[[_x509]]

=== X.509 Client Certificate User Authentication

{project_name} supports login with a X.509 client certificate if the server is configured for mutual SSL authentication.

A typical workflow is as follows:

- A client sends an authentication request over SSL/TLS channel
- During SSL/TLS handshake, the server and the client exchange their x.509/v3 certificates
- The container (WildFly) validates the certificate PKIX path and the certificate expiration
- The x.509 client certificate authenticator validates the client certificate as follows:
* Optionally checks the certificate revocation status using CRL and/or CRL Distribution Points
* Optionally checks the Certificate revocation status using OCSP (Online Certificate Status Protocol)
* Optionally validates whether the key usage in the certificate matches the expected key usage
* Optionally validates whether the extended key usage in the certificate matches the expected extended key usage
- If any of the above checks fails, the x.509 authentication fails
- Otherwise, the authenticator extracts the certificate identity and maps it to an existing user
- Once the certificate is mapped to an existing user, the behavior diverges depending on the authentication flow:
* In the Browser Flow, the server prompts the user to confirm identity or to ignore it and instead sign in with username/password 
* In the case of the Direct Grant Flow, the server signs in the user

==== Features

Supported Certificate Identity Sources::
- Match SubjectDN using regular expression
- X500 Subject's e-mail attribute
- X500 Subject's e-mail from Subject Alternative Name Extension (RFC822Name General Name)
- X500 Subject's Common Name attribute
- Match IssuerDN using regular expression
- X500 Issuer's e-mail attribute
- X500 Issuer's Common Name attribute
- Certificate Serial Number

Regular Expressions::
The certificate identity can be extracted from either Subject DN or Issuer DN using a regular expression as a filter. For example, the regular expression below will match the e-mail attribute:
```
emailAddress=(.*?)(?:,|$)
```
The regular expression filtering is applicable only if the `Identity Source` is set to either `Match SubjectDN using regular expression` or `Match IssuerDN using regular expression`. 

Mapping certificate identity to an existing user::

The certificate identity mapping can be configured to map the extracted user identity to an existing user's username or e-mail or to a custom attribute which value matches the certificate identity. For example, setting the `Identity source` to _Subject's e-mail_ and `User mapping method` to _Username or email_ will have the X.509 client certificate authenticator use the e-mail attribute in the certificate's Subject DN  as a search criteria to look up an existing user by username or by e-mail. 

IMPORTANT: Please notice that if we disable `Login with email` at realm settings, the same rules will be applied to certificate authentication. In other words, users won't be able to log in using e-mail attribute. 

Other Features: Extended Certificate Validation::
- Revocation status checking using CRL
- Revocation status checking using CRL/Distribution Point
- Revocation status checking using OCSP/Responder URI
- Certificate KeyUsage validation
- Certificate ExtendedKeyUsage validation

==== Enable X.509 Client Certificate User Authentication

The following sections describe how to configure WildFly/Undertow and the {project_name} Server to enable X.509 client certificate authentication.

[[_enable-mtls-wildfly]]
Enable mutual SSL in WildFly::
See link:https://docs.jboss.org/author/display/WFLY10/Admin+Guide#AdminGuide-EnableSSL[Enable SSL] and link:https://docs.jboss.org/author/display/WFLY10/Admin+Guide#AdminGuide-%7B%7B%3Cssl%2F%3E%7D%7D[SSL] for the instructions how to enable SSL in WildFly.

* Open {project_dirref}/standalone/configuration/standalone.xml and add a new realm:
```xml
<security-realms>
    <security-realm name="ssl-realm">
        <server-identities>
            <ssl>
                <keystore path="servercert.jks" 
                          relative-to="jboss.server.config.dir" 
                          keystore-password="servercert password"/>
            </ssl>
        </server-identities>
        <authentication>
            <truststore path="truststore.jks" 
                        relative-to="jboss.server.config.dir" 
                        keystore-password="truststore password"/>
        </authentication>
    </security-realm>
</security-realms>
```

`ssl/keystore`::
The `ssl` element contains the `keystore` element that defines how to load the server public key pair from a JKS keystore

`ssl/keystore/path`::
A path to a JKS keystore 

`ssl/keystore/relative-to`::
Defines a path the keystore path is relative to

`ssl/keystore/keystore-password`::
The password to open the keystore

`ssl/keystore/alias` (optional)::
The alias of the entry in the keystore. Set it if the keystore contains multiple entries

`ssl/keystore/key-password` (optional)::
The private key password, if different from the keystore password.

`authentication/truststore`::
Defines how to load a trust store to verify the certificate presented by the remote side of the inbound/outgoing connection. Typically, the truststore contains a collection of trusted CA certificates.   

`authentication/truststore/path`::
A path to a JKS keystore that contains the certificates of the trusted CAs (certificate authorities)

`authentication/truststore/relative-to`::
Defines a path the truststore path is relative to

`authentication/truststore/keystore-password`::
The password to open the truststore


Enable https listener::

See link:https://docs.jboss.org/author/display/WFLY10/Admin+Guide#AdminGuide-HTTPSlistener[HTTPS Listener] for the instructions how to enable HTTPS in WildFly.

* Add the <https-listener> element as shown below:

[source,xml,subs="attributes+"]
----
<subsystem xmlns="{subsystem_undertow_xml_urn}">
	....
    <server name="default-server">
	    <https-listener name="default"
                        socket-binding="https"
                        security-realm="ssl-realm"
                        verify-client="REQUESTED"/>
    </server>
</subsystem>
----

`https-listener/security-realm`::
The value must match the name of the realm from the previous section

`https-listener/verify-client`::
If set to `REQUESTED`, the server will optionally ask for a client certificate. Setting the attribute to `REQUIRED` will have the server to refuse inbound connections if no client certificate has been provided.

==== Adding X.509 Client Certificate Authentication to a Browser Flow

* Select a realm, click on Authentication link, select the "Browser" flow 
* Make a copy of the built-in "Browser" flow. You may want to give the new flow a distinctive name, i.e. "X.509 Browser"
* Using the drop down, select the copied flow, and click on "Add execution"
* Select "X509/Validate User Form" using the drop down and click on "Save"  

image:images/x509-execution.png[]

* Using the up/down arrows, change the order of the "X509/Validate Username Form" by moving it above the "Browser Forms" execution, and set the requirement to "ALTERNATIVE"

image:images/x509-browser-flow.png[]

* Select the "Bindings" tab, find the drop down for "Browser Flow". Select the newly created X509 browser flow from the drop down and click on "Save".

image:images/x509-browser-flow-bindings.png[]

Configuring X.509 Client Certificate Authentication::

image:images/x509-configuration.png[]

`User Identity Source`::
Defines how to extract the user identity from a client certificate.

`A regular expression` (optional)::
Defines a regular expression to use as a filter to extract the certificate identity. The regular expression must contain a single group.

`User Mapping Method`::
Defines how to match the certificate identity to an existing user. _Username or e-mail_ will search for an existing user by username or e-mail. _Custom Attribute Mapper_ will  search for an existing user with a custom attribute which value matches the certificate identity. The name of the custom attribute is configurable.

`A name of user attribute` (optional)::
A custom attribute which value will be matched against the certificate identity.

`CRL Checking Enabled` (optional)::
Defines whether to check the revocation status of the certificate using Certificate Revocation List.

`Enable CRL Distribution Point to check certificate revocation status` (optional)::
Defines whether to use CDP to check the certificate revocation status. Most PKI authorities include CDP in their certificates.

`CRL file path` (optional)::
Defines a path to a file that contains a CRL list. The value must be a path to a valid file if `CRL Checking Enabled` option is turned on.

`OCSP Checking Enabled`(optional)::
Defines whether to check the certificate revocation status using Online Certificate Status Protocol. 

`OCSP Responder URI` (optional)::
Allows to override a value of the OCSP responder URI in the certificate.

`Validate Key Usage` (optional)::
Verifies whether the certificate's KeyUsage extension bits are set. For example, "digitalSignature,KeyEncipherment" will verify if  bits 0 and 2 in the KeyUsage extension are asserted. Leave the parameter empty to disable the Key Usage validation. See link:https://tools.ietf.org/html/rfc5280#section-4.2.1.3[RFC5280, Section-4.2.1.3]. The server will raise an error only when flagged as critical by the issuing CA and there is a key usage extension mismatch. 

`Validate Extended Key Usage` (optional)::
Verifies one or more purposes as defined in the Extended Key Usage extension. See link:https://tools.ietf.org/html/rfc5280#section-4.2.1.12[RFC5280, Section-4.2.1.12]. Leave the parameter empty to disable the Extended Key Usage validation. The server will raise an error only when flagged as critical by the issuing CA and there is a key usage extension mismatch.

`Bypass identity confirmation`::
If set, X.509 client certificate authentication will not prompt the user to confirm the certificate identity and will automatically sign in the user upon successful authentication.

==== Adding X.509 Client Certificate Authentication to a Direct Grant Flow

* Using {project_name} admin console, click on "Authentication" and select the "Direct Grant" flow,
* Make a copy of the build-in "Direct Grant" flow. You may want to give the new flow a distinctive name, i.e. "X509 Direct Grant",
* Delete "Username Validation" and "Password" authenticators,
* Click on "Add execution" and add "X509/Validate Username" and click on "Save" to add the execution step to the parent flow.

image:images/x509-directgrant-execution.png[]

* Change the `Requirement` to _REQUIRED_.

image:images/x509-directgrant-flow.png[]

* Set up the x509 authentication configuration by following the steps described earlier in the x.509 Browser Flow section. 
* Select the "Bindings" tab, find the drop down for "Direct Grant Flow". Select the newly created X509 direct grant flow from the drop down and click on "Save".

image:images/x509-directgrant-flow-bindings.png[]

==== Client certificate lookup

When an HTTP request is sent directly to {project_name} server, the {appserver_name} undertow subsystem will establish an SSL handshake and extract the client certificate. The client certificate will be then saved to the attribute `javax.servlet.request.X509Certificate` of the HTTP request, as specified in the servlet specification. The {project_name} X509 authenticator will be then able to lookup the certificate from this attribute.

However, when the {project_name} server listens to HTTP requests behind a load balancer or reverse proxy, it may be the proxy server which extracts the client certificate and establishes the mutual SSL connection. A reverse proxy usually puts the authenticated client certificate in the HTTP header of the underlying request and forwards it to the back end {project_name} server. In this case, {project_name} must be able to look up the X.509 certificate chain from the HTTP headers instead of from the attribute of HTTP request, as is done for Undertow.

If {project_name} is behind a reverse proxy, you usually need to configure alternative provider of the `x509cert-lookup` SPI in {project_dirref}/standalone/configuration/standalone.xml. Along with the `default` provider, which looks up the certificate from the HTTP header, we also have two additional built-in providers: `haproxy` and `apache`, which are described next.

===== HAProxy certificate lookup provider

You can use this provider when your {project_name} server is behind an HAProxy reverse proxy. Configure the server like this:

[source,xml]
----
<spi name="x509cert-lookup">
    <default-provider>haproxy</default-provider>
    <provider name="haproxy" enabled="true">
        <properties>
            <property name="sslClientCert" value="SSL_CLIENT_CERT"/>
            <property name="sslCertChainPrefix" value="CERT_CHAIN"/>
            <property name="certificateChainLength" value="10"/>
        </properties>
    </provider>
</spi>
----

In this example configuration, the client certificate will be looked up from the HTTP header, `SSL_CLIENT_CERT`, and the other certificates from its chain will be looked up from HTTP headers like `CERT_CHAIN_0` , `CERT_CHAIN_1`, ..., `CERT_CHAIN_9` . The attribute `certificateChainLength` is the maximum length of the chain, so the last one tried attribute would be `CERT_CHAIN_9` .

Consult the HAProxy documentation for the details of how the HTTP Headers for the client certificate and client certificate chain can be configured and their proper names.

===== Apache certificate lookup provider

You can use this provider when your {project_name} server is behind an Apache reverse proxy. Configure the server like this:

[source,xml]
----
<spi name="x509cert-lookup">
    <default-provider>apache</default-provider>
    <provider name="apache" enabled="true">
        <properties>
            <property name="sslClientCert" value="SSL_CLIENT_CERT"/>
            <property name="sslCertChainPrefix" value="CERT_CHAIN"/>
            <property name="certificateChainLength" value="10"/>
        </properties>
    </provider>
</spi>
----

The configuration is same as for the `haproxy` provider. Consult the Apache documentation for the details of how the HTTP Headers for the client certificate and client certificate chain can be configured and their proper names.

===== Other reverse proxy implementations

We do not have built-in support for other reverse proxy implementations. However, it is possible that other reverse proxies can be made to behave in a similar way to `apache` or `haproxy` and that some of those providers can be used. If none of those works, you may need to create your own implementation of the `org.keycloak.services.x509.X509ClientCertificateLookupFactory` and `org.keycloak.services.x509.X509ClientCertificateLookup` provider. See the link:{developerguide_link}[{developerguide_name}] for the details on how to add your own provider.

==== Troubleshooting

Direct Grant authentication with X.509::
The following template can be used to request a token using the Resource Owner Password Credentials Grant: 

```
$ curl https://[host][:port]/auth/realms/master/protocol/openid-connect/token \
       --insecure \
       --data "grant_type=password&scope=openid profile&username=&password=&client_id=CLIENT_ID&client_secret=CLIENT_SECRET" \
       -E /path/to/client_cert.crt \
       --key /path/to/client_cert.key
```

`[host][:port]`::
The host and the port number of a remote {project_name} server that has been configured to allow users authenticate with x.509 client certificates using the Direct Grant Flow.

`CLIENT_ID`::
A client id.

`CLIENT_SECRET`::
For confidential clients, a client secret; otherwise, leave it empty.

`client_cert.crt`::
A public key certificate that will be used to verify the identity of the client in mutual SSL authentication. The certificate should be in PEM format.

`client_cert.key`::
A private key in the public key pair. Also expected in PEM format.

