1 files changed, 51 insertions, 0 deletions
diff --git a/TwoStepsAuthenticator/CounterAuthenticator.cs b/TwoStepsAuthenticator/CounterAuthenticator.cs
new file mode 100644
index 0000000..90b0ee4
--- /dev/null
+++ b/TwoStepsAuthenticator/CounterAuthenticator.cs
@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace TwoStepsAuthenticator {
+
+    /// <summary>
+    /// Implementation of RFC 4226 Counter-Based One-Time Password Algorithm
+    /// </summary>
+    public class CounterAuthenticator : Authenticator {
+        private readonly int WindowSize;
+
+        public CounterAuthenticator(int windowSize = 10) {
+            if (windowSize <= 0) {
+                throw new ArgumentException("look-ahead window size must be positive");
+            }
+
+            this.WindowSize = windowSize;
+        }
+
+        /// <summary>
+        /// Generates One-Time-Password.
+        /// </summary>
+        /// <param name="secret">Shared Secret</param>
+        /// <param name="counter">Current Counter</param>
+        /// <returns>OTP</returns>
+        public string GetCode(string secret, long counter) {
+            return GetCodeInternal(secret, counter);
+        }
+
+        /// <summary>
+        /// Checks if the passed code is valid.
+        /// </summary>
+        /// <param name="secret">Shared Secret</param>
+        /// <param name="code">OTP</param>
+        /// <param name="counter">Current Counter Position</param>
+        /// <returns>true if any code from counter to counter + WindowSize matches</returns>
+        public bool CheckCode(string secret, string code, long counter) {
+            var codeMatch = false;
+            for (int i = 0; i <= WindowSize; i++) {
+                if (ConstantTimeEquals(GetCode(secret, counter + i), code)) {
+                    codeMatch = true;
+                }
+            }
+
+            return codeMatch;
+        }
+    }
+}