(bugfix): fix mpfr error in hybrid-dual

[bencrts]

This PR fixes issue #38.

The problem was related to the exhaustive_search function creating an overflow in mpfr. In particular, when:

cost > 2**31 - 1

we see an mpfr error which crashes sage. To fix this quickly, we can add a condition on cost in the exhaustive search function which sends anything over 2**31 - 1 to oo. This is really slow, so we use 2**10000 instead. We might want to revisit this later, so I've left a todo in the codebase.

[bencrts]

For some reason I can't get the tests to run locally this time (via pytest or by using the old ./doctest.sh file method). Had to do it via the CI alone. Does the CI contain all of the tests we need? Everything seems to pass there.

[malb]

Hi Ben,

I was thinking we could make this slightly more general:

modified   estimator/cost.py
@@ -265,6 +265,8 @@ class Cost:
         """
         Perform basic checks.
         """
+        if self.get("rop", 0) > 2**10000:
+            setattr(self, "rop", oo)
         if self.get("beta", 0) > self.get("d", 0):
             raise RuntimeError(f"β = {self['beta']} > d = {self['d']}")
         if self.get("eta", 0) > self.get("d", 0):
modified   estimator/lwe_guess.py
@@ -217,7 +217,7 @@ class ExhaustiveSearch:
         cost = 2 * size * m
 
         ret = Cost(rop=cost, mem=cost / 2, m=m)
-        return ret
+        return ret.sanity_check()
 
     __name__ = "exhaustive_search"
 
@@ -428,7 +428,7 @@ class Distinguisher:
             raise InsufficientSamplesError(
                 "Not enough samples to distinguish with target advantage."
             )
-        return Cost(rop=m, mem=m, m=m)
+        return Cost(rop=m, mem=m, m=m).sanity_check()
 

with that applied I get

sage: from estimator import *
sage: LWE.estimate.rough(LWE.Parameters(n=4096, q=2^80, Xs=ND.CenteredBinomial(4), Xe=ND.CenteredBinomial(4)))
usvp                 :: rop: ≈2^147.5, red: ≈2^147.5, δ: 1.003380, β: 505, d: 7973, tag: usvp
dual_hybrid          :: rop: ≈2^148.0, mem: ≈2^133.5, m: ≈2^12.0, β: 507, d: 8207, ↻: 1, ζ: 9, tag: dual_hybrid
arora-gb             :: rop: ≈2^179.1, dreg: 9, mem: ≈2^179.1, t: 4, m: ≈2^89.5
{'usvp': rop: ≈2^147.5, red: ≈2^147.5, δ: 1.003380, β: 505, d: 7973, tag: usvp,
 'dual_hybrid': rop: ≈2^148.0, mem: ≈2^133.5, m: ≈2^12.0, β: 507, d: 8207, ↻: 1, ζ: 9, tag: dual_hybrid,
 'arora-gb': rop: ≈2^179.1, dreg: 9, mem: ≈2^179.1, t: 4, m: ≈2^89.5}

[bencrts]

Updated - thanks! I have now learned how sanity_check() works :).

[malb]

Thanks!