Fix name constraints check

There were two bugs. webpki didn't:

1. read the X.509 Name Constraints field in its entirety, nor

2. check the certificate subject against the constraints correctly

(1) is a simple fix, (2) requires reading the Common Name from the
certificate.

Closes #20, #134.

src/name/verify.rs

@@ -70,10 +70,7 @@ pub fn check_name_constraints(
         if !inner.peek(subtrees_tag.into()) {
             return Ok(None);
         }
-        let subtrees = der::nested(inner, subtrees_tag, Error::BadDer, |tagged| {
-            der::expect_tag_and_get_value(tagged, der::Tag::Sequence)
-        })?;
-        Ok(Some(subtrees))
+        der::expect_tag_and_get_value(inner, subtrees_tag).map(Some)
     }
 
     let permitted_subtrees = parse_subtrees(input, der::Tag::ContextSpecificConstructed0)?;
@@ -167,6 +164,10 @@ fn check_presented_id_conforms_to_constraints_in_subtree(
                 dns_name::presented_id_matches_constraint(name, base).ok_or(Error::BadDer)
             }
 
+            (GeneralName::DirectoryName(name), GeneralName::DnsName(base)) => {
+                common_name(name).map(|cn| cn == base)
+            }
+
             (GeneralName::DirectoryName(name), GeneralName::DirectoryName(base)) => Ok(
                 presented_directory_name_matches_constraint(name, base, subtrees),
             ),
@@ -326,3 +327,18 @@ fn general_name<'a>(input: &mut untrusted::Reader<'a>) -> Result<GeneralName<'a>
     };
     Ok(name)
 }
+
+static COMMON_NAME: untrusted::Input = untrusted::Input::from(&[85, 4, 3]);
+
+fn common_name(input: untrusted::Input) -> Result<untrusted::Input, Error> {
+    let inner = &mut untrusted::Reader::new(input);
+    der::nested(inner, der::Tag::Set, Error::BadDer, |tagged| {
+        der::nested(tagged, der::Tag::Sequence, Error::BadDer, |tagged| {
+            let value = der::expect_tag_and_get_value(tagged, der::Tag::OID)?;
+            if value != COMMON_NAME {
+                return Err(Error::BadDer);
+            }
+            der::expect_tag_and_get_value(tagged, der::Tag::UTF8String)
+        })
+    })
+}

tests/integration.rs

@@ -53,6 +53,25 @@ pub fn netflix() {
     );
 }
 
+#[cfg(feature = "alloc")]
+#[test]
+pub fn wpt() {
+    let ee: &[u8] = include_bytes!("wpt/ee.der");
+    let ca = include_bytes!("wpt/ca.der");
+
+    let anchors = vec![webpki::TrustAnchor::try_from_cert_der(ca).unwrap()];
+    let anchors = webpki::TlsServerTrustAnchors(&anchors);
+
+    #[allow(clippy::unreadable_literal)] // TODO: Make this clear.
+    let time = webpki::Time::from_seconds_since_unix_epoch(1619256684);
+
+    let cert = webpki::EndEntityCert::try_from(ee).unwrap();
+    assert_eq!(
+        Ok(()),
+        cert.verify_is_valid_tls_server_cert(ALL_SIGALGS, &anchors, &[], time)
+    );
+}
+
 #[test]
 pub fn ed25519() {
     let ee: &[u8] = include_bytes!("ed25519/ee.der");