feat: Add signature validation

lib/omemo_dart.dart

@@ -1,5 +1,6 @@
 library omemo_dart;
 
 export 'src/bundle.dart';
+export 'src/errors.dart';
 export 'src/key.dart';
 export 'src/x3dh.dart';

lib/src/errors.dart

@@ -0,0 +1,4 @@
+class InvalidSignatureException implements Exception {
+  @override
+  String errMsg() => 'The signature of the SPK does not match the provided signature';
+}

lib/src/x3dh.dart

@@ -2,6 +2,7 @@ import 'dart:convert';
 import 'dart:math';
 import 'package:cryptography/cryptography.dart';
 import 'package:omemo_dart/src/bundle.dart';
+import 'package:omemo_dart/src/errors.dart';
 import 'package:omemo_dart/src/key.dart';
 
 /// The overarching assumption is that we use Ed25519 keys for the identity keys
@@ -99,6 +100,19 @@ List<int> concat(List<List<int>> inputs) {
 /// Alice builds a session with Bob using his bundle [bundle] and Alice's identity key
 /// pair [ik].
 Future<X3DHAliceResult> x3dhFromBundle(OmemoBundle bundle, OmemoKeyPair ik) async {
+  // Check the signature first
+  final signatureValue = await Ed25519().verify(
+    await bundle.spk.getBytes(),
+    signature: Signature(
+      bundle.spkSignature,
+      publicKey: bundle.ik.asPublicKey(),
+    ),
+  );
+
+  if (!signatureValue) {
+    throw InvalidSignatureException();
+  }
+
   // Generate EK
   final ek = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
 

test/x3dh_test.dart

@@ -1,9 +1,10 @@
+import 'dart:convert';
 import 'package:cryptography/cryptography.dart';
 import 'package:omemo_dart/omemo_dart.dart';
 import 'package:test/test.dart';
 
 void main() {
-  test('X3DH', () async {
+  test('X3DH with correct signature', () async {
     // Generate keys
     final ikAlice = await OmemoKeyPair.generateNewPair(KeyPairType.ed25519);
     final ikBob = await OmemoKeyPair.generateNewPair(KeyPairType.ed25519);
@@ -13,8 +14,10 @@ void main() {
       '1',
       await spkBob.pk.asBase64(),
       '3',
-      // TODO(PapaTutuWawa): Do
+      base64Encode(
-      'n/a',
+        await sig(ikBob, await spkBob.pk.getBytes()),
+      ),
+      //'Q5in+/L4kJixEX692h6mJkPMyp4I3SlQ84L0E7ipPzqfPHOMiraUlqG2vG/O8wvFjLsKYZpPBraga9IvwhqVDA==',
       await ikBob.pk.asBase64(),
       {
         '2': await opkBob.pk.asBase64(),
@@ -42,4 +45,35 @@ void main() {
     expect(resultAlice.sk, resultBob.sk);
     expect(resultAlice.ad, resultBob.ad);
   });
+
+  test('X3DH with incorrect signature', () async {
+    // Generate keys
+    final ikAlice = await OmemoKeyPair.generateNewPair(KeyPairType.ed25519);
+    final ikBob = await OmemoKeyPair.generateNewPair(KeyPairType.ed25519);
+    final spkBob = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+    final opkBob = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+    final bundleBob = OmemoBundle(
+      '1',
+      await spkBob.pk.asBase64(),
+      '3',
+      // NOTE: A bit flakey, but it is highly unlikely that the same keypair as this one
+      //       gets generated.
+      'Q5in+/L4kJixEX692h6mJkPMyp4I3SlQ84L0E7ipPzqfPHOMiraUlqG2vG/O8wvFjLsKYZpPBraga9IvwhqVDA==',
+      await ikBob.pk.asBase64(),
+      {
+        '2': await opkBob.pk.asBase64(),
+      },
+    );
+    
+    // Alice does X3DH
+    var exception = false;
+    try {
+      await x3dhFromBundle(bundleBob, ikAlice);
+    } catch(e) {
+      exception = true;
+      expect(e is InvalidSignatureException, true, reason: 'Expected InvalidSignatureException, but got $e');
+    }
+
+    expect(exception, true, reason: 'Expected test failure');
+  });
 }