implemented loading of files with KeepassXC Challenge (#4).

requires write support, handling of Challenge/Response errors (or user cancels). Caution: saving corrupts the file at the moment!

src/KeePassLib2Android/Cryptography/KeyDerivation/AesKdf.cs

@@ -55,7 +55,10 @@ namespace KeePassLib.Cryptography.KeyDerivation
 			get { return "AES-KDF"; }
 		}
 
-		public AesKdf()
+        public override byte[] GetSeed(KdfParameters p)
+        { return p.GetByteArray(ParamSeed); }
+
+        public AesKdf()
 		{
 		}
 

src/KeePassLib2Android/Cryptography/KeyDerivation/Argon2Kdf.cs

@@ -68,7 +68,10 @@ namespace KeePassLib.Cryptography.KeyDerivation
 			get { return "Argon2"; }
 		}
 
-		public Argon2Kdf()
+        public override byte[] GetSeed(KdfParameters p)
+        { return p.GetByteArray(ParamSalt); }
+
+        public Argon2Kdf()
 		{
 		}
 

src/KeePassLib2Android/Cryptography/KeyDerivation/KdfEngine.cs

@@ -36,7 +36,9 @@ namespace KeePassLib.Cryptography.KeyDerivation
 			get;
 		}
 
-		public virtual KdfParameters GetDefaultParameters()
+        public abstract byte[] GetSeed(KdfParameters p);
+
+        public virtual KdfParameters GetDefaultParameters()
 		{
 			return new KdfParameters(this.Uuid);
 		}