IMPLEMENT X3DH (without OMEMO)

lib/src/x3dh/x3dh.dart

@@ -0,0 +1,107 @@
+import 'dart:convert';
+import 'package:cryptography/cryptography.dart';
+import 'package:cryptography/dart.dart';
+
+/// The overarching assumption is that we use Ed25519 keys for the identity keys
+
+class X3DHRun {
+
+  const X3DHRun(this.epk, this.sharedSecret);
+  final SimpleKeyPair epk;
+  final List<int> sharedSecret;
+}
+
+SimpleKeyPairData fromPublicKey(SimplePublicKey pk) {
+  return SimpleKeyPairData([], publicKey: pk, type: KeyPairType.x25519);
+}
+
+Future<List<int>> sig(SimpleKeyPair keyPair, List<int> message) async {
+  final signature = await Ed25519().sign(
+    message,
+    keyPair: keyPair,
+  );
+
+  return signature.bytes;
+}
+
+/// Performs X25519 with [pk1] and [pk2]. If [identityKey] is set, then
+/// it indicates which of [pk1] ([identityKey] == 1) or [pk2] ([identityKey] == 2)
+/// is the identity key.
+Future<List<int>> dh(SimpleKeyPair kp, SimplePublicKey pk, int identityKey) async {
+  var ckp = kp;
+  var cpk = pk;
+
+  /*
+  if (identityKey == 1) {
+    final pkc = await DartEd25519.publicKeyToCurve25519(kp);
+    final skc = await DartEd25519.privateKeyToCurve25519(kp);
+    ckp = SimpleKeyPairData(skc, publicKey: pkc, type: KeyPairType.x25519);
+  } else if (identityKey == 2) {
+    cpk = await DartEd25519.publicKeyToCurve25519(fromPublicKey(pk));
+  }
+  */
+
+  final shared = await Cryptography.instance.x25519().sharedSecretKey(
+    keyPair: ckp,
+    remotePublicKey: cpk,
+  );
+
+  return shared.extractBytes();
+}
+
+Future<List<int>> kdf(List<int> km) async {
+  final f = List<int>.filled(32, 0xFF);
+  final input = List<int>.empty(growable: true);
+  input
+    ..addAll(f)
+    ..addAll(km);
+
+  final algorithm = Hkdf(
+    hmac: Hmac(Sha256()),
+    outputLength: 32,
+  );
+  final output = await algorithm.deriveKey(
+    secretKey: SecretKey(input),
+    // TODO: Fix
+    nonce: List<int>.filled(32, 0x00),
+    info: utf8.encode('OMEMO X3DH'),
+  );
+
+  return output.extractBytes();
+}
+
+List<int> concat(List<List<int>> inputs) {
+  final tmp = List<int>.empty(growable: true);
+  for (final input in inputs) {
+    tmp.addAll(input);
+  }
+
+  return tmp;
+}
+
+// Alice -> Bob
+Future<X3DHRun> x3dhFromPrekeyBundle(SimplePublicKey ikb, SimplePublicKey spkb, SimplePublicKey opkb, SimpleKeyPair ika) async {
+  // Generate EPK
+  final epk = await Cryptography.instance.x25519().newKeyPair();
+
+  final dh1 = await dh(ika, spkb, 1);
+  final dh2 = await dh(epk, ikb,  2);
+  final dh3 = await dh(epk, spkb, 0);
+  final dh4 = await dh(epk, opkb, 0);
+
+  final sk = await kdf(concat([dh1, dh2, dh3, dh4]));
+
+  return X3DHRun(
+    epk,
+    sk,
+  );
+}
+
+Future<List<int>> x3dhFromInitialMessage(SimplePublicKey ika, SimplePublicKey epk, SimpleKeyPair opkb, SimpleKeyPair spk, SimpleKeyPair ikb) async {
+  final dh1 = await dh(spk, ika, 2);
+  final dh2 = await dh(ikb, epk, 1);
+  final dh3 = await dh(spk, epk, 0);
+  final dh4 = await dh(opkb, epk, 0);
+
+  return kdf(concat([dh1, dh2, dh3, dh4]));
+}