feat: support remote attachments

lib/src/client.dart

@@ -1,5 +1,8 @@
+import 'dart:typed_data';
+
 import 'package:web3dart/credentials.dart';
 import 'package:xmtp_proto/xmtp_proto.dart' as xmtp;
+import 'package:http/http.dart' as http;
 
 import 'auth.dart';
 import 'common/api.dart';
@@ -10,6 +13,7 @@ import 'content/codec.dart';
 import 'content/codec_registry.dart';
 import 'content/composite_codec.dart';
 import 'content/reaction_codec.dart';
+import 'content/remote_attachment_codec.dart';
 import 'content/reply_codec.dart';
 import 'content/text_codec.dart';
 import 'content/decoded.dart';
@@ -124,6 +128,7 @@ class Client implements Codec<DecodedContent> {
       CompositeCodec(),
       ReplyCodec(),
       ReactionCodec(),
+      RemoteAttachmentCodec(),
       AttachmentCodec(),
     ];
     for (var codec in commonCodecs..addAll(customCodecs)) {
@@ -257,6 +262,30 @@ class Client implements Codec<DecodedContent> {
   Future<bool> refreshContactConsentPreferences({bool fullRefresh = false}) =>
       _contacts.refreshConsents(_auth.keys, fullRefresh: fullRefresh);
 
+  /// This downloads the [attachment] and returns the [DecodedContent].
+  /// If [downloader] is specified then that will be used to fetch the payload.
+  Future<DecodedContent> download(
+    RemoteAttachment attachment, {
+    RemoteDownloader? downloader,
+  }) async {
+    downloader ??= (url) => http.readBytes(Uri.parse(url));
+    var decrypted = await attachment.download(downloader);
+    return decode(decrypted);
+  }
+
+  /// This uploads the [attachment] and returns the [RemoteAttachment].
+  /// The [uploader] will be used to upload the payload and produce the URL.
+  /// It will be uploaded after applying the specified [compression].
+  Future<RemoteAttachment> upload(
+    Attachment attachment,
+    RemoteUploader uploader, {
+    xmtp.Compression? compression = xmtp.Compression.COMPRESSION_GZIP,
+  }) async {
+    var content = DecodedContent(contentTypeAttachment, attachment);
+    var encoded = await _codecs.encode(content, compression: compression);
+    return RemoteAttachment.upload(attachment.filename, encoded, uploader);
+  }
+
   /// This lists messages sent to the [conversation].
   ///
   /// For listing multiple conversations, see [listBatchMessages].

lib/src/content/remote_attachment_codec.dart

@@ -0,0 +1,125 @@
+import 'dart:convert';
+
+import 'package:flutter/foundation.dart';
+import 'package:web3dart/crypto.dart';
+import 'package:xmtp_proto/xmtp_proto.dart' as xmtp;
+import '../common/crypto.dart' as crypto;
+
+import 'codec.dart';
+
+final contentTypeRemoteAttachment = xmtp.ContentTypeId(
+  authorityId: "xmtp.org",
+  typeId: "remoteStaticAttachment",
+  versionMajor: 1,
+  versionMinor: 0,
+);
+
+/// This should download the [url] and return the data.
+typedef RemoteDownloader = Future<List<int>> Function(String url);
+
+/// This should upload the [data] and return the URL.
+typedef RemoteUploader = Future<String> Function(List<int> data);
+
+/// This is a remote encrypted file URL.
+class RemoteAttachment {
+  final List<int> salt;
+  final List<int> nonce;
+  final List<int> secret;
+  final String scheme;
+  final String url;
+  final String filename;
+  final int contentLength;
+  final String contentDigest;
+
+  RemoteAttachment({
+    required this.salt,
+    required this.nonce,
+    required this.secret,
+    required this.scheme,
+    required this.url,
+    required this.filename,
+    required this.contentLength,
+    required this.contentDigest,
+  });
+
+  /// This uploads the [encoded] file using the [uploader].
+  /// See [Client.upload] for typical usage.
+  static Future<RemoteAttachment> upload(
+    String filename,
+    xmtp.EncodedContent encoded,
+    RemoteUploader uploader,
+  ) async {
+    var secret = crypto.generateRandomBytes(32);
+    var encrypted = await crypto.encrypt(
+      secret,
+      encoded.writeToBuffer(),
+    );
+    var url = await uploader(encrypted.aes256GcmHkdfSha256.payload);
+    return RemoteAttachment(
+      salt: encrypted.aes256GcmHkdfSha256.hkdfSalt,
+      nonce: encrypted.aes256GcmHkdfSha256.gcmNonce,
+      secret: secret,
+      scheme: "https://",
+      url: url,
+      filename: filename,
+      contentLength: encrypted.aes256GcmHkdfSha256.payload.length,
+      contentDigest:
+          bytesToHex(crypto.sha256(encrypted.aes256GcmHkdfSha256.payload)),
+    );
+  }
+
+  /// This downloads the file from the [url] and decrypts it.
+  /// See [Client.download] for typical usage.
+  Future<xmtp.EncodedContent> download(RemoteDownloader downloader) async {
+    var payload = await downloader(url);
+    var decrypted = await crypto.decrypt(
+        secret,
+        xmtp.Ciphertext(
+          aes256GcmHkdfSha256: xmtp.Ciphertext_Aes256gcmHkdfsha256(
+            hkdfSalt: salt,
+            gcmNonce: nonce,
+            payload: payload,
+          ),
+        ));
+    return xmtp.EncodedContent.fromBuffer(decrypted);
+  }
+}
+
+/// This is a [Codec] that encodes a remote encrypted file URL as the content.
+class RemoteAttachmentCodec extends Codec<RemoteAttachment> {
+  @override
+  xmtp.ContentTypeId get contentType => contentTypeRemoteAttachment;
+
+  @override
+  Future<RemoteAttachment> decode(xmtp.EncodedContent encoded) async =>
+      RemoteAttachment(
+        url: utf8.decode(encoded.content),
+        filename: encoded.parameters["filename"] ?? "",
+        salt: hexToBytes(encoded.parameters["salt"] ?? ""),
+        nonce: hexToBytes(encoded.parameters["nonce"] ?? ""),
+        secret: hexToBytes(encoded.parameters["secret"] ?? ""),
+        contentLength: int.parse(encoded.parameters["contentLength"] ?? "0"),
+        contentDigest: encoded.parameters["contentDigest"] ?? "",
+        scheme: "https://",
+      );
+
+  @override
+  Future<xmtp.EncodedContent> encode(RemoteAttachment decoded) async =>
+      xmtp.EncodedContent(
+        type: contentTypeRemoteAttachment,
+        parameters: {
+          "filename": decoded.filename,
+          "secret": bytesToHex(decoded.secret),
+          "salt": bytesToHex(decoded.salt),
+          "nonce": bytesToHex(decoded.nonce),
+          "contentLength": decoded.contentLength.toString(),
+          "contentDigest": decoded.contentDigest,
+        },
+        content: utf8.encode(decoded.url),
+      );
+
+  @override
+  String? fallback(RemoteAttachment content) {
+    return "Can’t display \"${content.filename}\". This app doesn’t support attachments.";
+  }
+}

test/client_test.dart

@@ -15,6 +15,7 @@ import 'package:xmtp/src/content/attachment_codec.dart';
 import 'package:xmtp/src/content/codec.dart';
 import 'package:xmtp/src/content/decoded.dart';
 import 'package:xmtp/src/content/reaction_codec.dart';
+import 'package:xmtp/src/content/remote_attachment_codec.dart';
 import 'package:xmtp/src/content/reply_codec.dart';
 import 'package:xmtp/src/content/text_codec.dart';
 import 'package:xmtp/src/client.dart';
@@ -560,6 +561,44 @@ void main() {
     },
   );
 
+  test(
+    skip: skipUnlessTestServerEnabled,
+    "remote attachments: uploading and downloading attachments should work",
+    () async {
+      var aliceWallet = EthPrivateKey.createRandom(Random.secure()).asSigner();
+      var aliceApi = createTestServerApi();
+      var bobWallet = EthPrivateKey.createRandom(Random.secure()).asSigner();
+      var bobApi = createTestServerApi();
+      var alice = await Client.createFromWallet(aliceApi, aliceWallet);
+      var bob = await Client.createFromWallet(bobApi, bobWallet);
+      var files = TestRemoteFiles();
+
+      // If Alice sends a message and then a remote attachment...
+      var convo = await alice.newConversation(bob.address.hex);
+      await alice.sendMessage(convo, "Here's an attachment for you:");
+      var attachment = Attachment("foo.txt", "text/plain", utf8.encode("bar"));
+      var remote = await alice.upload(attachment, files.upload);
+      await alice.sendMessage(convo, remote,
+          contentType: contentTypeRemoteAttachment);
+      await delayToPropagate();
+
+      // ... then Bob should see the messages.
+      var messages = await bob.listMessages(convo);
+      expect(messages.length, 2);
+      expect((messages[0].content as RemoteAttachment).filename, "foo.txt");
+      expect((messages[1].content as String), "Here's an attachment for you:");
+
+      // And he should be able to download the remote attachment.
+      var downloaded = await bob.download(
+        messages[0].content as RemoteAttachment,
+        downloader: files.download,
+      );
+      expect((downloaded.content as Attachment).filename, "foo.txt");
+      expect((downloaded.content as Attachment).mimeType, "text/plain");
+      expect(utf8.decode((downloaded.content as Attachment).data), "bar");
+    },
+  );
+
   test(
     skip: skipUnlessTestServerEnabled,
     "codecs: sending codec encoded message to user",
@@ -842,6 +881,19 @@ void main() {
   );
 }
 
+/// Helper to store and retrieve remote files.
+class TestRemoteFiles {
+  var files = {};
+
+  Future<String> upload(List<int> data) async {
+    var url = "https://example.com/${Random.secure().nextInt(1000000)}";
+    files[url] = data;
+    return url;
+  }
+
+  Future<List<int>> download(String url) async => files[url];
+}
+
 /// Helper to seed random conversations in a test account.
 Future _startRandomConversationsWith(
   String recipientAddress, {