feat: import Chinese-localized Buzz source snapshot

Signed-off-by: cls_宁波本机 <908705107@qq.com>

mobile/lib/shared/crypto/hkdf.dart

@@ -0,0 +1,44 @@
+import 'dart:typed_data';
+
+import 'package:pointycastle/api.dart';
+import 'package:pointycastle/digests/sha256.dart';
+import 'package:pointycastle/macs/hmac.dart';
+
+/// HKDF-SHA256 Extract: PRK = HMAC-SHA256(salt, ikm).
+Uint8List hkdfExtract(Uint8List salt, Uint8List ikm) {
+  final hmac = HMac(SHA256Digest(), 64);
+  hmac.init(KeyParameter(salt.isEmpty ? Uint8List(32) : salt));
+  final out = Uint8List(32);
+  hmac.update(ikm, 0, ikm.length);
+  hmac.doFinal(out, 0);
+  return out;
+}
+
+/// HKDF-SHA256 Expand: OKM = HKDF-Expand(prk, info, length).
+Uint8List hkdfExpand(Uint8List prk, Uint8List info, int length) {
+  final hmac = HMac(SHA256Digest(), 64);
+  hmac.init(KeyParameter(prk));
+
+  final n = (length + 31) ~/ 32; // ceil(length / hashLen)
+  final okm = Uint8List(n * 32);
+  var prev = Uint8List(0);
+
+  for (var i = 1; i <= n; i++) {
+    hmac.reset();
+    hmac.update(prev, 0, prev.length);
+    hmac.update(info, 0, info.length);
+    final counter = Uint8List.fromList([i]);
+    hmac.update(counter, 0, 1);
+    prev = Uint8List(32);
+    hmac.doFinal(prev, 0);
+    okm.setRange((i - 1) * 32, i * 32, prev);
+  }
+
+  return Uint8List.sublistView(okm, 0, length);
+}
+
+/// Convenience: HKDF-SHA256(ikm, salt, info, length).
+Uint8List hkdf(Uint8List ikm, Uint8List salt, Uint8List info, int length) {
+  final prk = hkdfExtract(salt, ikm);
+  return hkdfExpand(prk, info, length);
+}