How do I use constant-time comparisons in C++?

In C++, constant-time comparisons are useful for securely comparing sensitive data, such as passwords or cryptographic keys, to prevent timing attacks. A common approach is to use a method that ensures both inputs are processed in the same amount of time, regardless of their content.

The `std::equal` function can be employed for this purpose, where you compare two strings character by character. This ensures that the comparison takes the same time whether the strings match or not.

Below is a simple example demonstrating how to implement constant-time string comparison in C++:

#include <iostream> #include <string> #include <algorithm> bool constant_time_compare(const std::string& a, const std::string& b) { if (a.size() != b.size()) return false; volatile unsigned char result = 0; // Prevent optimization for (size_t i = 0; i < a.size(); ++i) { result |= a[i] ^ b[i]; } return result == 0; } int main() { std::string password = "secret"; std::string input; std::cout << "Enter password: "; std::cin >> input; if (constant_time_compare(password, input)) { std::cout << "Access granted!" << std::endl; } else { std::cout << "Access denied!" << std::endl; } return 0; }

C++ constant-time comparisons secure comparing timing attacks std::equal string comparison

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