refactor: Move the DH function into its own file

lib/src/crypto.dart

@@ -0,0 +1,24 @@
+import 'package:cryptography/cryptography.dart';
+import 'package:omemo_dart/src/key.dart';
+
+/// Performs X25519 with [kp] and [pk]. If [identityKey] is set, then
+/// it indicates which of [kp] ([identityKey] == 1) or [pk] ([identityKey] == 2)
+/// is the identity key. This is needed since the identity key pair/public key is
+/// an Ed25519 key, but we need them as X25519 keys for DH.
+Future<List<int>> omemoDH(OmemoKeyPair kp, OmemoPublicKey pk, int identityKey) async {
+  var ckp = kp;
+  var cpk = pk;
+
+  if (identityKey == 1) {
+    ckp = await kp.toCurve25519();
+  } else if (identityKey == 2) {
+    cpk = await pk.toCurve25519();
+  }
+
+  final shared = await Cryptography.instance.x25519().sharedSecretKey(
+    keyPair: await ckp.asKeyPair(),
+    remotePublicKey: cpk.asPublicKey(),
+  );
+
+  return shared.extractBytes();
+}