Add keyspecs interface and implementation

lib/src/keyspec.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2020
+// Author: Hugo Pointcheval
+
+/// This represents security parameters.
+abstract class KeySpec {
+  /// Returns the standard algorithm name for this key
+  String get algorithm;
+}
+
+class RSAKeySpec implements KeySpec {
+  int _size;
+
+  String get algorithm => "RSA";
+
+  /// Returns the size of RSA keys
+  int get size => _size;
+
+  RSAKeySpec(int size) {
+    _size = size;
+  }
+}