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randombit / botan / 5355576307

23 Jun 2023 11:22AM UTC coverage: 91.735% (-0.001%) from 91.736%
5355576307

Pull #3595

github

web-flow
Merge 92dd5d647 into 92171c524
Pull Request #3595: Improve clang-tidy coverage

78150 of 85191 relevant lines covered (91.74%)

12467959.48 hits per line

Source File
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98.08
/src/fuzzer/invert.cpp
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/*
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* (C) 2015,2016,2020 Jack Lloyd
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*
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* Botan is released under the Simplified BSD License (see license.txt)
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*/
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#include "fuzzers.h"
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#include <botan/numthry.h>
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namespace {
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Botan::BigInt ref_inverse_mod(const Botan::BigInt& n, const Botan::BigInt& mod) {
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   if(n == 0 || mod < 2) {
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      return 0;
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   }
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   if(n.is_even() && mod.is_even()) {
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      return 0;
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   }
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   Botan::BigInt u = mod, v = n;
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   Botan::BigInt A = 1, B = 0, C = 0, D = 1;
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   while(u.is_nonzero()) {
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      const size_t u_zero_bits = Botan::low_zero_bits(u);
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      u >>= u_zero_bits;
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      for(size_t i = 0; i != u_zero_bits; ++i) {
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         if(A.is_odd() || B.is_odd()) {
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            A += n;
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            B -= mod;
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         }
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         A >>= 1;
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         B >>= 1;
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      }
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      const size_t v_zero_bits = Botan::low_zero_bits(v);
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      v >>= v_zero_bits;
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      for(size_t i = 0; i != v_zero_bits; ++i) {
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         if(C.is_odd() || D.is_odd()) {
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            C += n;
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            D -= mod;
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         }
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         C >>= 1;
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         D >>= 1;
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      }
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      if(u >= v) {
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         u -= v;
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         A -= C;
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         B -= D;
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      } else {
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         v -= u;
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         C -= A;
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         D -= B;
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      }
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   }
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   if(v != 1) {
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      return 0;  // no modular inverse
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   }
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   while(D.is_negative()) {
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      D += mod;
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   }
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   while(D >= mod) {
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      D -= mod;
×
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   }
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   return D;
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}
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}  // namespace
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void fuzz(const uint8_t in[], size_t len) {
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   static const size_t max_bits = 4096;
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   if(len > 2 * max_bits / 8) {
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      return;
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   }
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   const Botan::BigInt x = Botan::BigInt::decode(in, len / 2);
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   Botan::BigInt mod = Botan::BigInt::decode(in + len / 2, len - len / 2);
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   if(mod < 2) {
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      return;
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   }
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   const Botan::BigInt lib = Botan::inverse_mod(x, mod);
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   const Botan::BigInt ref = ref_inverse_mod(x, mod);
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   if(ref != lib) {
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      FUZZER_WRITE_AND_CRASH("X = " << x << "\n"
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                                    << "Mod = " << mod << "\n"
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                                    << "GCD(X,Mod) = " << gcd(x, mod) << "\n"
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                                    << "RefInv(X,Mod) = " << ref << "\n"
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                                    << "LibInv(X,Mod)  = " << lib << "\n"
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                                    << "RefCheck = " << (x * ref) % mod << "\n"
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                                    << "LibCheck  = " << (x * lib) % mod << "\n");
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   }
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}
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