Use a simpler formula for Kyber division by q#3959
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Simpler approach than #3898
FAlbertDev
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Apr 2, 2024
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FAlbertDev
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I'm okay with using this more straightforward approach. We can stay with this simple method until we need other CT divisions in the future.
Since we already have a test for our ct_div function in #3898, we may also want to apply it here.
// src/tests/test_kyber.cpp
namespace {
std::vector<Test::Result> test_kyber_utilities() {
return {
Botan_Tests::CHECK("constant-time division",
[](Test::Result& result) {
constexpr auto Q = Botan::KyberConstants::Q;
// Check ct_int_div_kyber_q with all possible inputs that may appear in Kyber.
for(uint32_t i = 0; i < (1 << 18); ++i) {
result.test_is_eq(
Botan::fmt("{}/Q", i), Botan::ct_int_div_kyber_q(i), i / Q);
}
}),
};
}
} // namespace
BOTAN_REGISTER_TEST_FN("kyber", "kyber_utils", test_kyber_utilities);
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The function isn't currently exposed anywhere and I'm inclined not to expose it simply to test it, considering it is quite small and should already be very well tested as a side effect of the Kyber tests themselves. I did do an offline test that the expression works for all integers < 2**20. |
This was referenced Apr 5, 2024
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Simpler approach than #3898