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/*
* (C) 2024 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include "mp_fuzzers.h"
#include <botan/bigint.h>
#include <botan/internal/loadstor.h>
namespace {
consteval word crandall_C() {
if(sizeof(word) == 8) {
// secp256k1 modulus
return static_cast<word>(0x1000003d1);
} else {
// 128 bit prime with largest possible C
return 0xffffffe1;
}
}
} // namespace
void fuzz(std::span<const uint8_t> in) {
if(in.size() != 8 * sizeof(word)) {
return;
}
constexpr word C = crandall_C();
static const Botan::BigInt refp = Botan::BigInt::power_of_2(4 * 8 * sizeof(C)) - C;
static const Botan::BigInt refp2 = refp * refp;
const auto refz = Botan::BigInt::from_bytes(in);
if(refz >= refp2) {
return;
}
const auto refc = refz % refp;
std::array<word, 8> z = {};
for(size_t i = 0; i != 8; ++i) {
z[7 - i] = Botan::load_be<word>(in.subspan(sizeof(word) * i, sizeof(word)));
}
const auto rc = Botan::redc_crandall<word, 4, C>(z);
compare_word_vec(rc.data(), 4, refc._data(), refc.sig_words(), "Crandall reduction");
}