Permit trusted self-signed certificates

Self-signed certificates are both an end-entity as well as a certificate
authority in the eyes of webpki - since we have ultimate trust in the
supplied trust anchors, a certificate supplied with the same subject as a
trust anchor and is directly signed by the same trust anchor should also
be valid, even though the certificate supplied might still be a CA.

Cargo.toml

@@ -45,6 +45,7 @@ include = [
 
     "tests/dns_name_tests.rs",
     "tests/integration.rs",
+    "tests/misc/selfsigned.der",
     "tests/misc/serial_neg.der",
     "tests/misc/serial_zero.der",
     "tests/netflix/ca.der",

src/verify_cert.rs

@@ -28,13 +28,17 @@ pub fn build_chain(
 ) -> Result<(), Error> {
     let used_as_ca = used_as_ca(&cert.ee_or_ca);
 
-    check_issuer_independent_properties(
+    let must_be_self_signed = match check_issuer_independent_properties(
         cert,
         time,
         used_as_ca,
         sub_ca_count,
         required_eku_if_present,
-    )?;
+    ) {
+        Ok(()) => false,
+        Err(Error::CAUsedAsEndEntity) => true,
+        Err(e) => return Err(e),
+    };
 
     // TODO: HPKP checks.
 
@@ -71,6 +75,10 @@ pub fn build_chain(
 
         check_signatures(supported_sig_algs, cert, trust_anchor_spki)?;
 
+        if must_be_self_signed && trust_anchor_subject != cert.subject {
+            return Err(Error::CAUsedAsEndEntity);
+        }
+
         Ok(())
     }) {
         Ok(()) => {
@@ -81,6 +89,10 @@ pub fn build_chain(
         }
     }
 
+    if must_be_self_signed {
+        return Err(Error::CAUsedAsEndEntity);
+    }
+
     loop_while_non_fatal_error(intermediate_certs, |cert_der| {
         let potential_issuer =
             cert::parse_cert(untrusted::Input::from(*cert_der), EndEntityOrCA::CA(&cert))?;

tests/integration.rs

@@ -84,6 +84,23 @@ fn read_root_with_neg_serial() {
         .expect("idcat cert should parse as anchor");
 }
 
+#[test]
+fn selfsigned() {
+    let cert = include_bytes!("misc/selfsigned.der");
+
+    let anchors = vec![webpki::trust_anchor_util::cert_der_as_trust_anchor(cert).unwrap()];
+    let anchors = webpki::TLSServerTrustAnchors(&anchors);
+
+    #[allow(clippy::unreadable_literal)] // TODO: Make this clear.
+    let time = webpki::Time::from_seconds_since_unix_epoch(1610233207);
+
+    let cert = webpki::EndEntityCert::from(cert).unwrap();
+    assert_eq!(
+        Ok(()),
+        cert.verify_is_valid_tls_server_cert(ALL_SIGALGS, &anchors, &[], time)
+    );
+}
+
 #[cfg(feature = "std")]
 #[test]
 fn time_constructor() {