diff --git a/django_etesync/migrations/0032_auto_20200317_1513.py b/django_etesync/migrations/0032_auto_20200317_1513.py
new file mode 100644
index 0000000..0546711
--- /dev/null
+++ b/django_etesync/migrations/0032_auto_20200317_1513.py
@@ -0,0 +1,23 @@
+# Generated by Django 3.0.3 on 2020-03-17 15:13
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etesync', '0031_auto_20200317_1509'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='collectionitemrevision',
+            name='hmac',
+        ),
+        migrations.AlterField(
+            model_name='collectionitemrevision',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=44, unique=True, validators=[django.core.validators.RegexValidator(message='Expected a 256bit base64url.', regex='^[a-zA-Z0-9\\-_]{43}$')]),
+        ),
+    ]
diff --git a/django_etesync/models.py b/django_etesync/models.py
index 8df5d44..586c105 100644
--- a/django_etesync/models.py
+++ b/django_etesync/models.py
@@ -84,11 +84,10 @@ class CollectionItemChunk(models.Model):
 
 class CollectionItemRevision(models.Model):
     uid = models.CharField(db_index=True, unique=True, blank=False, null=False,
-                           max_length=44, validators=[UidValidator])
+                           max_length=44, validators=[Base64Url256BitValidator])
     item = models.ForeignKey(CollectionItem, related_name='revisions', on_delete=models.CASCADE)
     meta = models.BinaryField(editable=True, blank=True, null=True)
     chunks = models.ManyToManyField(CollectionItemChunk, related_name='items')
-    hmac = models.CharField(max_length=50, blank=False, null=False)
     current = models.BooleanField(db_index=True, default=True, null=True)
     deleted = models.BooleanField(default=False)
 
diff --git a/django_etesync/serializers.py b/django_etesync/serializers.py
index 7a2d3fd..33a09ac 100644
--- a/django_etesync/serializers.py
+++ b/django_etesync/serializers.py
@@ -66,7 +66,7 @@ class CollectionItemRevisionBaseSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.CollectionItemRevision
-        fields = ('chunks', 'hmac', 'deleted')
+        fields = ('chunks', 'uid', 'deleted')
 
 
 class CollectionItemRevisionSerializer(CollectionItemRevisionBaseSerializer):