diff --git a/data/api/client-server/definitions/encrypted_file.yaml b/data/api/client-server/definitions/encrypted_file.yaml
index 87dd7b19..329599c0 100644
--- a/data/api/client-server/definitions/encrypted_file.yaml
+++ b/data/api/client-server/definitions/encrypted_file.yaml
@@ -45,6 +45,7 @@ properties:
         example: A256CTR
       k:
         type: string
+        format: mx-urlsafe-unpadded-base64
         description: The key, encoded as [URL-safe unpadded Base64](/appendices/#url-safe-unpadded-base64).
         example: "aWF6-32KGYaC3A_FEUCk1Bt0JA37zP0wrStgmdCaW-0"
       ext:
diff --git a/data/string-formats.yaml b/data/string-formats.yaml
index 9d41c402..17c56882 100644
--- a/data/string-formats.yaml
+++ b/data/string-formats.yaml
@@ -71,6 +71,11 @@ mx-unpadded-base64:
   url: appendices#unpadded-base64
   # no regex
 
+mx-urlsafe-unpadded-base64:
+  title: URL-safe unpadded Base64
+  url: appendices/#url-safe-unpadded-base64
+  # no regex
+
 uri:
   title: URI
   url: https://datatracker.ietf.org/doc/html/rfc3986