fix: guard TOTP counter overflow

hmac_utils.cpp

@@ -1,6 +1,7 @@
 #include "hmac_utils.hpp"
 #include <ctime>
 #include <stdexcept>
+#include <limits>
 
 namespace hmac {
 
@@ -133,7 +134,9 @@ namespace hmac {
         }
         uint64_t counter = timestamp / period;
         if (token == get_hotp_code(key_ptr, key_len, counter, digits, hash_type)) return true;
-        if (token == get_hotp_code(key_ptr, key_len, counter + 1, digits, hash_type)) return true;
+        if (counter != std::numeric_limits<uint64_t>::max() &&
+            token == get_hotp_code(key_ptr, key_len, counter + 1, digits, hash_type))
+            return true;
         if (counter > 0 &&
             token == get_hotp_code(key_ptr, key_len, counter - 1, digits, hash_type))
             return true;
@@ -157,7 +160,9 @@ namespace hmac {
         uint64_t timestamp = static_cast<uint64_t>(std::time(nullptr));
         uint64_t counter = timestamp / period;
         if (token == get_hotp_code(key_ptr, key_len, counter, digits, hash_type)) return true;
-        if (token == get_hotp_code(key_ptr, key_len, counter + 1, digits, hash_type)) return true;
+        if (counter != std::numeric_limits<uint64_t>::max() &&
+            token == get_hotp_code(key_ptr, key_len, counter + 1, digits, hash_type))
+            return true;
         if (counter > 0 &&
             token == get_hotp_code(key_ptr, key_len, counter - 1, digits, hash_type))
             return true;

hmac_utils.hpp

@@ -175,7 +175,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Expected number of digits in the OTP (default: 6)
     /// \param hash_type Hash algorithm to use (SHA1, SHA256, SHA512). Default is SHA1
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     bool is_totp_token_valid(
             int token,
@@ -194,7 +196,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Number of digits in the OTP (default: 6)
     /// \param hash_type Hash function to use (default: SHA1)
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     template<typename T>
     inline bool is_totp_token_valid(
@@ -216,7 +220,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Number of digits in the OTP (default: 6)
     /// \param hash_type Hash function to use (default: SHA1)
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     inline bool is_totp_token_valid(
             int token,
@@ -235,7 +241,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Expected number of digits in the OTP (default: 6)
     /// \param hash_type Hash algorithm to use (default: SHA1)
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     bool is_totp_token_valid(
             int token,
@@ -252,7 +260,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Number of digits in the OTP (default: 6)
     /// \param hash_type Hash function to use (default: SHA1)
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     template<typename T>
     inline bool is_totp_token_valid(
@@ -272,7 +282,9 @@ namespace hmac {
     /// \param period Time step in seconds (default: 30)
     /// \param digits Number of digits in the OTP (default: 6)
     /// \param hash_type Hash function to use (default: SHA1)
-    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise
+    /// \return true if the token is valid within [-1, 0, +1] time step range; false otherwise.
+    ///         The +1 step check is skipped when the computed counter equals
+    ///         std::numeric_limits<uint64_t>::max().
     /// \throws std::invalid_argument if period <= 0 or digits not in [1,9]
     inline bool is_totp_token_valid(
             int token,

test_totp.cpp

@@ -20,6 +20,17 @@ int main() {
     bool valid_max = hmac::is_totp_token_valid(token_max, key.data(), key.size(), early_timestamp, period, digits, hmac::TypeHash::SHA1);
     assert(!valid_max);
 
-    std::cout << "TOTP early timestamp test passed" << std::endl;
+    // At maximum timestamp, ensure overflow does not validate counter 0 token
+    uint64_t max_timestamp = std::numeric_limits<uint64_t>::max();
+    int token_zero = hmac::get_hotp_code(key.data(), key.size(), 0, digits, hmac::TypeHash::SHA1);
+    bool valid_zero_at_max = hmac::is_totp_token_valid(token_zero, key.data(), key.size(), max_timestamp, 1, digits, hmac::TypeHash::SHA1);
+    assert(!valid_zero_at_max);
+
+    // Token for max counter should still be valid at that timestamp
+    int token_max_ts = hmac::get_hotp_code(key.data(), key.size(), max_counter, digits, hmac::TypeHash::SHA1);
+    bool valid_max_ts = hmac::is_totp_token_valid(token_max_ts, key.data(), key.size(), max_timestamp, 1, digits, hmac::TypeHash::SHA1);
+    assert(valid_max_ts);
+
+    std::cout << "TOTP tests passed" << std::endl;
     return 0;
 }