Add RSA KEM implementation

lib/src/asym/RSA.dart

@@ -0,0 +1,74 @@
+// Copyright (c) 2020
+// Author: Hugo Pointcheval
+
+import 'package:native_crypto/src/exceptions.dart';
+import 'package:native_crypto/src/kem.dart';
+import 'package:native_crypto/src/key.dart';
+
+/// This represents RSA Key Encapsulation Mechanism
+class RSAKeyEncapsulationMechanism implements KeyEncapsulationMechanism {
+  bool _isInit = false;
+  KemMode _mode;
+
+  PublicKey _publicKey;
+  PrivateKey _privateKey;
+
+  @override
+  String get algorithm => "RSA";
+
+  @override
+  Object get options => throw UnimplementedError();
+
+  @override
+  bool get isInitialized => _isInit;
+
+  @override
+  KemMode get mode => _mode;
+
+  @override
+  KeyPair get keypair => KeyPair.from(_publicKey, _privateKey);
+
+  RSAKeyEncapsulationMechanism(
+    KemMode mode, {
+    KeyPair keyPair,
+    PublicKey publicKey,
+  }) {
+    _mode = mode;
+    if (_mode == KemMode.ENCAPSULATION) {
+      if (publicKey != null) {
+        _isInit = true;
+        _publicKey = publicKey;
+      } else if (keyPair != null) {
+        if (keyPair.publicKey != null) {
+          _isInit = true;
+          _publicKey = keyPair.publicKey;
+        }
+      }
+    } else if (_mode == KemMode.DECAPSULATION) {
+      if (keyPair != null) {
+        if (keyPair.isComplete) {
+          _isInit = true;
+          _publicKey = keyPair.publicKey;
+          _privateKey = keyPair.privateKey;
+        } else {
+          throw KemInitException("Keypair must be complete for decapsulation.");
+        }
+      } else {
+        throw KemInitException("You must provide a keypair for decapsulation.");
+      }
+    }
+  }
+
+  @override
+  Future<Encapsulation> encapsulate() {
+    if (!_isInit) {
+      throw KemInitException("KEM not properly initialized.");
+    }
+    throw UnimplementedError();
+  }
+
+  @override
+  Future<SecretKey> decapsulate(Encapsulation encapsulation) {
+    throw UnimplementedError();
+  }
+}