der: add ErrorKind::TagNumberInvalid (#156)

Handles the specific case that the lower 5-bits of a tag are all `1`.

This is a bit different from the `ErrorKind::TagUnknown` previously
used, in that it specifically related to an unsupported BER encoding.

der/src/error.rs

@@ -229,6 +229,13 @@ pub enum ErrorKind {
     /// Unknown tag mode.
     TagModeUnknown,
 
+    /// Invalid tag number.
+    ///
+    /// The "tag number" is the lower 5-bits of a tag's octet.
+    /// This error occurs in the case that all 5-bits are set to `1`,
+    /// which indicates a multi-byte tag which is unsupported by this library.
+    TagNumberInvalid,
+
     /// Unexpected tag.
     TagUnexpected {
         /// Tag the decoder was expecting (if there is a single such tag).
@@ -328,6 +335,7 @@ impl fmt::Display for ErrorKind {
             #[cfg(feature = "std")]
             ErrorKind::PermissionDenied => f.write_str("permission denied"),
             ErrorKind::TagModeUnknown => write!(f, "unknown tag mode"),
+            ErrorKind::TagNumberInvalid => write!(f, "invalid tag number"),
             ErrorKind::TagUnexpected { expected, actual } => {
                 write!(f, "unexpected ASN.1 DER tag: ")?;
 

der/src/tag/number.rs

@@ -75,7 +75,7 @@ impl TryFrom<u8> for TagNumber {
     fn try_from(byte: u8) -> Result<Self> {
         match byte {
             0..=Self::MAX => Ok(Self(byte)),
-            _ => Err(ErrorKind::TagUnknown { byte }.into()),
+            _ => Err(ErrorKind::TagNumberInvalid.into()),
         }
     }
 }