diff --git a/docs/storage-transfer.md b/docs/storage-transfer.md
index 212439d..ac4dda4 100644
--- a/docs/storage-transfer.md
+++ b/docs/storage-transfer.md
@@ -16,7 +16,7 @@ Agent host, for sections 3 and 4:
 
 - Linux on AMD64 with a container runtime, Python 3, and the external storage mounted at `/mnt/ext-storage`.
 - A copy of this repository, a `git clone` or an archive unpacked as described in the section "Copy the repository to a server" of the main `README.md`, with `env.sh` created as described in the Configuration section of the main `README.md`. The commands run from its directory.
-- The client certificate renewed in `/var/lib/vault-cert/wif/` and `cred.json` for it in the repository directory, as described in section 4 and Next steps of `docs/vault-approle.md`.
+- With the Vault CA, the client certificate renewed in `/var/lib/vault-cert/wif/`, as described in section 4 of `docs/vault-approle.md`. With the OpenSSL CA, the certificate is installed in section 3.
 
 The names come from `env.sh`.
 
@@ -27,7 +27,7 @@ A transfer from a filesystem is agent based. It involves three identities:
 | Identity | Principal | Purpose | Roles |
 |---|---|---|---|
 | Job creator | `sa-test` through the `wif-test` configuration | Creates and runs transfer jobs | `roles/storagetransfer.user`, `roles/iam.roleViewer` |
-| Transfer agent | `sa-test` through Application Default Credentials (ADC) with the `cred.json` of the agent host in the agent container | Reads the files and writes the objects | `roles/storagetransfer.transferAgent`, `roles/storage.objectAdmin` on the bucket |
+| Transfer agent | `sa-test` through Application Default Credentials (ADC) with `/etc/sts-agent/cred.json` in the agent container | Reads the files and writes the objects | `roles/storagetransfer.transferAgent`, `roles/storage.objectAdmin` on the bucket |
 | Service agent | `project-PROJECT_NUMBER@storage-transfer-service.iam.gserviceaccount.com` | Manages the Pub/Sub topics of the pool, checks the transferred objects and writes the transfer logs | `roles/storagetransfer.serviceAgent`, `roles/storage.legacyBucketReader`, `roles/storage.objectViewer` and `roles/storage.objectCreator` on the bucket |
 
 The service agent is managed by Google and exists once per project. It cannot be replaced with a user-managed service account in agent based transfers.
@@ -52,18 +52,44 @@ bash make_sts.sh
 
 Role: **Server administrator**
 
-`make_sts_agent_creds.py` runs on the agent host. It creates the directory `/etc/sts-agent` with the following files, which hold no secrets:
+### Install the client certificate
 
-- `cred.json`, an unchanged copy of the `cred.json` of the agent host. The agent finds it through the `GOOGLE_APPLICATION_CREDENTIALS` environment variable. It refers to the default certificate configuration, which the agent finds through the `GOOGLE_API_CERTIFICATE_CONFIG` environment variable.
-- `certificate_config.json`, with the paths of the client certificate and key as seen inside the container, where the host directory `/var/lib/vault-cert/wif/` is mounted at `/transfer_root/var/lib/vault-cert/wif/`.
+The agent reads the client certificate and key from `/var/lib/vault-cert/wif/`. With the Vault CA, `vault-cert-renew@wif.timer` keeps them there and this step is skipped.
+
+With the OpenSSL CA, `make_certs.sh` created `client_cert.pem` and `client_key.pem` in the repository directory on the admin workstation. Copy them to the agent host, replacing `agent-host` with its name:
 
 ```bash
-sudo ./make_sts_agent_creds.py
+scp client_cert.pem client_key.pem agent-host:
 ```
 
-The script accepts the options `--cred` (default `cred.json`), `--cert` and `--key` (host paths, default the files in `/var/lib/vault-cert/wif/`), `--output` (default `/etc/sts-agent`) and `--prefix` (container path of the host paths, default `/transfer_root`).
+Install them on the agent host and delete the copies:
 
-The certificate and key are not copied. The agent reads the files renewed by `vault-cert-renew@wif.timer` from the mounted directory.
+```bash
+sudo install -d -m 0750 /var/lib/vault-cert/wif
+sudo install -m 0644 ~/client_cert.pem /var/lib/vault-cert/wif/client_cert.pem
+sudo install -m 0600 ~/client_key.pem /var/lib/vault-cert/wif/client_key.pem
+rm ~/client_cert.pem ~/client_key.pem
+```
+
+### Create the credentials configuration
+
+`make_sts_agent_creds.py` runs on the agent host. It checks that the client certificate and key exist, and creates the directory `/etc/sts-agent` with the following files, which hold no secrets:
+
+- `cred.json`, the credentials configuration for the X.509 provider and the impersonation of the service account. The agent finds it through the `GOOGLE_APPLICATION_CREDENTIALS` environment variable. It refers to the default certificate configuration, which the agent finds through the `GOOGLE_API_CERTIFICATE_CONFIG` environment variable.
+- `certificate_config.json`, with the paths of the client certificate and key.
+
+```bash
+source ./env.sh
+sudo ./make_sts_agent_creds.py \
+    --project-number="${PROJECT_NUMBER}" \
+    --pool="${POOL_ID}" \
+    --provider="${PROVIDER_ID}" \
+    --service-account="${SA_EMAIL}"
+```
+
+The options `--cert` and `--key` (default the files in `/var/lib/vault-cert/wif/`) and `--output` (default `/etc/sts-agent`) change the paths.
+
+The certificate and key are not copied into `/etc/sts-agent`. The agent reads them from the mounted directory `/var/lib/vault-cert/wif/`, so certificates renewed there by `vault-cert-renew@wif.timer` are used without changes to the configuration.
 
 ## 4. Start the transfer agent
 
@@ -74,11 +100,11 @@ The agent container runs with the following settings:
 | Setting | Value |
 |---|---|
 | Image | `gcr.io/cloud-ingest/tsop-agent:latest` |
-| Mount | `/mnt/ext-storage` to `/mnt/ext-storage`, read only |
-| Mount | `/etc/sts-agent` to `/transfer_root/etc/sts-agent`, read only |
-| Mount | `/var/lib/vault-cert/wif` to `/transfer_root/var/lib/vault-cert/wif`, read only |
-| Environment | `GOOGLE_APPLICATION_CREDENTIALS=/transfer_root/etc/sts-agent/cred.json` |
-| Environment | `GOOGLE_API_CERTIFICATE_CONFIG=/transfer_root/etc/sts-agent/certificate_config.json` |
+| Mount | `/mnt/ext-storage`, read only |
+| Mount | `/etc/sts-agent`, read only |
+| Mount | `/var/lib/vault-cert/wif`, read only |
+| Environment | `GOOGLE_APPLICATION_CREDENTIALS=/etc/sts-agent/cred.json` |
+| Environment | `GOOGLE_API_CERTIFICATE_CONFIG=/etc/sts-agent/certificate_config.json` |
 | Locked memory limit | `64000000` bytes |
 | Arguments | `--project-id=${PROJECT_ID} --hostname=$(hostname) --agent-pool=${STS_AGENT_POOL}` |
 
@@ -99,7 +125,7 @@ cd sts-agent
 sudo docker compose up -d
 ```
 
-The source directory is mounted at the same path as on the host, because the transfer job refers to it by that path.
+The directories are mounted at the same paths as on the host. The transfer job refers to the source directory by that path, and `certificate_config.json` holds the host paths of the certificate and key.
 
 The agent authenticates with ADC, not with `--creds-file`. The `--creds-file` option accepts only a service account key or user credentials, not a Workload Identity Federation credential configuration.
 
diff --git a/make_sts_agent_creds.py b/make_sts_agent_creds.py
index 1cec5f7..b1d53aa 100755
--- a/make_sts_agent_creds.py
+++ b/make_sts_agent_creds.py
@@ -2,31 +2,54 @@
 import argparse
 import json
 import os
-import shutil
+import sys
 
 parser = argparse.ArgumentParser(description="Create the credential configuration directory for the Storage Transfer agent container.")
-parser.add_argument("--cred", default="cred.json", help="credentials configuration file")
-parser.add_argument("--cert", default="/var/lib/vault-cert/wif/client_cert.pem", help="client certificate on the host")
-parser.add_argument("--key", default="/var/lib/vault-cert/wif/client_key.pem", help="client private key on the host")
-parser.add_argument("--output", default="/etc/sts-agent", help="credential configuration directory on the host")
-parser.add_argument("--prefix", default="/transfer_root", help="container path under which the host paths are mounted")
+parser.add_argument("--project-number", required=True, help="number of the project of the workload identity pool")
+parser.add_argument("--pool", required=True, help="workload identity pool ID")
+parser.add_argument("--provider", required=True, help="X.509 provider ID")
+parser.add_argument("--service-account", required=True, help="email of the impersonated service account")
+parser.add_argument("--cert", default="/var/lib/vault-cert/wif/client_cert.pem", help="client certificate")
+parser.add_argument("--key", default="/var/lib/vault-cert/wif/client_key.pem", help="client private key")
+parser.add_argument("--output", default="/etc/sts-agent", help="credential configuration directory")
 args = parser.parse_args()
 
+cert = os.path.abspath(args.cert)
+key = os.path.abspath(args.key)
+
+# The container mounts the certificate directory, so the files must exist on the host.
+for path in (cert, key):
+    if not os.path.isfile(path):
+        sys.exit(f"{path} does not exist")
+
 # Credential configuration directory mounted read only in the agent container, its files hold no secrets.
 os.makedirs(args.output, mode=0o755, exist_ok=True)
 
-shutil.copyfile(args.cred, os.path.join(args.output, "cred.json"))
+cred = {
+    "universe_domain": "googleapis.com",
+    "type": "external_account",
+    "audience": f"//iam.googleapis.com/projects/{args.project_number}/locations/global/workloadIdentityPools/{args.pool}/providers/{args.provider}",
+    "subject_token_type": "urn:ietf:params:oauth:token-type:mtls",
+    "token_url": "https://sts.mtls.googleapis.com/v1/token",
+    "credential_source": {
+        "certificate": {
+            "use_default_certificate_config": True,
+        }
+    },
+    "service_account_impersonation_url": f"https://iamcredentials.mtls.googleapis.com/v1/projects/-/serviceAccounts/{args.service_account}:generateAccessToken",
+}
 
-# Certificate and key paths as seen inside the container, where the host certificate directory is mounted under the prefix.
+# The container mounts the certificate directory at the same path as on the host.
 certificate_config = {
     "cert_configs": {
         "workload": {
-            "cert_path": args.prefix + os.path.abspath(args.cert),
-            "key_path": args.prefix + os.path.abspath(args.key),
+            "cert_path": cert,
+            "key_path": key,
         }
     }
 }
 
-with open(os.path.join(args.output, "certificate_config.json"), "w") as file:
-    json.dump(certificate_config, file, indent=2)
-    file.write("\n")
+for name, content in (("cred.json", cred), ("certificate_config.json", certificate_config)):
+    with open(os.path.join(args.output, name), "w") as file:
+        json.dump(content, file, indent=2)
+        file.write("\n")
diff --git a/sts-agent/compose.yaml b/sts-agent/compose.yaml
index 24c8bb2..b33238d 100644
--- a/sts-agent/compose.yaml
+++ b/sts-agent/compose.yaml
@@ -5,11 +5,11 @@ services:
       memlock: 64000000
     volumes:
       - /mnt/ext-storage:/mnt/ext-storage:ro
-      - /etc/sts-agent:/transfer_root/etc/sts-agent:ro
-      - /var/lib/vault-cert/wif:/transfer_root/var/lib/vault-cert/wif:ro
+      - /etc/sts-agent:/etc/sts-agent:ro
+      - /var/lib/vault-cert/wif:/var/lib/vault-cert/wif:ro
     environment:
-      GOOGLE_APPLICATION_CREDENTIALS: /transfer_root/etc/sts-agent/cred.json
-      GOOGLE_API_CERTIFICATE_CONFIG: /transfer_root/etc/sts-agent/certificate_config.json
+      GOOGLE_APPLICATION_CREDENTIALS: /etc/sts-agent/cred.json
+      GOOGLE_API_CERTIFICATE_CONFIG: /etc/sts-agent/certificate_config.json
     command:
       - --project-id=${PROJECT_ID}
       - --hostname=${HOSTNAME}