solution_16.cpp

#include "cryptopals/aes.h"
#include "cryptopals/answer.h"
#include "cryptopals/block.h"
#include "cryptopals/codec.h"

#include "absl/strings/str_cat.h"
#include "absl/strings/str_replace.h"
#include "absl/strings/str_split.h"

namespace cryptopals {
namespace {
class Helper {
 public:
  Helper() : key_(Block::RandomBlock()) {}

  std::string Oracle(const std::string &in) {
    std::string escaped = absl::StrReplaceAll(in, {{";", "%3B"}, {"=", "%3D"}});
    std::string plain = absl::StrCat("comment1=cooking%20MCs;userdata=", escaped,
                                     ";comment2=%20like%20a%20pound%20of%20bacon");
    PadPKCS7(&plain);
    return EncryptCBC(plain, key_);
  }

  bool IsAdmin(const std::string &cipher) {
    std::string plain = DecryptCBC(cipher, key_);
    UnpadPKCS7(&plain);
    for (std::string_view chunk : absl::StrSplit(plain, ";")) {
      if (chunk.empty()) {
        continue;
      }
      std::vector<std::string_view> chunks = absl::StrSplit(chunk, "=");
      if (chunks.size() == 2 && chunks[0] == "admin") {
        return chunks[1] == "true";
      }
    }
    return false;
  }

 private:
  const Block key_;
};

void FlipBit(char &c) {
  if (c & 1) {
    c &= 0xfe;
  } else {
    c |= 1;
  }
}
}  // namespace

void Solution16(Answer &&ans) {
  Helper h;
  std::string input = ":admin<true";
  std::string cipher = h.Oracle(input);
  FlipBit(cipher[16]);
  FlipBit(cipher[22]);
  ASSERT_ALWAYS(h.IsAdmin(cipher));
}
}  // namespace cryptopals