Initial commit

2020/puzzle-25-01.cc

@@ -0,0 +1,57 @@
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <iostream>
+#include <list>
+#include <map>
+#include <regex>
+#include <set>
+#include <string>
+#include <tuple>
+#include <vector>
+
+constexpr unsigned mod = 20201227;
+constexpr unsigned initial_subject = 7;
+
+unsigned lcg(unsigned i, unsigned subject) {
+  return ((unsigned long)i * (unsigned long)subject) % mod;
+}
+
+unsigned find_loop_length(unsigned pk) {
+  unsigned v = 1;
+  unsigned loop = 0;
+  while (v != pk) {
+    v = lcg(v, initial_subject);
+    ++loop;
+  }
+
+  return loop;
+}
+
+unsigned apply(unsigned pk, unsigned loop_size) {
+  unsigned v = 1;
+  for (unsigned i = 0; i < loop_size; ++i) {
+    v = lcg(v, pk);
+  }
+
+  return v;
+}
+
+int main(void) {
+  std::string line;
+  std::getline(std::cin, line);
+  unsigned pk1 = std::stoul(line);
+  std::getline(std::cin, line);
+  unsigned pk2 = std::stoul(line);
+  std::cout << "Public keys: " << pk1 << " " << pk2 << "\n";
+  auto l1 = find_loop_length(pk1);
+  auto l2 = find_loop_length(pk2);
+  std::cout << "Loop lengths: " << l1 << " " << l2 << "\n";
+
+  auto ek1 = apply(pk1, l2);
+  auto ek2 = apply(pk2, l1);
+
+  std::cout << "Encryption key: " << ek1 << " " << ek2 << "\n";
+
+  return 0;
+}