???: Move code around

lib/src/double_ratchet/crypto.dart

@@ -0,0 +1,109 @@
+import 'dart:convert';
+import 'package:cryptography/cryptography.dart';
+import 'package:omemo_dart/protobuf/schema.pb.dart';
+import 'package:omemo_dart/src/helpers.dart';
+
+/// Info string for ENCRYPT
+const encryptHkdfInfoString = 'OMEMO Message Key Material';
+
+/// cryptography _really_ wants to check the MAC output from AES-256-CBC. Since
+/// we don't have it, we need the MAC check to always "pass".
+class NoMacSecretBox extends SecretBox {
+  NoMacSecretBox(super.cipherText, { required super.nonce }) : super(mac: Mac.empty);
+
+  @override
+  Future<void> checkMac({
+    required MacAlgorithm macAlgorithm,
+    required SecretKey secretKey,
+    required List<int> aad,
+  }) async {}
+}
+
+/// Signals ENCRYPT function as specified by OMEMO 0.8.0.
+/// Encrypt [plaintext] using the message key [mk], given associated_data [associatedData]
+/// and the AD output from the X3DH [sessionAd].
+Future<List<int>> encrypt(List<int> mk, List<int> plaintext, List<int> associatedData, List<int> sessionAd) async {
+  final hkdf = Hkdf(
+    hmac: Hmac(Sha256()),
+    outputLength: 80,
+  );
+  final hkdfResult = await hkdf.deriveKey(
+    secretKey: SecretKey(mk),
+    nonce: List<int>.filled(32, 0x0),
+    info: utf8.encode(encryptHkdfInfoString),
+  );
+  final hkdfBytes = await hkdfResult.extractBytes();
+
+  // Split hkdfBytes into encryption, authentication key and IV
+  final encryptionKey = hkdfBytes.sublist(0, 32);
+  final authenticationKey = hkdfBytes.sublist(32, 64);
+  final iv = hkdfBytes.sublist(64, 80);
+
+  final aesResult = await AesCbc.with256bits(
+    macAlgorithm: MacAlgorithm.empty,
+  ).encrypt(
+    plaintext,
+    secretKey: SecretKey(encryptionKey),
+    nonce: iv,
+  );
+  
+  final header = OMEMOMessage.fromBuffer(associatedData.sublist(sessionAd.length))
+    ..ciphertext = aesResult.cipherText;
+  final headerBytes = header.writeToBuffer();
+  final hmacInput = concat([sessionAd, headerBytes]);
+  final hmacResult = (await Hmac.sha256().calculateMac(
+    hmacInput,
+    secretKey: SecretKey(authenticationKey),
+  )).bytes.sublist(0, 16);
+  
+  final message = OMEMOAuthenticatedMessage()
+    ..mac = hmacResult
+    ..message = headerBytes;
+  return message.writeToBuffer();
+}
+
+/// Signals DECRYPT function as specified by OMEMO 0.8.0.
+/// Decrypt [ciphertext] with the message key [mk], given the associated_data [associatedData]
+/// and the AD output from the X3DH.
+Future<List<int>> decrypt(List<int> mk, List<int> ciphertext, List<int> associatedData, List<int> sessionAd) async {
+  // Generate the keys and iv from mk
+  final hkdf = Hkdf(
+    hmac: Hmac(Sha256()),
+    outputLength: 80,
+  );
+  final hkdfResult = await hkdf.deriveKey(
+    secretKey: SecretKey(mk),
+    nonce: List<int>.filled(32, 0x0),
+    info: utf8.encode(encryptHkdfInfoString),
+  );
+  final hkdfBytes = await hkdfResult.extractBytes();
+
+  // Split hkdfBytes into encryption, authentication key and IV
+  final encryptionKey = hkdfBytes.sublist(0, 32);
+  final authenticationKey = hkdfBytes.sublist(32, 64);
+  final iv = hkdfBytes.sublist(64, 80);
+  
+  // Assumption ciphertext is a OMEMOAuthenticatedMessage
+  final message = OMEMOAuthenticatedMessage.fromBuffer(ciphertext);
+  final header = OMEMOMessage.fromBuffer(message.message);
+
+  final hmacInput = concat([sessionAd, header.writeToBuffer()]);
+  final hmacResult = (await Hmac.sha256().calculateMac(
+    hmacInput,
+    secretKey: SecretKey(authenticationKey),
+  )).bytes.sublist(0, 16);
+
+  // TODO(PapaTutuWawa): Check the HMAC result
+  
+  final plaintext = await AesCbc.with256bits(
+    macAlgorithm: MacAlgorithm.empty,
+  ).decrypt(
+    NoMacSecretBox(
+      header.ciphertext,
+      nonce: iv,
+    ),
+    secretKey: SecretKey(encryptionKey),
+  );
+
+  return plaintext;
+}

lib/src/double_ratchet/kdf.dart

@@ -0,0 +1,35 @@
+import 'dart:convert';
+import 'package:cryptography/cryptography.dart';
+
+/// Info string for KDF_RK
+const kdfRkInfoString = 'OMEMO Root Chain';
+
+/// Flags for KDF_CK
+const kdfCkNextMessageKey = 0x01;
+const kdfCkNextChainKey = 0x02;
+
+/// Signals KDF_CK function as specified by OMEMO 0.8.0.
+Future<List<int>> kdfCk(List<int> ck, int constant) async {
+  final hkdf = Hkdf(hmac: Hmac(Sha256()), outputLength: 32);
+  final result = await hkdf.deriveKey(
+    secretKey: SecretKey(ck),
+    nonce: [constant],
+  );
+
+  return result.extractBytes();
+}
+
+/// Signals KDF_RK function as specified by OMEMO 0.8.0.
+Future<List<int>> kdfRk(List<int> rk, List<int> dhOut) async {
+  final algorithm = Hkdf(
+    hmac: Hmac(Sha256()),
+    outputLength: 32,
+  );
+  final result = await algorithm.deriveKey(
+    secretKey: SecretKey(dhOut),
+    nonce: rk,
+    info: utf8.encode(kdfRkInfoString),
+  );
+
+  return result.extractBytes();
+}