Pass EL JWT secret key via cli flag (#3568)

## Proposed Changes

In this change I've added a new beacon_node cli flag `--execution-jwt-secret-key` for passing the JWT secret directly as string.

Without this flag, it was non-trivial to pass a secrets file containing a JWT secret key without compromising its contents into some management repo or fiddling around with manual file mounts for cloud-based deployments.

When used in combination with environment variables, the secret can be injected into container-based systems like docker & friends quite easily.

It's both possible to either specify the file_path to the JWT secret or pass the JWT secret directly.

I've modified the docs and attached a test as well.

## Additional Info

The logic has been adapted a bit so that either one of `--execution-jwt` or `--execution-jwt-secret-key` must be set when specifying `--execution-endpoint` so that it's still compatible with the semantics before this change and there's at least one secret provided.

beacon_node/src/cli.rs

@@ -440,7 +440,6 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        JSON-RPC connection. Uses the same endpoint to populate the \
                        deposit cache.")
                 .takes_value(true)
-                .requires("execution-jwt")
         )
         .arg(
             Arg::with_name("execution-jwt")
@@ -452,6 +451,17 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .requires("execution-endpoint")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("execution-jwt-secret-key")
+                .long("execution-jwt-secret-key")
+                .value_name("EXECUTION-JWT-SECRET-KEY")
+                .alias("jwt-secret-key")
+                .help("Hex-encoded JWT secret for the \
+                       execution endpoint provided in the --execution-endpoint flag.")
+                .requires("execution-endpoint")
+                .conflicts_with("execution-jwt")
+                .takes_value(true)
+        )
         .arg(
             Arg::with_name("execution-jwt-id")
                 .long("execution-jwt-id")

beacon_node/src/config.rs

@@ -3,6 +3,7 @@ use clap_utils::flags::DISABLE_MALLOC_TUNING_FLAG;
 use client::{ClientConfig, ClientGenesis};
 use directory::{DEFAULT_BEACON_NODE_DIR, DEFAULT_NETWORK_DIR, DEFAULT_ROOT_DIR};
 use environment::RuntimeContext;
+use execution_layer::DEFAULT_JWT_FILE;
 use genesis::Eth1Endpoint;
 use http_api::TlsConfig;
 use lighthouse_network::{multiaddr::Protocol, Enr, Multiaddr, NetworkConfig, PeerIdSerialized};
@@ -288,12 +289,34 @@ pub fn get_config<E: EthSpec>(
         let execution_endpoint =
             parse_only_one_value(endpoints, SensitiveUrl::parse, "--execution-endpoint", log)?;
 
-        // Parse a single JWT secret, logging warnings if multiple are supplied.
-        //
-        // JWTs are required if `--execution-endpoint` is supplied.
-        let secret_files: String = clap_utils::parse_required(cli_args, "execution-jwt")?;
-        let secret_file =
-            parse_only_one_value(&secret_files, PathBuf::from_str, "--execution-jwt", log)?;
+        // JWTs are required if `--execution-endpoint` is supplied. They can be either passed via
+        // file_path or directly as string.
+
+        let secret_file: PathBuf;
+        // Parse a single JWT secret from a given file_path, logging warnings if multiple are supplied.
+        if let Some(secret_files) = cli_args.value_of("execution-jwt") {
+            secret_file =
+                parse_only_one_value(secret_files, PathBuf::from_str, "--execution-jwt", log)?;
+
+        // Check if the JWT secret key is passed directly via cli flag and persist it to the default
+        // file location.
+        } else if let Some(jwt_secret_key) = cli_args.value_of("execution-jwt-secret-key") {
+            use std::fs::File;
+            use std::io::Write;
+            secret_file = client_config.data_dir.join(DEFAULT_JWT_FILE);
+            let mut jwt_secret_key_file = File::create(secret_file.clone())
+                .map_err(|e| format!("Error while creating jwt_secret_key file: {:?}", e))?;
+            jwt_secret_key_file
+                .write_all(jwt_secret_key.as_bytes())
+                .map_err(|e| {
+                    format!(
+                        "Error occured while writing to jwt_secret_key file: {:?}",
+                        e
+                    )
+                })?;
+        } else {
+            return Err("Error! Please set either --execution-jwt file_path or --execution-jwt-secret-key directly via cli when using --execution-endpoint".to_string());
+        }
 
         // Parse and set the payload builder, if any.
         if let Some(endpoint) = cli_args.value_of("builder") {

book/src/merge-migration.md

@@ -48,6 +48,10 @@ If you set up an execution engine with `--execution-endpoint` then you *must* pr
 using `--execution-jwt`. This is a mandatory form of authentication that ensures that Lighthouse
 has authority to control the execution engine.
 
+> Tip: the --execution-jwt-secret-key <STRING> flag can be used instead of --execution-jwt <FILE>.
+> This is useful, for example, for users who wish to inject the value into a Docker container without
+> needing to pass a jwt secret file.
+
 The execution engine connection must be **exclusive**, i.e. you must have one execution node
 per beacon node. The reason for this is that the beacon node _controls_ the execution node. Please
 see the [FAQ](#faq) for further information about why many:1 and 1:many configurations are not

lighthouse/tests/beacon_node.rs

@@ -4,7 +4,7 @@ use crate::exec::{CommandLineTestExec, CompletedTest};
 use eth1::Eth1Endpoint;
 use lighthouse_network::PeerId;
 use std::fs::File;
-use std::io::Write;
+use std::io::{Read, Write};
 use std::net::IpAddr;
 use std::path::PathBuf;
 use std::process::Command;
@@ -386,6 +386,27 @@ fn run_merge_execution_endpoints_flag_test(flag: &str) {
         });
 }
 #[test]
+fn run_execution_jwt_secret_key_is_persisted() {
+    let jwt_secret_key = "0x3cbc11b0d8fa16f3344eacfd6ff6430b9d30734450e8adcf5400f88d327dcb33";
+    CommandLineTest::new()
+        .flag("execution-endpoint", Some("http://localhost:8551/"))
+        .flag("execution-jwt-secret-key", Some(jwt_secret_key))
+        .run_with_zero_port()
+        .with_config(|config| {
+            let config = config.execution_layer.as_ref().unwrap();
+            assert_eq!(
+                config.execution_endpoints[0].full.to_string(),
+                "http://localhost:8551/"
+            );
+            let mut file_jwt_secret_key = String::new();
+            File::open(config.secret_files[0].clone())
+                .expect("could not open jwt_secret_key file")
+                .read_to_string(&mut file_jwt_secret_key)
+                .expect("could not read from file");
+            assert_eq!(file_jwt_secret_key, jwt_secret_key);
+        });
+}
+#[test]
 fn merge_execution_endpoints_flag() {
     run_merge_execution_endpoints_flag_test("execution-endpoints")
 }