# cf.tls\_client\_auth.cert\_ski

`cf.tls_client_auth.cert_ski` String

The Subject Key Identifier (SKI) of the certificate in the request.

This field is only filled in if the request includes a client certificate for [mTLS authentication](https://developers.cloudflare.com/ssl/client-certificates/enable-mtls/).

Example value:

```txt
"27846FAE6EAC4A8DAD9101B519CF1EB460242831"
```

Categories:

* Request
* mTLS
