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Change Key::validate to use variable time division to fix performance regression #2389

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Change Key::validate to use variable time division to fix performance regression #2389

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Change Key::validate to use variable time division to fix performance…
kaie May 7, 2026
a0b3128
fix linting
kaie May 19, 2026
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10 changes: 6 additions & 4 deletions src/lib/crypto/elgamal.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,6 @@
#include <botan/ffi.h>
#include <botan/bigint.h>
#include <botan/numthry.h>
#include <botan/reducer.h>
#include "botan_utils.hpp"
#include <rnp/rnp_def.h>
#include "elgamal.h"
Expand Down Expand Up @@ -101,10 +100,13 @@ Key::validate(bool secret) const noexcept
return false;
}
/* check for small order subgroups */
Botan::Modular_Reducer reducer(bp);
Botan::BigInt v = bg;
/* Note: we use (v * bg) % bp instead of Modular_Reducer::multiply() because
* Botan >= 3.8.0 changed Modular_Reducer::reduce() to use constant-time
* ct_modulo(), causing a ~190x slowdown.
* BigInt::operator% uses variable-time division. */
Botan::BigInt v = bg;
for (size_t i = 2; i < (1 << 17); i++) {
v = reducer.multiply(v, bg);
v = (v * bg) % bp;
if (!v.cmp_word(1)) {
RNP_LOG("Small subgroup detected. Order %zu", i);
return false;
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