Add hotp counter window

2 files changed, 16 insertions, 4 deletions

diff --git a/src/Otp.php b/src/Otp.php
index ed9b671..e5bff8f 100644
--- a/src/Otp.php
+++ b/src/Otp.php
@@ -91,9 +91,20 @@ class Otp implements OtpInterface
     /* (non-PHPdoc)
      * @see Otp.OtpInterface::checkHotp()
     */
-    public function checkHotp($secret, $counter, $key)
+    public function checkHotp($secret, $counter, $key, $counterwindow = 5)
     {
-        return $this->safeCompare($this->hotp($secret, $counter), $key);
+        if(!is_numeric($counterwindow) || $counterwindow < 0){
+          throw new \InvalidArgumentException('Invalid counterwindow supplied');
+        }
+
+        for($c = 0; $c <= $counterwindow; $c = $c + 1) {
+
+          if($this->safeCompare($this->hotp($secret, $counter + $c), $key)){
+              return $counter + $c;
+          }
+        }
+
+        return false;
     }
     
     /* (non-PHPdoc)
diff --git a/src/OtpInterface.php b/src/OtpInterface.php
index 62e60c8..88241cb 100644
--- a/src/OtpInterface.php
+++ b/src/OtpInterface.php
@@ -47,10 +47,11 @@ interface OtpInterface
      * @param string  $secret  Base32 Secret String
      * @param integer $counter Counter
      * @param string  $key     User supplied key
+     * @param integer $counterwindow Size of the look-ahead window
      *
-     * @return boolean True if key is correct
+     * @return int|boolean the counter if key is correct else false
      */
-    function checkHotp($secret, $counter, $key);
+    function checkHotp($secret, $counter, $key, $counterwindow);
 
     /**
      * Checks Totp agains a key