diff --git a/lib/encryption/cross_signing.dart b/lib/encryption/cross_signing.dart
index 6d243f21..60d9a64c 100644
--- a/lib/encryption/cross_signing.dart
+++ b/lib/encryption/cross_signing.dart
@@ -16,9 +16,9 @@
  *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-import 'dart:convert';
 import 'dart:typed_data';
 
+import 'package:matrix/encryption/utils/base64_unpadded.dart';
 import 'package:olm/olm.dart' as olm;
 
 import '../matrix.dart';
@@ -33,7 +33,7 @@ class CrossSigning {
         (String secret) async {
       final keyObj = olm.PkSigning();
       try {
-        return keyObj.init_with_seed(base64.decode(secret)) ==
+        return keyObj.init_with_seed(base64decodeUnpadded(secret)) ==
             client.userDeviceKeys[client.userID]!.selfSigningKey!.ed25519Key;
       } catch (_) {
         return false;
@@ -45,7 +45,7 @@ class CrossSigning {
         (String secret) async {
       final keyObj = olm.PkSigning();
       try {
-        return keyObj.init_with_seed(base64.decode(secret)) ==
+        return keyObj.init_with_seed(base64decodeUnpadded(secret)) ==
             client.userDeviceKeys[client.userID]!.userSigningKey!.ed25519Key;
       } catch (_) {
         return false;
@@ -87,8 +87,8 @@ class CrossSigning {
       );
       await handle.maybeCacheAll();
     }
-    final masterPrivateKey =
-        base64.decode(await handle.getStored(EventTypes.CrossSigningMasterKey));
+    final masterPrivateKey = base64decodeUnpadded(
+        await handle.getStored(EventTypes.CrossSigningMasterKey));
     final keyObj = olm.PkSigning();
     String? masterPubkey;
     try {
@@ -153,7 +153,7 @@ class CrossSigning {
           // we don't care about signing other cross-signing keys
         } else {
           // okay, we'll sign a device key with our self signing key
-          selfSigningKey ??= base64.decode(await encryption.ssss
+          selfSigningKey ??= base64decodeUnpadded(await encryption.ssss
                   .getCached(EventTypes.CrossSigningSelfSigning) ??
               '');
           if (selfSigningKey.isNotEmpty) {
@@ -163,7 +163,7 @@ class CrossSigning {
         }
       } else if (key is CrossSigningKey && key.usage.contains('master')) {
         // we are signing someone elses master key
-        userSigningKey ??= base64.decode(await encryption.ssss
+        userSigningKey ??= base64decodeUnpadded(await encryption.ssss
                 .getCached(EventTypes.CrossSigningUserSigning) ??
             '');
         if (userSigningKey.isNotEmpty) {
diff --git a/lib/encryption/key_manager.dart b/lib/encryption/key_manager.dart
index 15977bb9..5f9f6547 100644
--- a/lib/encryption/key_manager.dart
+++ b/lib/encryption/key_manager.dart
@@ -18,6 +18,7 @@
 
 import 'dart:convert';
 
+import 'package:matrix/encryption/utils/base64_unpadded.dart';
 import 'package:matrix/encryption/utils/stored_inbound_group_session.dart';
 import 'package:olm/olm.dart' as olm;
 import 'package:collection/collection.dart';
@@ -49,7 +50,7 @@ class KeyManager {
             BackupAlgorithm.mMegolmBackupV1Curve25519AesSha2) {
           return false;
         }
-        return keyObj.init_with_private_key(base64.decode(secret)) ==
+        return keyObj.init_with_private_key(base64decodeUnpadded(secret)) ==
             info.authData['public_key'];
       } catch (_) {
         return false;
@@ -579,7 +580,7 @@ class KeyManager {
       return;
     }
     final privateKey =
-        base64.decode((await encryption.ssss.getCached(megolmKey))!);
+        base64decodeUnpadded((await encryption.ssss.getCached(megolmKey))!);
     final decryption = olm.PkDecryption();
     final info = await getRoomKeysBackupInfo();
     String backupPubKey;
@@ -723,7 +724,7 @@ class KeyManager {
         return; // nothing to do
       }
       final privateKey =
-          base64.decode((await encryption.ssss.getCached(megolmKey))!);
+          base64decodeUnpadded((await encryption.ssss.getCached(megolmKey))!);
       // decryption is needed to calculate the public key and thus see if the claimed information is in fact valid
       final decryption = olm.PkDecryption();
       final info = await getRoomKeysBackupInfo(false);
diff --git a/lib/encryption/ssss.dart b/lib/encryption/ssss.dart
index fc591522..60e4dabf 100644
--- a/lib/encryption/ssss.dart
+++ b/lib/encryption/ssss.dart
@@ -24,6 +24,7 @@ import 'dart:typed_data';
 import 'package:base58check/base58.dart';
 import 'package:crypto/crypto.dart';
 import 'package:collection/collection.dart';
+import 'package:matrix/encryption/utils/base64_unpadded.dart';
 
 import '../matrix.dart';
 import '../src/utils/crypto/crypto.dart' as uc;
@@ -85,7 +86,7 @@ class SSSS {
       [String? ivStr]) async {
     Uint8List iv;
     if (ivStr != null) {
-      iv = base64.decode(ivStr);
+      iv = base64decodeUnpadded(ivStr);
     } else {
       iv = Uint8List.fromList(uc.secureRandomBytes(16));
     }
@@ -108,15 +109,15 @@ class SSSS {
   static Future<String> decryptAes(
       _Encrypted data, Uint8List key, String name) async {
     final keys = deriveKeys(key, name);
-    final cipher = base64.decode(data.ciphertext);
+    final cipher = base64decodeUnpadded(data.ciphertext);
     final hmac = base64
         .encode(Hmac(sha256, keys.hmacKey).convert(cipher).bytes)
         .replaceAll(RegExp(r'=+$'), '');
     if (hmac != data.mac.replaceAll(RegExp(r'=+$'), '')) {
       throw Exception('Bad MAC');
     }
-    final decipher =
-        await uc.aesCtr.encrypt(cipher, keys.aesKey, base64.decode(data.iv));
+    final decipher = await uc.aesCtr
+        .encrypt(cipher, keys.aesKey, base64decodeUnpadded(data.iv));
     return String.fromCharCodes(decipher);
   }
 
diff --git a/lib/encryption/utils/base64_unpadded.dart b/lib/encryption/utils/base64_unpadded.dart
new file mode 100644
index 00000000..c66b64e8
--- /dev/null
+++ b/lib/encryption/utils/base64_unpadded.dart
@@ -0,0 +1,13 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+/// decodes base64
+///
+/// Dart's native [base64.decode] requires a padded base64 input String.
+/// This function allows unpadded base64 too.
+///
+/// See: https://github.com/dart-lang/sdk/issues/39510
+Uint8List base64decodeUnpadded(String s) {
+  final needEquals = (4 - (s.length % 4)) % 4;
+  return base64.decode(s + ('=' * needEquals));
+}
diff --git a/lib/encryption/utils/bootstrap.dart b/lib/encryption/utils/bootstrap.dart
index aef6518c..515d0744 100644
--- a/lib/encryption/utils/bootstrap.dart
+++ b/lib/encryption/utils/bootstrap.dart
@@ -26,6 +26,7 @@ import '../encryption.dart';
 import '../ssss.dart';
 import '../key_manager.dart';
 import '../../matrix.dart';
+import 'base64_unpadded.dart';
 
 enum BootstrapState {
   /// Is loading.
@@ -385,7 +386,7 @@ class Bootstrap {
         }
       } else {
         Logs().v('Get stored key...');
-        masterSigningKey = base64.decode(
+        masterSigningKey = base64decodeUnpadded(
             await newSsssKey?.getStored(EventTypes.CrossSigningMasterKey) ??
                 '');
         if (masterSigningKey.isEmpty) {
diff --git a/lib/src/utils/crypto/encrypted_file.dart b/lib/src/utils/crypto/encrypted_file.dart
index 754e5ced..d3dd874b 100644
--- a/lib/src/utils/crypto/encrypted_file.dart
+++ b/lib/src/utils/crypto/encrypted_file.dart
@@ -18,6 +18,8 @@
 import 'dart:typed_data';
 import 'dart:convert';
 
+import 'package:matrix/encryption/utils/base64_unpadded.dart';
+
 import 'crypto.dart';
 
 class EncryptedFile {
@@ -52,7 +54,7 @@ Future<Uint8List?> decryptFile(EncryptedFile input) async {
     return null;
   }
 
-  final key = base64.decode(base64.normalize(input.k));
-  final iv = base64.decode(base64.normalize(input.iv));
+  final key = base64decodeUnpadded(base64.normalize(input.k));
+  final iv = base64decodeUnpadded(base64.normalize(input.iv));
   return await aesCtr.encrypt(input.data, key, iv);
 }
diff --git a/test/encryption/utils_test.dart b/test/encryption/utils_test.dart
new file mode 100644
index 00000000..242703ac
--- /dev/null
+++ b/test/encryption/utils_test.dart
@@ -0,0 +1,43 @@
+/*
+ *   Famedly Matrix SDK
+ *   Copyright (C) 2022 Famedly GmbH
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as
+ *   published by the Free Software Foundation, either version 3 of the
+ *   License, or (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+import 'dart:convert';
+
+import 'package:matrix/encryption/utils/base64_unpadded.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Utils', () {
+    const base64input = 'foobar';
+    final utf8codec = Utf8Codec();
+    test('base64 padded', () {
+      final paddedBase64 = base64.encode(base64input.codeUnits);
+
+      final decodedPadded =
+          utf8codec.decode(base64decodeUnpadded(paddedBase64));
+      expect(decodedPadded, base64input, reason: 'Padded base64 decode');
+    });
+
+    test('base64 unpadded', () {
+      const unpaddedBase64 = 'Zm9vYmFy';
+      final decodedUnpadded =
+          utf8codec.decode(base64decodeUnpadded(unpaddedBase64));
+      expect(decodedUnpadded, base64input, reason: 'Unpadded base64 decode');
+    });
+  });
+}