Add optional secret for authenticating clients (#1)

* Add optional secret for authenticating clients

* Add server challenge to authentication

* Refactor and simplify code, reduce dependencies

* Update README to describe HMAC authentication

Co-authored-by: Eric Zhang <ekzhang1@gmail.com>

Cargo.toml

@@ -19,8 +19,11 @@ path = "src/main.rs"
 anyhow = { version = "1.0.56", features = ["backtrace"] }
 clap = { version = "3.1.8", features = ["derive"] }
 dashmap = "5.2.0"
+hex = "0.4.3"
+hmac = "0.12.1"
 serde = { version = "1.0.136", features = ["derive"] }
 serde_json = "1.0.79"
+sha2 = "0.10.2"
 tokio = { version = "1.17.0", features = ["full"] }
 tracing = "0.1.32"
 tracing-subscriber = "0.3.10"

README.md

@@ -19,7 +19,7 @@ This will expose your local port at `localhost:8000` to the public internet at `
 
 Similar to [localtunnel](https://github.com/localtunnel/localtunnel) and [ngrok](https://ngrok.io/), except `bore` is intended to be a highly efficient, unopinionated tool for forwarding TCP traffic that is simple to install and easy to self-host, with no frills attached.
 
-(`bore` totals less than 300 lines of safe, async Rust code and is trivial to set up — just run a single binary for the client and server.)
+(`bore` totals less than 400 lines of safe, async Rust code and is trivial to set up — just run a single binary for the client and server.)
 
 ## Detailed Usage
 
@@ -38,7 +38,7 @@ You can optionally pass in a `--port` option to pick a specific port on the remo
 The full options are shown below.
 
 ```shell
-bore-local 0.1.0
+bore-local 0.1.1
 Starts a local proxy to the remote server
 
 USAGE:
@@ -48,10 +48,11 @@ ARGS:
     <LOCAL_PORT>    The local port to listen on
 
 OPTIONS:
-    -h, --help           Print help information
-    -p, --port <PORT>    Optional port on the remote server to select [default: 0]
-    -t, --to <TO>        Address of the remote server to expose local ports to
-    -V, --version        Print version information
+    -h, --help               Print help information
+    -p, --port <PORT>        Optional port on the remote server to select [default: 0]
+    -s, --secret <SECRET>    Optional secret for authentication
+    -t, --to <TO>            Address of the remote server to expose local ports to
+    -V, --version            Print version information
 ```
 
 ### Self-Hosting
@@ -67,7 +68,7 @@ That's all it takes! After the server starts running at a given address, you can
 The full options for the `bore server` command are shown below.
 
 ```shell
-bore-server 0.1.0
+bore-server 0.1.1
 Runs the remote proxy server
 
 USAGE:
@@ -76,6 +77,7 @@ USAGE:
 OPTIONS:
     -h, --help                   Print help information
         --min-port <MIN_PORT>    Minimum TCP port number to accept [default: 1024]
+    -s, --secret <SECRET>        Optional secret for authentication
     -V, --version                Print version information
 ```
 
@@ -87,6 +89,18 @@ Whenever the server obtains a connection on the remote port, it generates a secu
 
 For correctness reasons and to avoid memory leaks, incoming connections are only stored by the server for up to 10 seconds before being discarded if the client does not accept them.
 
+## Authentication
+
+On a custom deployment of `bore server`, you can optionally require a _secret_ to prevent the server from being used by others. The protocol requires clients to verify possession of the secret on each TCP connection by answering random challenges in the form of HMAC codes. (This secret is only used for the initial handshake, and no further traffic is encrypted by default.)
+
+```shell
+# on the server
+bore server --secret my_secret_string
+
+# on the client
+bore local <LOCAL_PORT> --to <TO> --secret my_secret_string
+```
+
 ## Acknowledgements
 
 Created by Eric Zhang ([@ekzhang1](https://twitter.com/ekzhang1)). Licensed under the [MIT license](LICENSE).

src/auth.rs

@@ -0,0 +1,79 @@
+//! Auth implementation for bore client and server.
+
+use anyhow::{bail, ensure, Result};
+use hmac::{Hmac, Mac};
+use sha2::{Digest, Sha256};
+use tokio::io::{AsyncBufRead, AsyncWrite};
+use uuid::Uuid;
+
+use crate::shared::{recv_json, send_json, ClientMessage, ServerMessage};
+
+/// Wrapper around a MAC used for authenticating clients that have a secret.
+pub struct Authenticator(Hmac<Sha256>);
+
+impl Authenticator {
+    /// Generate an authenticator from a secret.
+    pub fn new(secret: &str) -> Self {
+        let hashed_secret = Sha256::new().chain_update(secret).finalize();
+        Self(Hmac::new_from_slice(&hashed_secret).expect("HMAC can take key of any size"))
+    }
+
+    /// Generate a reply message for a challenge.
+    pub fn answer(&self, challenge: &Uuid) -> String {
+        let mut hmac = self.0.clone();
+        hmac.update(challenge.as_bytes());
+        hex::encode(hmac.finalize().into_bytes())
+    }
+
+    /// Validate a reply to a challenge.
+    ///
+    /// ```
+    /// use uuid:Uuid;
+    /// use crate::auth::Authenticator;
+    ///
+    /// let auth = Authenticator::new("secret");
+    /// let challenge = Uuid::new_v4();
+    ///
+    /// assert!(auth.validate(&challenge, auth.answer(&challenge)));
+    /// assert!(!auth.validate(&challenge, "wrong answer"));
+    /// ```
+    pub fn validate(&self, challenge: &Uuid, tag: &str) -> bool {
+        if let Ok(tag) = hex::decode(tag) {
+            let mut hmac = self.0.clone();
+            hmac.update(challenge.as_bytes());
+            hmac.verify_slice(&tag).is_ok()
+        } else {
+            false
+        }
+    }
+
+    /// As the server, send a challenge to the client and validate their response.
+    pub async fn server_handshake(
+        &self,
+        stream: &mut (impl AsyncBufRead + AsyncWrite + Unpin),
+    ) -> Result<()> {
+        let challenge = Uuid::new_v4();
+        send_json(stream, ServerMessage::Challenge(challenge)).await?;
+        match recv_json(stream, &mut Vec::new()).await? {
+            Some(ClientMessage::Authenticate(tag)) => {
+                ensure!(self.validate(&challenge, &tag), "invalid secret");
+                Ok(())
+            }
+            _ => bail!("server requires secret, but no secret was provided"),
+        }
+    }
+
+    /// As the client, answer a challenge to attempt to authenticate with the server.
+    pub async fn client_handshake(
+        &self,
+        stream: &mut (impl AsyncBufRead + AsyncWrite + Unpin),
+    ) -> Result<()> {
+        let challenge = match recv_json(stream, &mut Vec::new()).await? {
+            Some(ServerMessage::Challenge(challenge)) => challenge,
+            _ => bail!("expected authentication challenge, but no secret was required"),
+        };
+        let tag = self.answer(&challenge);
+        send_json(stream, ClientMessage::Authenticate(tag)).await?;
+        Ok(())
+    }
+}

src/client.rs

@@ -7,6 +7,7 @@ use tokio::{io::BufReader, net::TcpStream};
 use tracing::{error, info, info_span, warn, Instrument};
 use uuid::Uuid;
 
+use crate::auth::Authenticator;
 use crate::shared::{proxy, recv_json, send_json, ClientMessage, ServerMessage, CONTROL_PORT};
 
 /// State structure for the client.
@@ -22,18 +23,31 @@ pub struct Client {
 
     /// Port that is publicly available on the remote.
     remote_port: u16,
+
+    /// Optional secret used to authenticate clients.
+    auth: Option<Authenticator>,
 }
 
 impl Client {
     /// Create a new client.
-    pub async fn new(local_port: u16, to: &str, port: u16) -> Result<Self> {
-        let stream = TcpStream::connect((to, CONTROL_PORT)).await?;
+    pub async fn new(local_port: u16, to: &str, port: u16, secret: Option<&str>) -> Result<Self> {
+        let stream = TcpStream::connect((to, CONTROL_PORT))
+            .await
+            .with_context(|| format!("could not connect to {to}:{CONTROL_PORT}"))?;
         let mut stream = BufReader::new(stream);
 
+        let auth = secret.map(Authenticator::new);
+        if let Some(auth) = &auth {
+            auth.client_handshake(&mut stream).await?;
+        }
+
         send_json(&mut stream, ClientMessage::Hello(port)).await?;
         let remote_port = match recv_json(&mut stream, &mut Vec::new()).await? {
             Some(ServerMessage::Hello(remote_port)) => remote_port,
             Some(ServerMessage::Error(message)) => bail!("server error: {message}"),
+            Some(ServerMessage::Challenge(_)) => {
+                bail!("server requires authentication, but no client secret was provided");
+            }
             Some(_) => bail!("unexpected initial non-hello message"),
             None => bail!("unexpected EOF"),
         };
@@ -45,6 +59,7 @@ impl Client {
             to: to.to_string(),
             local_port,
             remote_port,
+            auth,
         })
     }
 
@@ -62,6 +77,7 @@ impl Client {
             let msg = recv_json(&mut conn, &mut buf).await?;
             match msg {
                 Some(ServerMessage::Hello(_)) => warn!("unexpected hello"),
+                Some(ServerMessage::Challenge(_)) => warn!("unexpected challenge"),
                 Some(ServerMessage::Heartbeat) => (),
                 Some(ServerMessage::Connection(id)) => {
                     let this = Arc::clone(&this);
@@ -86,9 +102,15 @@ impl Client {
         let local_conn = TcpStream::connect(("localhost", self.local_port))
             .await
             .context("failed TCP connection to local port")?;
-        let mut remote_conn = TcpStream::connect((&self.to[..], CONTROL_PORT))
-            .await
-            .context("failed TCP connection to remote port")?;
+        let mut remote_conn = BufReader::new(
+            TcpStream::connect((&self.to[..], CONTROL_PORT))
+                .await
+                .context("failed TCP connection to remote port")?,
+        );
+
+        if let Some(auth) = &self.auth {
+            auth.client_handshake(&mut remote_conn).await?;
+        }
 
         send_json(&mut remote_conn, ClientMessage::Accept(id)).await?;
         proxy(local_conn, remote_conn).await?;

src/lib.rs

@@ -15,6 +15,7 @@
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]
 
+pub mod auth;
 pub mod client;
 pub mod server;
 pub mod shared;