1 files changed, 42 insertions, 0 deletions
diff --git a/src/Otp.NET/KeyGeneration.cs b/src/Otp.NET/KeyGeneration.cs
index 7e5b613..543fef6 100644
--- a/src/Otp.NET/KeyGeneration.cs
+++ b/src/Otp.NET/KeyGeneration.cs
@@ -23,6 +23,9 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 */
 
+using System;
+using System.Security.Cryptography;
+
 namespace OtpNet
 {
     /// <summary>
@@ -55,6 +58,45 @@ namespace OtpNet
             return GenerateRandomKey(LengthForMode(mode));
         }
 
+        /// <summary>
+        /// Uses the procedure defined in RFC 4226 section 7.5 to derive a key from the master key
+        /// </summary>
+        /// <param name="masterKey">The master key from which to derive a device specific key</param>
+        /// <param name="publicIdentifier">The public identifier that is unique to the authenticating device</param>
+        /// <param name="mode">The hash mode to use.  This will determine the resulting key lenght.  The default is sha-1 (as per the RFC) which is 20 bytes</param>
+        /// <returns>Derived key</returns>
+        public static byte[] DeriveKeyFromMaster(IKeyProvider masterKey, byte[] publicIdentifier, OtpHashMode mode = OtpHashMode.Sha1)
+        {
+            if(masterKey == null)
+                throw new ArgumentNullException("masterKey");
+            return masterKey.ComputeHmac(mode, publicIdentifier);
+        }
+
+        /// <summary>
+        /// Uses the procedure defined in RFC 4226 section 7.5 to derive a key from the master key
+        /// </summary>
+        /// <param name="masterKey">The master key from which to derive a device specific key</param>
+        /// <param name="serialNumber">A serial number that is unique to the authenticating device</param>
+        /// <param name="mode">The hash mode to use.  This will determine the resulting key lenght.  The default is sha-1 (as per the RFC) which is 20 bytes</param>
+        /// <returns>Derived key</returns>
+        public static byte[] DeriveKeyFromMaster(IKeyProvider masterKey, int serialNumber, OtpHashMode mode = OtpHashMode.Sha1)
+        {
+            return DeriveKeyFromMaster(masterKey, KeyUtilities.GetBigEndianBytes(serialNumber), mode);
+        }
+
+        private static HashAlgorithm GetHashAlgorithmForMode(OtpHashMode mode)
+        {
+            switch(mode)
+            {
+                case OtpHashMode.Sha256:
+                    return SHA256.Create();
+                case OtpHashMode.Sha512:
+                    return SHA512.Create();
+                default: //case OtpHashMode.Sha1:
+                    return SHA1.Create();
+            }
+        }
+
         private static int LengthForMode(OtpHashMode mode)
         {
             switch(mode)