feat: custom TLS certificate verification

src/net/tls/danger.rs

@@ -1,26 +1,85 @@
-//! Dangerous TLS implementation of accepting invalid certificates for Rustls.
+//! Custom TLS verification.
+//!
+//! We want to accept expired certificates.
 
+use rustls::RootCertStore;
+use rustls::client::{verify_server_cert_signed_by_trust_anchor, verify_server_name};
 use rustls::pki_types::{CertificateDer, ServerName, UnixTime};
+use rustls::server::ParsedCertificate;
 use tokio_rustls::rustls;
 
-#[derive(Debug)]
-pub(super) struct NoCertificateVerification();
+use crate::net::tls::spki::spki_hash;
 
-impl NoCertificateVerification {
-    pub(super) fn new() -> Self {
-        Self()
+#[derive(Debug)]
+pub(super) struct CustomCertificateVerifier {
+    /// Root certificates.
+    root_cert_store: RootCertStore,
+
+    /// Expected SPKI hash as a base64 of SHA-256.
+    spki_hash: Option<String>,
+}
+
+impl CustomCertificateVerifier {
+    pub(super) fn new(spki_hash: Option<String>) -> Self {
+        let mut root_cert_store = rustls::RootCertStore::empty();
+        root_cert_store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
+        Self {
+            root_cert_store,
+            spki_hash,
+        }
     }
 }
 
-impl rustls::client::danger::ServerCertVerifier for NoCertificateVerification {
+impl rustls::client::danger::ServerCertVerifier for CustomCertificateVerifier {
     fn verify_server_cert(
         &self,
-        _end_entity: &CertificateDer<'_>,
-        _intermediates: &[CertificateDer<'_>],
-        _server_name: &ServerName<'_>,
+        end_entity: &CertificateDer<'_>,
+        intermediates: &[CertificateDer<'_>],
+        server_name: &ServerName<'_>,
         _ocsp_response: &[u8],
-        _now: UnixTime,
+        now: UnixTime,
     ) -> Result<rustls::client::danger::ServerCertVerified, rustls::Error> {
+        let parsed_certificate = ParsedCertificate::try_from(end_entity)?;
+
+        let spki = parsed_certificate.subject_public_key_info();
+
+        let provider = rustls::crypto::ring::default_provider();
+
+        if let ServerName::DnsName(dns_name) = server_name
+            && dns_name.as_ref().starts_with("_")
+        {
+            // Do not verify certificates for hostnames starting with `_`.
+            // They are used for servers with self-signed certificates, e.g. for local testing.
+            // Hostnames starting with `_` can have only self-signed TLS certificates or wildcard certificates.
+            // It is not possible to get valid non-wildcard TLS certificates because CA/Browser Forum requirements
+            // explicitly state that domains should start with a letter, digit or hyphen:
+            // https://github.com/cabforum/servercert/blob/24f38fd4765e019db8bb1a8c56bf63c7115ce0b0/docs/BR.md
+        } else if let Some(hash) = &self.spki_hash
+            && spki_hash(&spki) == *hash
+        {
+            // Last time we successfully connected to this hostname with TLS checks,
+            // SPKI had this hash.
+            // It does not matter if certificate has now expired.
+        } else {
+            // verify_server_cert_signed_by_trust_anchor does no revocation checking:
+            // <https://docs.rs/rustls/0.23.37/rustls/client/fn.verify_server_cert_signed_by_trust_anchor.html>
+            // We don't do it either.
+            verify_server_cert_signed_by_trust_anchor(
+                &parsed_certificate,
+                &self.root_cert_store,
+                intermediates,
+                now,
+                provider.signature_verification_algorithms.all,
+            )?;
+        }
+
+        // Verify server name unconditionally.
+        //
+        // We do this even for self-signed certificates when hostname starts with `_`
+        // so we don't try to connect to captive portals
+        // and fail on MITM certificates if they are generated once
+        // and reused for all hostnames.
+        verify_server_name(&parsed_certificate, server_name)?;
         Ok(rustls::client::danger::ServerCertVerified::assertion())
     }