X <-- H(V_j) | B
> <-- X
>
> At least I call it Anthony Ferrara's scrypt tradeoff defeater, and
> it was in my code leading up to yescrypt probably before Gambit
> appeared.
>
> BTW, the Gambit submission doesn't appear to credit any prior work
> that it builds upon (it's not the first memory-hard function, not
> the first to suggest use of a ROM array, and not the first to XOR
> things onto memory), except for Keccak.  I wouldn't care much, but
> maybe you're bashing RIG for the same kind of things too much... or
> would you do it to Gambit too, when you reach it?

The Gambit author took issue with me calling the cache-timing-resistant
category the "Catena inspired" category.  I believe him when he says he has
been thinking of a cache-resistant algorithm before Catena's paper came
out.  His algorithm also does not seem to rip-off any of Catena's ideas at
all.  It is very minimalistic - just the sponge and a memory access
pattern, without all the framework stuff from Catena.  Adding the ROM seems
odd to me, given that the algorithm is minimalistic.  You must have made a
really good case for your ROM idea at some point :-)

Gambit did combine a sponge and cache-timing resistance, which seems to be
his main point, and his memory access pattern proved very hard to pebble
below 1/8th memory TMTO (unlike Lyra2's).  His code is painfully slow, but
easy to read and verify.  I can't put Gambit in the same category as Catena
for having a lot of good new ideas, but it's not offensive at all.
However, he did not convince me to stop calling it the "Catena inspired"
category :-)

RIG, on the other hand, did everything but copy Catena's code and paper,
without mentioning anywhere that they were based on Catena.  Their hash
matches Lyra2's stripped-down Blake2b round character-for-character...
hopefully just copied from the same paper, but surely they got they idea
from Lyra2, and those guys aren't even in the bibliography.  The XOR
connection to Gambit is a weak, so I'll stop complaining about that.

>> Now that I've found that writing to a memory location just read
>> from is quite fast compared to writing to a different location, I
>> am a fan.
>
> Yes, I like this too.  It works great for our typical defender, but
> what would it mean for an attacker with custom memory chips?  That
> attacker can implement the XOR near memory rather than near the
> hashing cores.
>
> Luckily, this doesn't save them any bandwidth: they still need to
> pass the data to XOR with to memory, and the data read back from
> memory (before or after XOR, depending on algorithm) back to a
> hashing core. ...Unless a given password hashing scheme only XORs
> things onto memory, without using that memory's contents in any
> other way at the same time. yescrypt is fine in this respect (it
> also uses the data), but some other PHC candidates might not be -
> we could want to check them for this risk.

Lyra2 gets this detail right.  Gambit gets it wrong.  I'll check for this
detail when I read the code.

I haven't actually done a proper analysis of the RIG code, yet.  I am
waiting for them to explain why I'm wrong to find their paper and code
offensively similar to Catena.  I make a lot of mistakes, and if this is
one, I'll apologize and properly analyze their work.

> Then, what if the attacker implements more or all of the hashing
> logic, not just the XOR, near memory?  The concept of memory
> bandwidth stops to make full sense, so we can't reasonably say that
> such attacker would e.g. halve their required memory bandwidth, but
> perhaps there are still some savings due to use of possible custom
> memory cells that support a write-XOR-read operation?  I guess such
> savings might exceed the speedup we're seeing, relative to
> unrelated read/write locations, on our target CPU+RAM systems.
> Hopefully, this risk is rather theoretical, and clearly its worst
> case impact is limited to 2x (or actually less, if we consider it
> relative to defender's speedup).

A DRAM cell is "one transistor", which is a bit bogus, but everyone
says that.  The cheapest XOR I've seen is 4 transistors, so I don't
think there's much chance a custom cell will be used to make this
operation faster in any high capacity DRAM chip.

If algorithms are XORing onto sequential addresses, I'd just make it two
separate memory operations, put the XOR on the DRAM chip near the
interface, and have two DRAM chips alternate addresses between them.  While
one is doing a read-XOR operation, the other one would be doing a write of
the previous read-XOR result.

Bill

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