[curves] Distribution-ready optimized code

[Diego Aranha]

I've been meaning to write about this for a while, but I think now is good
timing. While LLVM has plenty of problems regarding generation of
constant-time code (or actually any sophisticated enough compiler), it
simultaneously provides plenty of resources for tracking information flow
and building a static analysis tool to check for variable-time behavior in
compiled code. That's the approach we used in FlowTracker:

http://cuda.dcc.ufmg.br/flowtracker/

You can type or upload your code (currently, we only deal with algorithms
implemented in C), and we can point out if the implementation's run time
depends on secret information. We use an XML file to describe sensitive
data. A tutorial and examples of use are available at
http://cuda.dcc.ufmg.br/flowtracker/example.html. Of course, not having
information about exactly what instructions take constant time is still a
limitation.

PS: Beware this is ongoing work, with lots of space for improvement.

Best,
--
Diego de Freitas Aranha
Institute of Computing - University of Campinas
http://www.ic.unicamp.br/~dfaranha

On Fri, Apr 3, 2015 at 3:53 PM Irene Knapp <ireneista at gmail.com> wrote:

> Well, my revised suggestion is actually to skip its codegen and go
> directly to the class that knows how to output x86-64.  It's conceptually
> an internal class, but it's still part of the documented API (to the extent
> that any of LLVM can be said to be documented).  You'd be hooking in as if
> your code were the codegen, and yes, it'd be specific to the architecture
> but that's what you wanted.
>
> I definitely don't want to push this too hard; I understand that I'm not
> the one who would actually be implementing any solution. :)  Coming from
> the compilers background, it's the appealing route to me.
>
> Irene
>
> On Fri, Apr 3, 2015 at 11:48 AM Michael Hamburg <mike at shiftleft.org>
> wrote:
>
>>
>> On Apr 3, 2015, at 11:41 AM, Tony Arcieri <bascule at gmail.com> wrote:
>>
>> On Fri, Apr 3, 2015 at 11:35 AM, Irene Knapp <ireneista at gmail.com> wrote:
>>
>>> Surely, what you are describing is a lightweight tool that either
>>> generates LLVM bitcode, or hooks into the LLVM backends at a slightly lower
>>> level than that to output particular instructions when that's what you
>>> really, really want - but I suspect its hinting system already makes that
>>> unnecessary for this use-case.  LLVM bitcode is precisely this "mostly
>>> concrete assembly" concept that you're describing.
>>>
>>
>> The problem with using LLVM in this context is robust cryptographic
>> implementations need to follow a very specific set of rules to avoid cache
>> timing attacks, and LLVM is not designed to follow these rules:
>>
>> https://cryptocoding.net/index.php/Coding_rules
>>
>> LLVM has not been designed to support the generation of constant time
>> code and is instead rather eager to do things like insert branches in
>> otherwise branch free code if it thinks the code can be better optimized.
>>
>> --
>> Tony Arcieri
>>
>>
>> It may be that if your tool chooses carefully the optimization passes —
>> or even avoids most of them entirely — you could get constant-time
>> operation.  But I don’t know enough about LLVM’s codegen to be sure one way
>> or the other.  At least until recently, though, it was absolutely terrible
>> at things like add-with-carry intrinsics.  (Not necessarily making them
>> variable time, but lowering add; addc to add; setc; zext; add; add.)
>>
>> — Mike
>>
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