feat(tls): add an option for optional TLS client authentication

Previously there were only two options for client authentication –
either no authentication or mandatory authentication. With this change,
a server can allow for optional authentication with a given root CA
certificate and enforce client authentication on a per-request basis.

Refs: #687

tonic/src/transport/server/tls.rs

@@ -11,6 +11,7 @@ use std::fmt;
 pub struct ServerTlsConfig {
     identity: Option<Identity>,
     client_ca_root: Option<Certificate>,
+    client_auth_optional: bool,
 }
 
 #[cfg(feature = "tls")]
@@ -27,6 +28,7 @@ impl ServerTlsConfig {
         ServerTlsConfig {
             identity: None,
             client_ca_root: None,
+            client_auth_optional: false,
         }
     }
 
@@ -46,7 +48,18 @@ impl ServerTlsConfig {
         }
     }
 
+    /// Sets whether client certificate verification is optional.
+    /// 
+    /// This option has effect only if CA certificate is set.
+    pub fn client_auth_optional(self, optional: bool) -> Self {
+        ServerTlsConfig {
+            client_auth_optional: optional,
+            ..self
+        }
+    }
+
+
     pub(crate) fn tls_acceptor(&self) -> Result<TlsAcceptor, crate::Error> {
-        TlsAcceptor::new(self.identity.clone().unwrap(), self.client_ca_root.clone())
+        TlsAcceptor::new(self.identity.clone().unwrap(), self.client_ca_root.clone(), self.client_auth_optional)
     }
-}
+}

tonic/src/transport/service/tls.rs

@@ -118,12 +118,19 @@ impl TlsAcceptor {
     pub(crate) fn new(
         identity: Identity,
         client_ca_root: Option<Certificate>,
+        client_auth_optional: bool,
     ) -> Result<Self, crate::Error> {
         let builder = ServerConfig::builder().with_safe_defaults();
 
-        let builder = match client_ca_root {
-            None => builder.with_no_client_auth(),
-            Some(cert) => {
+        let builder = match (client_ca_root, client_auth_optional) {
+            (None, _) => builder.with_no_client_auth(),
+            (Some(cert), true) => {
+                use tokio_rustls::rustls::server::AllowAnyAnonymousOrAuthenticatedClient;
+                let mut roots = RootCertStore::empty();
+                rustls_keys::add_certs_from_pem(std::io::Cursor::new(&cert.pem[..]), &mut roots)?;
+                builder.with_client_cert_verifier(AllowAnyAnonymousOrAuthenticatedClient::new(roots))
+            }
+            (Some(cert), false) => {
                 use tokio_rustls::rustls::server::AllowAnyAuthenticatedClient;
                 let mut roots = RootCertStore::empty();
                 rustls_keys::add_certs_from_pem(std::io::Cursor::new(&cert.pem[..]), &mut roots)?;