Add RCS Strong Verification Documentation

Author: jfreden

deploy-manage/remote-clusters/remote-clusters-api-key.md

@@ -26,6 +26,7 @@ To add a remote cluster using API key authentication:
 2. [Establish trust with a remote cluster](#remote-clusters-security-api-key)
 3. [Connect to a remote cluster](#remote-clusters-connect-api-key)
 4. [Configure roles and users](#remote-clusters-privileges-api-key)
+5. (Optional) [Configure remote cluster strong verification](#remote-cluster-strong-verification) for additional security
 
 If you run into any issues, refer to [Troubleshooting](/troubleshoot/elasticsearch/remote-clusters.md).
 
@@ -405,3 +406,72 @@ POST /_security/user/cross-search-user
 ```
 
 Note that you only need to create this user on the local cluster.
+
+
+## Remote cluster strong verification [remote-cluster-strong-verification]
+preview::[]
+
+Cross-cluster API keys can be configured with strong verification to provide an additional layer of security. To enable this feature, 
+the cross-cluster API key is created with a certificate identity pattern. The local cluster is then required to be configured to sign 
+the API key for every request to the remote cluster and provide a trusted certificate with a subject matching the certificate identity 
+pattern that was used when the cross-cluster API key was created.  
+
+### How strong verification works [_how_strong_verification_works]
+
+When a local cluster makes a request to a remote cluster using a cross-cluster API key:
+
+1. The local cluster signs the request headers with its configured private key and sends the signature and certificate chain as header 
+   in the request to the remote cluster 
+2. The remote cluster verifies that the API key is valid
+3. If the API key has a certificate identity pattern configured, the remote cluster extracts the Distinguished Name (DN) from the 
+   certificate chain's leaf certificate and matches it against the certificate identity pattern
+4. The remote cluster validates that the provided certificate chain is trusted
+5. The remote cluster validates the signature and checks that the certificate is not expired
+
+If any of these validation steps fail, the request is rejected.
+
+### Configure strong verification [_configure_strong_verification]
+
+To use certificate identity validation, the local and remote clusters must be configured to sign request headers and to verify request 
+headers.
+
+#### On the local cluster [_certificate_identity_local_cluster]
+
+The local cluster must be configured to sign cross-cluster requests with a certificate-private key pair.
+
+```yaml
+cluster.remote.my_remote_cluster.signing.certificate: "path/to/signing/certificate.crt"
+cluster.remote.my_remote_cluster.signing.key: "path/to/signing/key.key"
+```
+
+#### On the remote cluster [_certificate_identity_remote_cluster]
+
+The remote cluster must be configured with a certificate authority that trusts the certificate that was used to sign the request headers. 
+
+```yaml
+cluster.remote.signing.certificate_authorities: "path/to/signing/certificate_authorities.crt"
+```
+
+When creating a cross-cluster API key on the remote cluster, specify a `certificate_identity` pattern that matches the Distinguished 
+Name (DN) of the local cluster's certificate. Use the [Create Cross-Cluster API key](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-security-create-cross-cluster-api-key) API:
+
+```console
+POST /_security/cross_cluster/api_key
+{
+  "name": "my-cross-cluster-api-key",
+  "access": {
+    "search": [
+      {
+        "names": ["logs-*"]
+      }
+    ]
+  },
+  "certificate_identity": "CN=local-cluster.example.com,O=Example Corp,C=US"
+}
+```
+
+The `certificate_identity` field supports regular expressions. For example:
+
+* `"CN=.*.example.com,O=Example Corp,C=US"` matches any certificate with a CN starting with any subdomain of example.com
+* `"CN=local-cluster.*,O=Example Corp,C=US"` matches any certificate with a CN starting with "local-cluster"
+* `"CN=.*"` matches any certificate (not recommended for production)