refactor: Move the implementations into their own directory

lib/omemo_dart.dart

@@ -1,7 +1,7 @@
 library omemo_dart;
 
 export 'src/bundle.dart';
-export 'src/double_ratchet.dart';
+export 'src/double_ratchet/double_ratchet.dart';
 export 'src/errors.dart';
 export 'src/helpers.dart';
 export 'src/key.dart';

lib/src/x3dh.dart

@@ -1,124 +0,0 @@
-import 'dart:convert';
-import 'dart:math';
-import 'package:cryptography/cryptography.dart';
-import 'package:omemo_dart/src/bundle.dart';
-import 'package:omemo_dart/src/crypto.dart';
-import 'package:omemo_dart/src/errors.dart';
-import 'package:omemo_dart/src/helpers.dart';
-import 'package:omemo_dart/src/key.dart';
-
-/// The overarching assumption is that we use Ed25519 keys for the identity keys
-const omemoX3DHInfoString = 'OMEMO X3DH';
-
-/// Performed by Alice
-class X3DHAliceResult {
-
-  const X3DHAliceResult(this.ek, this.sk, this.opkId, this.ad);
-  final OmemoKeyPair ek;
-  final List<int> sk;
-  final String opkId;
-  final List<int> ad;
-}
-
-/// Received by Bob
-class X3DHMessage {
-
-  const X3DHMessage(this.ik, this.ek, this.opkId);
-  final OmemoPublicKey ik;
-  final OmemoPublicKey ek;
-  final String opkId;
-}
-
-class X3DHBobResult {
-
-  const X3DHBobResult(this.sk, this.ad);
-  final List<int> sk;
-  final List<int> ad;
-}
-
-/// Sign [message] using the keypair [keyPair]. Note that [keyPair] must be
-/// a Ed25519 keypair.
-Future<List<int>> sig(OmemoKeyPair keyPair, List<int> message) async {
-  assert(keyPair.type == KeyPairType.ed25519, 'Signature keypair must be Ed25519');
-  final signature = await Ed25519().sign(
-    message,
-    keyPair: await keyPair.asKeyPair(),
-  );
-
-  return signature.bytes;
-}
-
-/// Derive a secret from the key material [km].
-Future<List<int>> kdf(List<int> km) async {
-  final f = List<int>.filled(32, 0xFF);
-  final input = List<int>.empty(growable: true)
-    ..addAll(f)
-    ..addAll(km);
-
-  final algorithm = Hkdf(
-    hmac: Hmac(Sha256()),
-    outputLength: 32,
-  );
-  final output = await algorithm.deriveKey(
-    secretKey: SecretKey(input),
-    nonce: List<int>.filled(32, 0x00),
-    info: utf8.encode(omemoX3DHInfoString),
-  );
-
-  return output.extractBytes();
-}
-
-/// 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);
-
-  final random = Random.secure();
-  final opkIndex = random.nextInt(bundle.opksEncoded.length);
-  final opkId = bundle.opksEncoded.keys.elementAt(opkIndex);
-  final opk = bundle.getOpk(opkId);
-  
-  final dh1 = await omemoDH(ik, bundle.spk, 1);
-  final dh2 = await omemoDH(ek, bundle.ik,  2);
-  final dh3 = await omemoDH(ek, bundle.spk, 0);
-  final dh4 = await omemoDH(ek, opk, 0);
-
-  final sk = await kdf(concat([dh1, dh2, dh3, dh4]));
-  final ad = concat([
-    await ik.pk.getBytes(),
-    await bundle.ik.getBytes(),
-  ]);
-
-  return X3DHAliceResult(ek, sk, opkId, ad);
-}
-
-/// Bob builds the X3DH shared secret from the inital message [msg], the SPK [spk], the
-/// OPK [opk] that was selected by Alice and our IK [ik]. Returns the shared secret.
-Future<X3DHBobResult> x3dhFromInitialMessage(X3DHMessage msg, OmemoKeyPair spk, OmemoKeyPair opk, OmemoKeyPair ik) async {
-  final dh1 = await omemoDH(spk, msg.ik, 2);
-  final dh2 = await omemoDH(ik,  msg.ek, 1);
-  final dh3 = await omemoDH(spk, msg.ek, 0);
-  final dh4 = await omemoDH(opk, msg.ek, 0);
-
-  final sk = await kdf(concat([dh1, dh2, dh3, dh4]));
-  final ad = concat([
-    await msg.ik.getBytes(),
-    await ik.pk.getBytes(),
-  ]);
-
-  return X3DHBobResult(sk, ad);
-}

lib/src/x3dh/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/crypto.dart';
 import 'package:omemo_dart/src/errors.dart';
 import 'package:omemo_dart/src/helpers.dart';
 import 'package:omemo_dart/src/key.dart';
@@ -47,28 +48,6 @@ Future<List<int>> sig(OmemoKeyPair keyPair, List<int> message) async {
   return signature.bytes;
 }
 
-/// 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>> dh(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();
-}
-
 /// Derive a secret from the key material [km].
 Future<List<int>> kdf(List<int> km) async {
   final f = List<int>.filled(32, 0xFF);
@@ -113,10 +92,10 @@ Future<X3DHAliceResult> x3dhFromBundle(OmemoBundle bundle, OmemoKeyPair ik) asyn
   final opkId = bundle.opksEncoded.keys.elementAt(opkIndex);
   final opk = bundle.getOpk(opkId);
   
-  final dh1 = await dh(ik, bundle.spk, 1);
+  final dh1 = await omemoDH(ik, bundle.spk, 1);
-  final dh2 = await dh(ek, bundle.ik,  2);
+  final dh2 = await omemoDH(ek, bundle.ik,  2);
-  final dh3 = await dh(ek, bundle.spk, 0);
+  final dh3 = await omemoDH(ek, bundle.spk, 0);
-  final dh4 = await dh(ek, opk, 0);
+  final dh4 = await omemoDH(ek, opk, 0);
 
   final sk = await kdf(concat([dh1, dh2, dh3, dh4]));
   final ad = concat([
@@ -130,10 +109,10 @@ Future<X3DHAliceResult> x3dhFromBundle(OmemoBundle bundle, OmemoKeyPair ik) asyn
 /// Bob builds the X3DH shared secret from the inital message [msg], the SPK [spk], the
 /// OPK [opk] that was selected by Alice and our IK [ik]. Returns the shared secret.
 Future<X3DHBobResult> x3dhFromInitialMessage(X3DHMessage msg, OmemoKeyPair spk, OmemoKeyPair opk, OmemoKeyPair ik) async {
-  final dh1 = await dh(spk, msg.ik, 2);
+  final dh1 = await omemoDH(spk, msg.ik, 2);
-  final dh2 = await dh(ik,  msg.ek, 1);
+  final dh2 = await omemoDH(ik,  msg.ek, 1);
-  final dh3 = await dh(spk, msg.ek, 0);
+  final dh3 = await omemoDH(spk, msg.ek, 0);
-  final dh4 = await dh(opk, msg.ek, 0);
+  final dh4 = await omemoDH(opk, msg.ek, 0);
 
   final sk = await kdf(concat([dh1, dh2, dh3, dh4]));
   final ad = concat([