## X.509 Client Certificate User Authentication
Keycloak 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)
* Optinally 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 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.
#### 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 Keycloak Server to enable X.509 client certificate authentication.
#### 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 $KEYCLOAK_HOME/standalone/configuration/standalone.xml and add a new realm:
```
```
`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 element as shown below:
[source,xml,subs="attributes+"]
----
....
----
`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 buit-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:{project_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:{project_images}/x509-browser-flow.png[]
#### Configuring X.509 Client Certificate Authentication
image:{project_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 validaion. See link:https://tools.ietf.org/html/rfc5280#section-4.2.1.3[RFC5280, Section-4.2.1.3]
`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.
`Bypass identity confirmation`::
If set, X.509 client certificate authentication will not prompt the user to confirm the certificate identity and will automatiocally sign in the user upon successful authentication.
### Adding X.509 Client Certificate Authentication to a Direct Grant Flow
* Using keycloak 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 "Validate Username" and "Password" authenticators,
* Click on "Execution" and add "X509/Validate Username" and click on "Save" to add the execution step to the parent flow.
image:{project_images}/x509-directgrant-execution.png[]
* Change the `Requirement` to _REQUIRED_.
image:{project_images}/x509-directgrant-flow.png[]
* Set up the x509 authentication configuration by following the steps described earlier in the x.509 Browser Flow section.
### 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 Keycloak 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.