1 files changed, 74 insertions, 0 deletions
diff --git a/TwoStepsAuthenticator/CounterAuthenticator.cs b/TwoStepsAuthenticator/CounterAuthenticator.cs
new file mode 100644
index 0000000..fffae73
--- /dev/null
+++ b/TwoStepsAuthenticator/CounterAuthenticator.cs
@@ -0,0 +1,74 @@
+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;
+        private readonly IUsedCodesManager UsedCodeManager;
+
+        public CounterAuthenticator(int windowSize = 10, IUsedCodesManager usedCodeManager = null) {
+            if (windowSize <= 0) {
+                throw new ArgumentException("look-ahead window size must be positive");
+            }
+
+            this.UsedCodeManager = (usedCodeManager == null) ? Authenticator.DefaultUsedCodeManager.Value : usedCodeManager;
+            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, ulong 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, ulong counter) {
+            ulong successfulSequenceNumber = 0uL;
+
+            return CheckCode(secret, code, counter, out successfulSequenceNumber);
+        }
+
+        /// <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>
+        /// <param name="usedCounter">Matching counter value if successful</param>
+        /// <returns>true if any code from counter to counter + WindowSize matches</returns>
+        public bool CheckCode(string secret, string code, ulong counter, out ulong usedCounter) {
+            var codeMatch = false;
+            ulong successfulSequenceNumber = 0uL;
+
+            for (uint i = 0; i <= WindowSize; i++) {
+                ulong checkCounter = counter + i;
+                if (ConstantTimeEquals(GetCode(secret, checkCounter), code) && !UsedCodeManager.IsCodeUsed(checkCounter, code)) {
+                    codeMatch = true;
+                    successfulSequenceNumber = checkCounter;
+
+                    UsedCodeManager.AddCode(successfulSequenceNumber, code);
+                }
+            }
+
+            usedCounter = successfulSequenceNumber;
+            return codeMatch;
+        }
+    }
+}