cdh/kms/aliyun: add docs for sealed secret guide

Signed-off-by: Xynnn007 <xynnn@linux.alibaba.com>

confidential-data-hub/docs/kms-providers/alibaba.md

@@ -45,3 +45,90 @@ The client `AliyunKmsClient` supports both `Encrypter` and `Decrypter` api. When
 user side, the credential files can be directly given by the user.
 
 When in Tee, the credential files is supposed to be placed under `/run/confidential-containers/cdh/kms-credential/aliyun` directory.
+
+## Sealed Secrets
+
+This section introduces how to use `aliyun` KMS to seal a secret.
+
+Suppose that we login on an ECS machine where we can connect to a KMS instance.
+
+Prepare the following files
+- `kms-key-id.txt`: The content is the key id of the symmetric key. Example
+```
+key-bjj...
+```
+- `kms-instance-id.txt`: The content is the instance id of the kms. Example
+```
+kst-bjj652d0b1dkngwgt63bl
+```
+- `ClientKeyPassword.json`: The content is the password of the password of the ClientKey. Example
+```json
+{
+    "ClientKeyPassword": "1e367c6b24..."
+}
+```
+- `ClientKeyContent.json`: The content is the ClientKeyContent. Example
+```json
+{
+  "KeyId": "KAAP.e9692...",
+  "PrivateKeyData": "MIIJ2wIBAzCC..."
+}
+```
+- `ca.pem`: The public key certificate of the KMS instance. Example
+```
+-----BEGIN CERTIFICATE-----
+MIIDuzCCAqOgAwIBAgIJALTKwWAjvbMiMA0GCSqGSIb3DQEBCwUAMHQxCzAJBgNV
+...
+nc8BTncWI0KGWIzTQasuSEye50R6gc9wZCGIElmhWcu3NYk=
+-----END CERTIFICATE-----
+```
+- `plaintext`: The file whose content will be sealed.
+
+Then, let's 
+```bash
+# define the parameters
+KEY_ID=$(cat kms-key-id.txt)
+KMS_INSTANCE_ID=$(cat kms-instance-id.txt)
+CLIENT_KEY_PASSWORD_FILE_PATH=$(pwd)/ClientKeyPassword.txt
+CERT_PATH=$(pwd)/ca.pem
+CLIENT_KEY_FILE_PATH=$(pwd)/ClientKeyContent.json
+
+git clone https://github.com/confidential-containers/guest-components.git && cd guest-components
+
+cargo build --bin secret_cli --release
+
+target/release/secret_cli seal --file-path plaintext \
+    envelope --key-id $KEY_ID ali \
+    --password-file-path $CLIENT_KEY_PASSWORD_FILE_PATH \
+    --cert-path $CERT_PATH \
+    --kms-instance-id $KMS_INSTANCE_ID \
+    --client-key-file-path $CLIENT_KEY_FILE_PATH \
+    > sealed_secret.json
+```
+
+Finally the sealed secret will be output to `sealed_secret.json`.
+
+```bash
+cat sealed_secret.json | python -m json.tool
+```
+
+And the output
+```json
+{
+    "version": "0.1.0",
+    "type": "envelope",
+    "key_id": "key-bj...",
+    "encrypted_key": "XTXa...",
+    "encrypted_data": "7vfE...",
+    "wrap_type": "A256GCM",
+    "iv": "Q/g...",
+    "provider": "aliyun",
+    "provider_settings": {
+        "client_key_id": "KAAP.e9...",
+        "kms_instance_id": "kst-bj..."
+    },
+    "annotations": {
+        "iv": "s2O..."
+    }
+}
+```