diff --git a/client/fido/additional_ca.c b/client/fido/additional_ca.c
index 21d9d80f6..7ea103004 100644
--- a/client/fido/additional_ca.c
+++ b/client/fido/additional_ca.c
@@ -36,6 +36,7 @@
 
 // Name: Yubico U2F Root CA Serial 457200631
 // Issued: 2014-08-01
+// https://github.com/Yubico/developers.yubico.com/tree/master/static/U2F
 #define YUBICO_CA \
     "-----BEGIN CERTIFICATE-----\r\n" \
     "MIIDHjCCAgagAwIBAgIEG0BT9zANBgkqhkiG9w0BAQsFADAuMSwwKgYDVQQDEyNZ\r\n" \