Avoid a small timing channel in Barrett reduction

No known exploit for this but no point taking chances.

1 files changed, 19 insertions, 17 deletions

diff --git a/src/cli/speed.cpp b/src/cli/speed.cpp
index d549253a7..ebf0fe466 100644
--- a/src/cli/speed.cpp
+++ b/src/cli/speed.cpp
@@ -1603,29 +1603,31 @@ class Speed final : public Command
 
       void bench_bn_redc(const std::chrono::milliseconds runtime)
          {
-         Botan::BigInt p;
-         p.set_bit(521);
-         p--;
+         for(size_t bitsize : { 512, 1024, 2048, 4096 })
+            {
+            Botan::BigInt p(rng(), bitsize);
 
-         std::unique_ptr<Timer> barrett_timer = make_timer("Barrett");
-         std::unique_ptr<Timer> schoolbook_timer = make_timer("Schoolbook");
+            std::string bit_str = std::to_string(bitsize);
+            std::unique_ptr<Timer> barrett_timer = make_timer("Barrett-" + bit_str);
+            std::unique_ptr<Timer> schoolbook_timer = make_timer("Schoolbook-" + bit_str);
 
-         Botan::Modular_Reducer mod_p(p);
+            Botan::Modular_Reducer mod_p(p);
 
-         while(schoolbook_timer->under(runtime))
-            {
-            const Botan::BigInt x(rng(), p.bits() * 2 - 2);
+            while(schoolbook_timer->under(runtime))
+               {
+               const Botan::BigInt x(rng(), p.bits() * 2 - 2);
 
-            const Botan::BigInt r1 = barrett_timer->run(
-               [&] { return mod_p.reduce(x); });
-            const Botan::BigInt r2 = schoolbook_timer->run(
-               [&] { return x % p; });
+               const Botan::BigInt r1 = barrett_timer->run(
+                  [&] { return mod_p.reduce(x); });
+               const Botan::BigInt r2 = schoolbook_timer->run(
+                  [&] { return x % p; });
 
-            BOTAN_ASSERT(r1 == r2, "Computed different results");
-            }
+               BOTAN_ASSERT(r1 == r2, "Computed different results");
+               }
 
-         record_result(barrett_timer);
-         record_result(schoolbook_timer);
+            record_result(barrett_timer);
+            record_result(schoolbook_timer);
+            }
          }
 
       void bench_inverse_mod(const std::chrono::milliseconds runtime)