feat: add GOOGLE_API_USE_CLIENT_CERTIFICATE support (#592)
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
index 9c1367f..46a8926 100644
--- a/google/auth/environment_vars.py
+++ b/google/auth/environment_vars.py
@@ -53,3 +53,9 @@
GCE_METADATA_IP = "GCE_METADATA_IP"
"""Environment variable providing an alternate ip:port to be used for ip-only
GCE metadata requests."""
+
+GOOGLE_API_USE_CLIENT_CERTIFICATE = "GOOGLE_API_USE_CLIENT_CERTIFICATE"
+"""Environment variable controlling whether to use client certificate or not.
+
+The default value is false. Users have to explicitly set this value to true
+in order to use client certificate to establish a mutual TLS channel."""