We use base64url without padding.

django_etesync/migrations/0031_auto_20200317_1509.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.0.3 on 2020-03-17 15:09
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etesync', '0030_auto_20200312_1859'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collectionitemchunk',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=44, validators=[django.core.validators.RegexValidator(message='Expected a 256bit base64url.', regex='^[a-zA-Z0-9\\-_]{43}$')]),
+        ),
+    ]

django_etesync/models.py

@@ -20,7 +20,7 @@ from django.core.validators import RegexValidator
 from django.utils.functional import cached_property
 
 
-Base64Url256BitValidator = RegexValidator(regex=r'^[a-zA-Z0-9\-_]{43}=?$', message='Expected a 256bit base64url.')
+Base64Url256BitValidator = RegexValidator(regex=r'^[a-zA-Z0-9\-_]{43}$', message='Expected a 256bit base64url.')
 UidValidator = RegexValidator(regex=r'[a-zA-Z0-9]', message='Not a valid UID')