invert.cpp - mozsearch

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/*

* (C) 2015,2016,2020 Jack Lloyd

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include "fuzzers.h"

#include <botan/numthry.h>

namespace {

Botan::BigInt ref_inverse_mod(const Botan::BigInt& n, const Botan::BigInt& mod)

   if(n == 0 || mod < 2)

      return 0;

   if(n.is_even() && mod.is_even())

      return 0;

   Botan::BigInt u = mod, v = n;

   Botan::BigInt A = 1, B = 0, C = 0, D = 1;

   while(u.is_nonzero())

      const size_t u_zero_bits = Botan::low_zero_bits(u);

      u >>= u_zero_bits;

      for(size_t i = 0; i != u_zero_bits; ++i)

         if(A.is_odd() || B.is_odd())

            { A += n; B -= mod; }

         A >>= 1; B >>= 1;

      const size_t v_zero_bits = Botan::low_zero_bits(v);

      v >>= v_zero_bits;

      for(size_t i = 0; i != v_zero_bits; ++i)

         if(C.is_odd() || D.is_odd())

            { C += n; D -= mod; }

         C >>= 1; D >>= 1;

      if(u >= v) { u -= v; A -= C; B -= D; }

      else       { v -= u; C -= A; D -= B; }

   if(v != 1)

      return 0; // no modular inverse

   while(D.is_negative()) D += mod;

   while(D >= mod) D -= mod;

   return D;

void fuzz(const uint8_t in[], size_t len)

   static const size_t max_bits = 4096;

   if(len > 2*max_bits/8)

      return;

   const Botan::BigInt x = Botan::BigInt::decode(in, len / 2);

   Botan::BigInt mod = Botan::BigInt::decode(in + len / 2, len - len / 2);

   if(mod < 2)

      return;

   const Botan::BigInt lib = Botan::inverse_mod(x, mod);

   const Botan::BigInt ref = ref_inverse_mod(x, mod);

   if(ref != lib)

      FUZZER_WRITE_AND_CRASH("X = " << x << "\n"

                             << "Mod = " << mod << "\n"

                             << "GCD(X,Mod) = " << gcd(x, mod) << "\n"

                             << "RefInv(X,Mod) = " << ref << "\n"

                             << "LibInv(X,Mod)  = " << lib << "\n"

                             << "RefCheck = " << (x*ref)%mod << "\n"

                             << "LibCheck  = " << (x*lib)%mod << "\n");