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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] New side channel attack that can recover
Bitcoin keys
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You've heard of TRESOR?
No, not Trezor.
https://en.wikipedia.org/wiki/TRESOR
Signing on the CPU, without touching RAM.
- Sent from my phone
Den 6 mar 2014 09:41 skrev "Mike Hearn" <mike@plan99.net>:
> I'm wondering about whether (don't laugh) moving signing into the kernel
> and then using the MTRRs to disable caching entirely for a small scratch
> region of memory would also work. You could then disable pre-emption and
> prevent anything on the same core from interrupting or timing the signing
> operation.
>
> However I suspect just making a hardened secp256k1 signer implementation
> in userspace would be of similar difficulty, in which case it would
> naturally be preferable.
>
>
> On Wed, Mar 5, 2014 at 11:25 PM, Gregory Maxwell <gmaxwell@gmail.com>wrot=
e:
>
>> On Wed, Mar 5, 2014 at 2:14 PM, Eric Lombrozo <elombrozo@gmail.com>
>> wrote:
>> > Everything you say is true.
>> >
>> > However, branchless does reduce the attack surface considerably - if
>> nothing else, it significantly ups the difficulty of an attack for a
>> relatively low cost in program complexity, and that might still make it
>> worth doing.
>>
>> Absolutely. I believe these things are worth doing.
>>
>> My comment on it being insufficient was only that "my signer is
>> branchless" doesn't make other defense measures (avoiding reuse,
>> multsig with multiple devices, not sharing hardware, etc.)
>> unimportant.
>>
>> > As for uniform memory access, if we avoided any kind of heap
>> allocation, wouldn't we avoid such issues?
>>
>> No. At a minimum to hide a memory timing side-channel you must perform
>> no data dependent loads (e.g. no operation where an offset into memory
>> is calculated). A strategy for this is to always load the same values,
>> but then mask out the ones you didn't intend to read... even that I'd
>> worry about on sufficiently advanced hardware, since I would very much
>> not be surprised if the processor was able to determine that the load
>> had no effect and eliminate it! :) )
>>
>> Maybe in practice if your data dependencies end up only picking around
>> in the same cache-line it doesn't actually matter... but it's hard to
>> be sure, and unclear when a future optimization in the rest of the
>> system might leave it exposed again.
>>
>> (In particular, you can't generally write timing sign-channel immune
>> code in C (or other high level language) because the compiler is
>> freely permitted to optimize things in a way that break the property.
>> ... It may be _unlikely_ for it to do this, but its permitted=E2=80=94 a=
nd
>> will actually do so in some cases=E2=80=94, so you cannot be completely =
sure
>> unless you check and freeze the toolchain)
>>
>> > Anyhow, without having gone into the full details of this particular
>> attack, it seems the main attack point is differences in how squaring an=
d
>> multiplication (in the case of field exponentiation) or doubling and poi=
nt
>> addition (in the case of ECDSA) are performed. I believe using a branchl=
ess
>> implementation where each phase of the operation executes the exact same
>> code and accesses the exact same stack frames would not be vulnerable to
>> FLUSH+RELOAD.
>>
>> I wouldn't be surprised.
>>
>>
>> ------------------------------------------------------------------------=
------
>> Subversion Kills Productivity. Get off Subversion & Make the Move to
>> Perforce.
>> With Perforce, you get hassle-free workflows. Merge that actually works.
>> Faster operations. Version large binaries. Built-in WAN optimization an=
d
>> the
>> freedom to use Git, Perforce or both. Make the move to Perforce.
>>
>> http://pubads.g.doubleclick.net/gampad/clk?id=3D122218951&iu=3D/4140/ost=
g.clktrk
>> _______________________________________________
>> Bitcoin-development mailing list
>> Bitcoin-development@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>>
>
>
>
> -------------------------------------------------------------------------=
-----
> Subversion Kills Productivity. Get off Subversion & Make the Move to
> Perforce.
> With Perforce, you get hassle-free workflows. Merge that actually works.
> Faster operations. Version large binaries. Built-in WAN optimization and
> the
> freedom to use Git, Perforce or both. Make the move to Perforce.
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<p dir=3D"ltr">You've heard of TRESOR? </p>
<p dir=3D"ltr">No, not Trezor. </p>
<p dir=3D"ltr"><a href=3D"https://en.wikipedia.org/wiki/TRESOR">https://en.=
wikipedia.org/wiki/TRESOR</a></p>
<p dir=3D"ltr">Signing on the CPU, without touching RAM. </p>
<p dir=3D"ltr">- Sent from my phone</p>
<div class=3D"gmail_quote">Den 6 mar 2014 09:41 skrev "Mike Hearn"=
; <<a href=3D"mailto:mike@plan99.net">mike@plan99.net</a>>:<br type=
=3D"attribution"><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8=
ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir=3D"ltr">I'm wondering about whether (don't laugh) moving s=
igning into the kernel and then using the MTRRs to disable caching entirely=
for a small scratch region of memory would also work. You could then disab=
le pre-emption and prevent anything on the same core from interrupting or t=
iming the signing operation.<div>
<br></div><div>However I suspect just making a hardened secp256k1 signer im=
plementation in userspace would be of similar difficulty, in which case it =
=C2=A0would naturally be preferable.</div></div><div class=3D"gmail_extra">=
<br>
<br><div class=3D"gmail_quote">On Wed, Mar 5, 2014 at 11:25 PM, Gregory Max=
well <span dir=3D"ltr"><<a href=3D"mailto:gmaxwell@gmail.com" target=3D"=
_blank">gmaxwell@gmail.com</a>></span> wrote:<br><blockquote class=3D"gm=
ail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-le=
ft:1ex">
<div>On Wed, Mar 5, 2014 at 2:14 PM, Eric Lombrozo <<a href=3D"mailto:el=
ombrozo@gmail.com" target=3D"_blank">elombrozo@gmail.com</a>> wrote:<br>
> Everything you say is true.<br>
><br>
> However, branchless does reduce the attack surface considerably - if n=
othing else, it significantly ups the difficulty of an attack for a relativ=
ely low cost in program complexity, and that might still make it worth doin=
g.<br>
<br>
</div>Absolutely. I believe these things are worth doing.<br>
<br>
My comment on it being insufficient was only that "my signer is<br>
branchless" doesn't make other defense measures (avoiding reuse,<b=
r>
multsig with multiple devices, not sharing hardware, etc.)<br>
unimportant.<br>
<div><br>
> As for uniform memory access, if we avoided any kind of heap allocatio=
n, wouldn't we avoid such issues?<br>
<br>
</div>No. At a minimum to hide a memory timing side-channel you must perfor=
m<br>
no data dependent loads (e.g. no operation where an offset into memory<br>
is calculated). A strategy for this is to always load the same values,<br>
but then mask out the ones you didn't intend to read... even that I'=
;d<br>
worry about on sufficiently advanced hardware, since I would very much<br>
not be surprised if the processor was able to determine that the load<br>
had no effect and eliminate it! :) )<br>
<br>
Maybe in practice if your data dependencies end up only picking around<br>
in the same cache-line it doesn't actually matter... but it's hard =
to<br>
be sure, and unclear when a future optimization in the rest of the<br>
system might leave it exposed again.<br>
<br>
(In particular, you can't generally write timing sign-channel immune<br=
>
code in C (or other high level language) because the compiler is<br>
freely permitted to optimize things in a way that break the property.<br>
... It may be _unlikely_ for it to do this, but its permitted=E2=80=94 and<=
br>
will actually do so in some cases=E2=80=94, so you cannot be completely sur=
e<br>
unless you check and freeze the toolchain)<br>
<div><br>
> Anyhow, without having gone into the full details of this particular a=
ttack, it seems the main attack point is differences in how squaring and mu=
ltiplication (in the case of field exponentiation) or doubling and point ad=
dition (in the case of ECDSA) are performed. I believe using a branchless i=
mplementation where each phase of the operation executes the exact same cod=
e and accesses the exact same stack frames would not be vulnerable to FLUSH=
+RELOAD.<br>
<br>
</div>I wouldn't be surprised.<br>
<div><div><br>
---------------------------------------------------------------------------=
---<br>
Subversion Kills Productivity. Get off Subversion & Make the Move to Pe=
rforce.<br>
With Perforce, you get hassle-free workflows. Merge that actually works.<br=
>
Faster operations. Version large binaries. =C2=A0Built-in WAN optimization =
and the<br>
freedom to use Git, Perforce or both. Make the move to Perforce.<br>
<a href=3D"http://pubads.g.doubleclick.net/gampad/clk?id=3D122218951&iu=
=3D/4140/ostg.clktrk" target=3D"_blank">http://pubads.g.doubleclick.net/gam=
pad/clk?id=3D122218951&iu=3D/4140/ostg.clktrk</a><br>
_______________________________________________<br>
Bitcoin-development mailing list<br>
<a href=3D"mailto:Bitcoin-development@lists.sourceforge.net" target=3D"_bla=
nk">Bitcoin-development@lists.sourceforge.net</a><br>
<a href=3D"https://lists.sourceforge.net/lists/listinfo/bitcoin-development=
" target=3D"_blank">https://lists.sourceforge.net/lists/listinfo/bitcoin-de=
velopment</a><br>
</div></div></blockquote></div><br></div>
<br>-----------------------------------------------------------------------=
-------<br>
Subversion Kills Productivity. Get off Subversion & Make the Move to Pe=
rforce.<br>
With Perforce, you get hassle-free workflows. Merge that actually works.<br=
>
Faster operations. Version large binaries. =C2=A0Built-in WAN optimization =
and the<br>
freedom to use Git, Perforce or both. Make the move to Perforce.<br>
<a href=3D"http://pubads.g.doubleclick.net/gampad/clk?id=3D122218951&iu=
=3D/4140/ostg.clktrk" target=3D"_blank">http://pubads.g.doubleclick.net/gam=
pad/clk?id=3D122218951&iu=3D/4140/ostg.clktrk</a><br>__________________=
_____________________________<br>
Bitcoin-development mailing list<br>
<a href=3D"mailto:Bitcoin-development@lists.sourceforge.net">Bitcoin-develo=
pment@lists.sourceforge.net</a><br>
<a href=3D"https://lists.sourceforge.net/lists/listinfo/bitcoin-development=
" target=3D"_blank">https://lists.sourceforge.net/lists/listinfo/bitcoin-de=
velopment</a><br>
<br></blockquote></div>
--089e01493de02d55f804f3ed34cd--
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