-Added support for variable length challenges

-New "lt64" field in xml file handled properly
-KeeChallengeProv is no longer a static class but object oriented

src/keepass2android/ChallengeInfo.cs

@@ -31,6 +31,8 @@ namespace KeeChallenge
 {
 	public class ChallengeInfo
 	{
+        private bool m_LT64;
+
 		public byte[] EncryptedSecret {
 			get;
 			private set;
@@ -51,16 +53,24 @@ namespace KeeChallenge
 			private set;
 		}
 
-        private ChallengeInfo()
+        public bool LT64
         {
+            get { return m_LT64;  }
+            private set { m_LT64 = value; }
         }
 
-		public ChallengeInfo(byte[] encryptedSecret, byte[] iv, byte[] challenge, byte[] verification)
+        private ChallengeInfo()
+        {
+            LT64 = false;
+        }
+
+		public ChallengeInfo(byte[] encryptedSecret, byte[] iv, byte[] challenge, byte[] verification, bool lt64)
 		{
 			EncryptedSecret = encryptedSecret;
 			IV = iv;
 			Challenge = challenge;
 			Verification = verification;
+            LT64 = lt64;
 		}
 
 		public static ChallengeInfo Load(IOConnectionInfo ioc)
@@ -125,6 +135,10 @@ namespace KeeChallenge
 							xml.Read();
 							Verification = Convert.FromBase64String(xml.Value.Trim());
 							break;
+                        case "lt64":
+                            xml.Read();
+                            if (!bool.TryParse(xml.Value.Trim(), out m_LT64)) throw new Exception("Unable to parse LT64 flag");
+                            break;
 						}
 					}
 				}
@@ -184,6 +198,7 @@ namespace KeeChallenge
 
 				xml.WriteElementString("challenge", Convert.ToBase64String(Challenge));
 				xml.WriteElementString("verification", Convert.ToBase64String(Verification));
+                xml.WriteElementString("lt64", LT64.ToString());
 
 				xml.WriteEndElement();
 				xml.WriteEndDocument();

src/keepass2android/KeeChallenge.cs

@@ -46,19 +46,40 @@ namespace KeeChallenge
         public const int responseLenBytes = 20;
         public const int secretLenBytes = 20;
 
-		private KeeChallengeProv()
+        //If variable length challenges are enabled, a 63 byte challenge is sent instead.
+        //See GenerateChallenge() and http://forum.yubico.com/viewtopic.php?f=16&t=1078
+        //This field is automatically set by calling GetSecret(). However, when creating 
+        //a new database it will need to be set manually based on the user's yubikey settings
+        public bool LT64
         {
-        }         
+            get;
-
+            set;
-        private static byte[] GenerateChallenge()
-        {
-            CryptoRandom rand = CryptoRandom.Instance;
-            return CryptoRandom.Instance.GetRandomBytes(challengeLenBytes);            
         }
 
-        private static byte[] GenerateResponse(byte[] challenge, byte[] key)
+		private KeeChallengeProv()
+        {
+            LT64 = false;
+        }         
+
+        private byte[] GenerateChallenge()
+        {
+            CryptoRandom rand = CryptoRandom.Instance;
+            byte[] chal = CryptoRandom.Instance.GetRandomBytes(challengeLenBytes);
+            if (LT64)
+            {
+                chal[challengeLenBytes - 2] = (byte)~chal[challengeLenBytes - 1];
+            }
+
+            return chal;  
+        }
+
+        private byte[] GenerateResponse(byte[] challenge, byte[] key)
         {
             HMACSHA1 hmac = new HMACSHA1(key);
+
+            if (LT64)
+                challenge = challenge.Take(challengeLenBytes - 1).ToArray();
+
             byte[] resp = hmac.ComputeHash(challenge);
             hmac.Clear();
             return resp;
@@ -71,7 +92,7 @@ namespace KeeChallenge
         /// </summary>
         /// <param name="secret">The un-encrypted secret</param>
         /// <returns>A fully populated ChallengeInfo object ready to be saved</returns>
-		public static ChallengeInfo Encrypt(byte[] secret)
+		public ChallengeInfo Encrypt(byte[] secret)
         {
             //generate a random challenge for use next time
             byte[] challenge = GenerateChallenge();
@@ -101,14 +122,14 @@ namespace KeeChallenge
                 msEncrypt.Close();
             }
 
-			ChallengeInfo inf = new ChallengeInfo (encrypted, IV, challenge, secretHash);
+			ChallengeInfo inf = new ChallengeInfo (encrypted, IV, challenge, secretHash, LT64);
 
 			sha.Clear();
 
 			return inf;
         }
                 
-		private static bool DecryptSecret(byte[] yubiResp, ChallengeInfo inf, out byte[] secret)
+		private bool DecryptSecret(byte[] yubiResp, ChallengeInfo inf, out byte[] secret)
         {
             secret = new byte[keyLenBytes];
 
@@ -156,7 +177,7 @@ namespace KeeChallenge
         ///                   This should be populated from the database.xml auxilliary file</param>
         /// <param name="resp"	>The Yubikey's response to the issued challenge</param>
         /// <returns>The common secret, used as a composite key to encrypt a Keepass database</returns>
-		public static byte[] GetSecret(ChallengeInfo inf, byte[] resp)
+		public byte[] GetSecret(ChallengeInfo inf, byte[] resp)
         {
 			if (resp.Length != responseLenBytes)
 				return null;
@@ -165,6 +186,8 @@ namespace KeeChallenge
             if (inf.Challenge == null ||
                 inf.Verification == null)
                 return null;
+            
+            LT64 = inf.LT64;
 
 			byte[] secret;
                       

src/keepass2android/PasswordActivity.cs

@@ -129,6 +129,7 @@ namespace keepass2android
 		private OtpInfo _otpInfo;
         private ChallengeInfo _chalInfo;
         private byte[] _challengeSecret;
+        private KeeChallengeProv _challengeProv;
 		private readonly int[] _otpTextViewIds = new[] {Resource.Id.otp1, Resource.Id.otp2, Resource.Id.otp3, Resource.Id.otp4, Resource.Id.otp5, Resource.Id.otp6};
 		private const string OtpInfoKey = "OtpInfoKey";
 		private const string EnteredOtpsKey = "EnteredOtpsKey";
@@ -325,8 +326,9 @@ namespace keepass2android
 			{
 				try
 				{
+                    _challengeProv = new KeeChallengeProv();
 					byte[] challengeResponse = data.GetByteArrayExtra("response");
-					_challengeSecret = KeeChallengeProv.GetSecret(_chalInfo, challengeResponse);
+					_challengeSecret = _challengeProv.GetSecret(_chalInfo, challengeResponse);
 					Array.Clear(challengeResponse, 0, challengeResponse.Length);
 				}
 				catch (Exception e)
@@ -348,7 +350,7 @@ namespace keepass2android
 						//save aux file
 						try
 						{
-							ChallengeInfo temp = KeeChallengeProv.Encrypt(_challengeSecret);
+							ChallengeInfo temp = _challengeProv.Encrypt(_challengeSecret);
 							IFileStorage fileStorage = App.Kp2a.GetOtpAuxFileStorage(_ioConnection);
 							IOConnectionInfo iocAux = fileStorage.GetFilePath(fileStorage.GetParentPath(_ioConnection),
 								fileStorage.GetFilenameWithoutPathAndExt(_ioConnection) + ".xml");