doc: Rotation the signer key for Rekor service

docs/rekor-key-rotation.md

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+# Rotating the Signer Key for Rekor Service
+
+This document provides detailed steps on how to rotate the signer key for the Rekor service. The process involves
+sharding the Rekor log and then updating the signer key.
+
+## Prerequisites
+
+Before you begin, ensure you have the necessary access to your Kubernetes cluster and the Rekor CLI.
+
+## Part 1: Freezing the Current Tree
+
+In order to rotate the signer key effectively, it's crucial to transition the current tree into a frozen state, ensuring
+it's only accessible for reading purposes. Simultaneously, a new tree needs to be created to serve as the active tree
+for signing new records with the updated key. This process is facilitated through the sharding feature of Rekor, which
+allows the log to be divided into multiple manageable parts. By following the initial 8 steps outlined in the
+[Sharding the Rekor Log documentation](rekor-sharding.md), you will freeze the current log tree and establish a new log
+tree ready for operations.
+
+## Part 2: Rotating the Signer Key
+
+Before proceeding with the rotation of the signer key, it's essential to complete **Part 1** to ensure the Rekor service
+is prepared with a frozen current tree and a newly established active tree for continued operations with the updated
+key. Once Part 1 is completed, ensure you have the following environment variables set:
+
+```bash
+CURRENT_SHARD_LENGTH=<length_of_frozen_shard>
+CURRENT_TREE_ID=<frozen_tree_id>
+NEW_TREE_ID=<new_tree_id>
+CURRENT_SHARD_PUBLIC_KEY=<public_key_of_frozen_shard>
+```
+
+These variables are necessary for the subsequent steps to successfully rotate the signer key.
+
+1. **Create New Private Key:** 
+   Generate a new private key and store it in a Kubernetes secret. You can use the following commands:
+
+   ```bash
+   openssl ecparam -genkey -name secp384r1 -noout -out rekor.pem
+   kubectl create secret generic rekor-signer-key --from-file=private=rekor.pem
+   ```
+
+1. **Update Securesign Resource:**
+
+   Patch the Securesign resource to use the newly created secret, update the tree ID, and configure the sharding details for frozen log.
+
+   ```bash
+   read -r -d '' SECURESIGN_PATCH <<EOF
+   [
+       {
+           "op": "replace",
+           "path": "/spec/rekor/treeID",
+           "value": "$NEW_TREE_ID"
+       },
+       {
+           "op": "add",
+           "path": "/spec/rekor/sharding",
+           "value": {
+               "treeID": "$CURRENT_TREE_ID",
+               "treeLength": "$CURRENT_SHARD_LENGTH",
+               "encodedPublicKey": "$CURRENT_SHARD_PUBLIC_KEY"
+           }
+       },
+       {
+           "op": "replace",
+           "path": "/spec/rekor/signer/keyRef",
+           "value": {"name": "rekor-signer-key", "key": "private"}
+       },
+       {
+           "op": "remove",
+           "path": "/spec/rekor/signer/keyPasswordRef"
+       },
+   ]
+   EOF
+   kubectl patch securesign securesign-sample --type='json' -p="$SECURESIGN_PATCH"
+   ```
+
+3. **Wait for Rekor Server Redeployment:**
+
+   Monitor the Kubernetes deployment to ensure the Rekor server is redeployed with the updated configuration.
+
+   ```bash
+   kubectl get pods -w -l app.kubernetes.io/name=rekor-server
+   ```
+
+4. **Retrieve New Rekor Public Key:**
+
+   After the Rekor server is redeployed, retrieve the new public key of the signer.
+
+   ```bash
+   NEW_SHARD_PUBLIC_KEY=$(curl $(oc get rekor -o jsonpath='{.items[0].status.url}')/api/v1/log/publicKey | base64)
+   ```
+
+5. **Update TUF Service:**
+
+   Follow the [TUF key rotation documentation](TODO) to add the new public key into TUF service.