On Wed, Mar 5, 2014 at 2:14 PM, Eric Lombrozo <elombr...@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— and will actually do so in some cases—, 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 and > multiplication (in the case of field exponentiation) or doubling and point > addition (in the case of ECDSA) are performed. I believe using a branchless > 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 and the freedom to use Git, Perforce or both. Make the move to Perforce. http://pubads.g.doubleclick.net/gampad/clk?id=122218951&iu=/4140/ostg.clktrk _______________________________________________ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
