eqaf is a small library which wants to provide a /constant-time/ equal function to protect such computation
against a timing attack. Currently, the distribution comes with a tool check/check.exe which benchmarks
our function and introspects results.
For more information about this tool, you should check the README.md of eqaf.
However, as noticed in our Continuous Integration, we can not fully rely on this tool where recorded samples
are disturbed by the underlying scheduler of the operating-system, the virtualization, the CPU cache, etc.
From all of these parameters, it's hard to really know time spend by our equal function.
By this way, it's hard to check if our equal function respects our predicate: the constant-time.
By this fact, when we want to improve eqaf, we mostly introspect GAS assembler and see if any jump don't
leak any information (such as which character starts to differ from the secret value). However, this kind of
development requires that user trusts on us - and should not trust on the CI as we don't really trust on the
Continuous Integration.
This little project wants to go further about our introspection and it provides a way to parse and execute
a GAS assembler. At the end, we want to simulate the execution of our equal function. By this way, we are
able to record correctly samples without noises (such as the cache).
The goal is to fuzz the execution of our equal function into our Virtual Machine and see that for any
random inputs, we surely spend a theoric and expected amount of tick. An GAS instruction spends 1 tick.
However, even if this approach is interesting for us, it does not assert our predicates. At least, we need
to trust on our Virtual Machine which is naive and smaller than a real CPU. By this fact, abstract simulation can
help us but it's not a proof that eqaf provides a /constant-time/ function.
The current project is a bit huge comparing to eqaf and the trade-off is not really interesting to integrate
such tool into our distribution of eqaf. However, the provided binary is useful enough to go further. It's why
it's not a part of the initial eqaf distribution.
Currently, the project is small and only test Eqaf.equal (the first implemented function). You must pin the project and it provides a tool, gas, which launchs the fuzzer on Eqaf.equal:
$ opam pin add eqaf-gas https://github.com/dinosaure/gas-eqaf.git
$ opam install eqaf-gas
$ gas
[x] /tmp/eqaf-97a390/eqaf.cmi compiled.
[x] /tmp/eqaf-97a390/eqaf.cmx compiled.
[x] /tmp/eqaf-97a390/eqaf.s generated.
[x] eqaf.s generated.
camlEqaf__equal_155: ....
camlEqaf__equal_155: PASS
However, gas handles only x86 assembler (depending on your computer, so).
You can see how many ticks we need to execute the function from given inputs with:
$ gas run "toto" "tata"
results: false.
ticks: 70.
$ gas run "toto" "toto"
results: true.
ticks: 70.At least, the project is interesting but it still is experimental and does not respect any release cycle or roadmap. In others words, it's a toy.