feat(utils): export constant_time_equal

Author: LimiNode

README-RU.md

@@ -112,6 +112,13 @@ auto sig = hmac::get_hmac(key, payload, hmac::TypeHash::SHA256);
 secure_zero(key); // при необходимости: очистить после использования
 ```
 
+Чтобы сравнить два токена напрямую, используйте
+`hmac::constant_time_equal` для защиты от атак по времени:
+
+```cpp
+bool same = hmac::constant_time_equal(expected_token, user_token); // длины публичны
+```
+
 ### HMAC (сырые бинарные данные)
 
 ```cpp
@@ -285,21 +292,27 @@ try {
 ```cpp
 #include <iostream>
 #include <hmac_cpp/hmac.hpp>
+#include <hmac_cpp/hmac_utils.hpp>
 
 int main() {
     std::string input = "grape";
     std::string key = "12345";
 
-    std::string hmac_sha256 = hmac::get_hmac(key, input, hmac::TypeHash::SHA256);
-    std::cout << "HMAC-SHA256: " << hmac_sha256 << std::endl;
-
-    std::string hmac_sha512 = hmac::get_hmac(key, input, hmac::TypeHash::SHA512);
-    std::cout << "HMAC-SHA512: " << hmac_sha512 << std::endl;
+    std::string mac = hmac::get_hmac(key, input, hmac::TypeHash::SHA256);
+    if (hmac::constant_time_equal(mac,
+            "7632ac2e8ddedaf4b3e7ab195fefd17571c37c970e02e169195a158ef59e53ca")) {
+        std::cout << "MAC проверен\n";
+    }
 
     return 0;
 }
 ```
 
+**Примечание:** `constant_time_equal` считает длину входных данных публичной и
+время работы зависит от максимальной длины. Не проверяйте длины отдельно —
+ранние проверки могут выдать информацию через побочные каналы времени
+выполнения.
+
 ## 📚 Полезные ссылки
 
 * Исходный код [SHA256](http://www.zedwood.com/article/cpp-sha256-function)

README.md

@@ -141,6 +141,13 @@ std::vector<uint8_t> sig =
 secure_zero(key); // optional: wipe after use
 ```
 
+To compare two tokens directly, use `hmac::constant_time_equal` for a
+timing-safe check:
+
+```cpp
+bool same = hmac::constant_time_equal(expected_token, user_token); // lengths are public
+```
+
 ### HMAC (binary data: raw buffer)
 
 ```cpp
@@ -344,8 +351,9 @@ int main() {
 }
 ```
 
-**Note:** avoid checking input lengths before calling `constant_time_equal`.
-Early length comparisons can leak information through timing side channels.
+**Note:** `constant_time_equal` treats input lengths as public and may run
+longer for longer inputs. Avoid checking lengths separately—early length
+comparisons can leak information through timing side channels.
 
 ## 📚 Resources
 

include/hmac_cpp/hmac_utils.hpp

@@ -24,6 +24,17 @@ namespace hmac_cpp {
     HMAC_CPP_API bool constant_time_equals(const uint8_t* a, size_t a_len,
                               const uint8_t* b, size_t b_len);
 
+    /// \brief Alias for \c constant_time_equals.
+    /// \param a Pointer to first array.
+    /// \param a_len Length of the first array.
+    /// \param b Pointer to second array.
+    /// \param b_len Length of the second array.
+    /// \return true if both arrays are equal.
+    /// \note Avoids early length checks; input lengths are treated as public
+    ///       and may influence timing.
+    HMAC_CPP_API bool constant_time_equal(const uint8_t* a, size_t a_len,
+                                          const uint8_t* b, size_t b_len);
+
     /// \brief Compare vectors in constant time.
     /// \param a First vector.
     /// \param b Second vector.
@@ -42,19 +53,7 @@ namespace hmac_cpp {
                                     reinterpret_cast<const uint8_t*>(b.data()), b.size());
     }
 
-    /// \brief Alias for constant_time_equals.
-    /// \param a Pointer to first array.
-    /// \param a_len Length of the first array.
-    /// \param b Pointer to second array.
-    /// \param b_len Length of the second array.
-    /// \return true if both arrays are equal.
-    /// \note Avoids early length checks to mitigate timing attacks.
-    inline bool constant_time_equal(const uint8_t* a, size_t a_len,
-                                    const uint8_t* b, size_t b_len) {
-        return constant_time_equals(a, a_len, b, b_len);
-    }
-
-    /// \brief Alias for constant_time_equals on vectors.
+    /// \brief Alias for \c constant_time_equal on vectors.
     /// \param a First vector.
     /// \param b Second vector.
     /// \return true if both vectors are equal.
@@ -63,7 +62,7 @@ namespace hmac_cpp {
         return constant_time_equal(a.data(), a.size(), b.data(), b.size());
     }
 
-    /// \brief Alias for constant_time_equals on strings.
+    /// \brief Alias for \c constant_time_equal on strings.
     /// \param a First string.
     /// \param b Second string.
     /// \return true if both strings are equal.

src/hmac_utils.cpp

@@ -21,6 +21,11 @@ namespace hmac_cpp {
         return diff == 0;
     }
 
+    bool constant_time_equal(const uint8_t* a, size_t a_len,
+                             const uint8_t* b, size_t b_len) {
+        return constant_time_equals(a, a_len, b, b_len);
+    }
+
     static TypeHash to_type_hash(Pbkdf2Hash prf) {
         switch (prf) {
             case Pbkdf2Hash::Sha1: return TypeHash::SHA1;