c++ - 通用常数时间比较函数 C++

Question

我正在编写一个使用 Windows Crypto API 保护内存中对象的 ProtectedPtr 类，但我在创建通用常量时间比较函数时遇到了问题。我当前的代码:

template <class T>
bool operator==(volatile const ProtectedPtr& other)
{
    std::size_t thisDataSize = sizeof(*protectedData) / sizeof(T);
    std::size_t otherDataSize = sizeof(*other) / sizeof(T);

    volatile auto thisData = (byte*)getEncyptedData();
    volatile auto otherData = (byte*)other.getEncyptedData();

    if (thisDataSize != otherDataSize)
        return false;

    volatile int result = 0;
    for (int i = 0; i < thisDataSize; i++)
        result |= thisData[i] ^ otherData[i];

    return result == 0;
}

getEncryptedData 函数:

std::unique_ptr<T> protectedData;
const T& getEncyptedData() const
{
    ProtectMemory(true);
    return *protectedData;
}

问题是转换为 byte*。将此类与字符串一起使用时，我的编译器会提示无法将字符串强制转换为字节指针。我在想也许我的函数基于 Go 的 ConstantTimeByteEq 函数，但它仍然让我回到我最初的问题，即将模板类型转换为 int 或我可以执行二进制操作的东西。

Go 的 ConstantTimeByteEq 函数:

func ConstantTimeByteEq(x, y uint8) int {
    z := ^(x ^ y)
    z &= z >> 4
    z &= z >> 2
    z &= z >> 1

    return int(z)
}

我怎样才能轻松地将模板类型转换成可以轻松执行二进制操作的类型？

更新 根据 lockcmpxchg8b 的建议工作通用常量比较函数:

//only works on primative types, and types that don't have
//internal pointers pointing to dynamically allocated data
byte* serialize()
{
    const size_t size = sizeof(*protectedData);
    byte* out = new byte[size];

    ProtectMemory(false);
    memcpy(out, &(*protectedData), size);
    ProtectMemory(true);

    return out;
}

bool operator==(ProtectedPtr& other)
{
    if (sizeof(*protectedData) != sizeof(*other))
        return false;

    volatile auto thisData = serialize();
    volatile auto otherData = other.serialize();

    volatile int result = 0;
    for (int i = 0; i < sizeof(*protectedData); i++)
        result |= thisData[i] ^ otherData[i];

    //wipe the unencrypted copies of the data
    SecureZeroMemory(thisData, sizeof(thisData));
    SecureZeroMemory(otherData, sizeof(otherData));

    return result == 0;
}

Answer 1

通常，您要在当前代码中完成的工作称为 Format Preserving Encryption .即，加密 std::string，使得生成的密文也是有效的 std::string。这比让加密过程从原始类型转换为平面字节数组要困难得多。

要转换为平面数组，请为“Serializer”对象声明第二个模板参数，该对象知道如何将类型 T 的对象序列化为 unsigned char 数组。您可以将其默认为适用于所有原始类型的通用 sizeof/memcpy 序列化程序。

这是 std::string 的示例。

template <class T>
class Serializer
{
  public:
    virtual size_t serializedSize(const T& obj) const = 0;
    virtual size_t serialize(const T& obj, unsigned char *out, size_t max) const = 0;
    virtual void deserialize(const unsigned char *in, size_t len, T& out) const = 0;
};

class StringSerializer : public Serializer<std::string>
{
public:

  size_t serializedSize(const std::string& obj) const {
    return obj.length();
  };

  size_t serialize(const std::string& obj, unsigned char *out, size_t max) const {
    if(max >= obj.length()){
      memcpy(out, obj.c_str(), obj.length());
      return obj.length();
    }
    throw std::runtime_error("overflow");
  }

  void deserialize(const unsigned char *in, size_t len, std::string& out) const {
    out = std::string((const char *)in, (const char *)(in+len));
  }
};

一旦您将对象缩减为 unsigned char 的平面数组，那么您给定的恒定时间比较算法就可以正常工作。

这是使用上述序列化程序的示例代码的精简版。

template <class T, class S>
class Test
{
  std::unique_ptr<unsigned char[]> protectedData;
  size_t serSize;
public:
  Test(const T& obj) : protectedData() {
    S serializer;
    size_t size = serializer.serializedSize(obj);

    protectedData.reset(new unsigned char[size]);
    serSize = serializer.serialize(obj, protectedData.get(), size);

    // "Encrypt"
    for(size_t i=0; i< size; i++)
      protectedData.get()[i] ^= 0xa5;
  }

  size_t getEncryptedLen() const {
    return serSize;
  }
  const unsigned char *getEncryptedData() const {
    return protectedData.get();
  }

  const T getPlaintextData() const {
    S serializer;
    T target;

    //"Decrypt"
    for(size_t i=0; i< serSize; i++)
      protectedData.get()[i] ^= 0xa5;

    serializer.deserialize(protectedData.get(), serSize, target);
    return target;
  }
};

int main(int argc, char *argv[])
{
  std::string data = "test";
  Test<std::string, StringSerializer> tester(data);

  const unsigned char *ptr = tester.getEncryptedData();
  std::cout << "\"Encrypted\" bytes: ";
  for(size_t i=0; i<tester.getEncryptedLen(); i++)
    std::cout << std::setw(2) << std::hex << std::setfill('0') << (unsigned int)ptr[i] << " ";
  std::cout << std::endl;

  std::string recov = tester.getPlaintextData();

  std::cout << "Recovered: " << recov << std::endl;
}

输出:

$ ./a.out
"Encrypted" bytes: d1 c0 d6 d1
Recovered: test

编辑:回答对原始/平面类型的通用序列化程序的请求。将其视为伪代码，因为我在没有测试的情况下将其输入浏览器。我不确定这是否是正确的模板语法。

template<class T>
class PrimitiveSerializer : public Serializer<T>
{
public:

  size_t serializedSize(const T& obj) const {
    return sizeof obj;
  };

  size_t serialize(const T& obj, unsigned char *out, size_t max) const {
    if(max >= sizeof obj){
      memcpy(out, &obj, sizeof obj);
      return sizeof obj;
    }
    throw std::runtime_error("overflow");
  }

  void deserialize(const unsigned char *in, size_t len, T& out) const {
    if(len < sizeof out) {
      throw std::runtime_error("underflow");
    }
    memcpy(&out, in, sizeof out);
  }
};