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feat(utils): add constant-time token comparison #22

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Merged
LimiNode merged 1 commit into from
Sep 3, 2025
Merged

feat(utils): add constant-time token comparison #22

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21 changes: 15 additions & 6 deletions hmac_utils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,15 @@
#include <stdexcept>

namespace hmac {

bool constant_time_equals(const std::string &a, const std::string &b) {
if (a.size() != b.size()) return false;
unsigned char diff = 0;
for (size_t i = 0; i < a.size(); ++i) {
diff |= static_cast<unsigned char>(a[i]) ^ static_cast<unsigned char>(b[i]);
}
return diff == 0;
}

std::string generate_time_token(const std::string &key, int interval_sec, TypeHash hash_type) {
if (interval_sec <= 0) {
Expand All @@ -25,9 +34,9 @@ namespace hmac {
throw std::runtime_error("std::time failed");
}
std::time_t rounded = (now / interval_sec) * interval_sec;
if (token == get_hmac(key, std::to_string(rounded), hash_type)) return true;
if (token == get_hmac(key, std::to_string(rounded - interval_sec), hash_type)) return true;
if (token == get_hmac(key, std::to_string(rounded + interval_sec), hash_type)) return true;
if (constant_time_equals(token, get_hmac(key, std::to_string(rounded), hash_type))) return true;
if (constant_time_equals(token, get_hmac(key, std::to_string(rounded - interval_sec), hash_type))) return true;
if (constant_time_equals(token, get_hmac(key, std::to_string(rounded + interval_sec), hash_type))) return true;
return false;
}

Expand Down Expand Up @@ -55,11 +64,11 @@ namespace hmac {
std::time_t rounded = (now / interval_sec) * interval_sec;
std::string prefix = "|" + fingerprint;
std::string payload = std::to_string(rounded) + prefix;
if (token == get_hmac(key, payload, hash_type)) return true;
if (constant_time_equals(token, get_hmac(key, payload, hash_type))) return true;
payload = std::to_string(rounded - interval_sec) + prefix;
if (token == get_hmac(key, payload, hash_type)) return true;
if (constant_time_equals(token, get_hmac(key, payload, hash_type))) return true;
payload = std::to_string(rounded + interval_sec) + prefix;
if (token == get_hmac(key, payload, hash_type)) return true;
if (constant_time_equals(token, get_hmac(key, payload, hash_type))) return true;
return false;
}

Expand Down
6 changes: 6 additions & 0 deletions hmac_utils.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,12 @@

namespace hmac {

/// \brief Compares two strings in constant time
/// \param a First string
/// \param b Second string
/// \return true if both strings are equal
bool constant_time_equals(const std::string &a, const std::string &b);

/// \brief Generates a time-based HMAC-SHA256 token
/// \param key Secret key used for HMAC
/// \param interval_sec Interval in seconds that defines token rotation. Must be positive. Default is 60 seconds
Expand Down
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