der: add TagNumber type, make Tag structured (#464)

Adds a `TagNumber` type for representing the lower 5 bits of a Tag,
where bits 8/7 identify the class and bit 6 identifies primitive vs
constructed forms.

Changes `Tag` from being `repr(u8)` to a structured representation with
`Application`, `ContextSpecific`, and `Private` variants, each of which
includes a `TagNumber`. This enables support for all of these classes
for tag numbers up to 30, which is the maximum value which can be
represented with a single leading identifier octet.

This change enables the `ContextSpecific` struct to have public members,
as the tag number invariant can now be maintained by the `TagNumber`
type instead.

der/derive/src/choice.rs

@@ -224,9 +224,6 @@ impl DeriveChoice {
 /// ASN.1 `CHOICE` alternative: one of the ASN.1 types comprising the `CHOICE`
 /// which maps to an enum variant.
 struct Alternative {
-    /// ASN.1 type for this alternative.
-    pub asn1_type: Asn1Type,
-
     /// [`Ident`] for the corresponding enum variant.
     pub ident: Ident,
 
@@ -246,7 +243,6 @@ impl Alternative {
         };
 
         let alternative = Self {
-            asn1_type,
             ident: variant.ident.clone(),
             field_type,
         };

der/src/asn1/context_specific.rs

@@ -2,61 +2,32 @@
 
 use crate::{
     Any, Choice, Decodable, Encodable, Encoder, Error, ErrorKind, Header, Length, Result, Tag,
+    TagNumber,
 };
 use core::convert::TryFrom;
 
 /// Context-specific field.
 ///
 /// This type encodes a field which is specific to a particular context,
-/// and has a special "context-specific tag" (presently 0-15 supported).
+/// and is identified by a [`TagNumber`].
 ///
 /// Any context-specific field can be decoded/encoded with this type.
 /// The intended use is to dynamically dispatch off of the context-specific
-/// tag when decoding, which allows support for extensions, which are denoted
-/// in an ASN.1 schema using the `...` ellipsis extension marker.
-///
-///
+/// tag number when decoding, which allows support for extensions, which are
+/// denoted in an ASN.1 schema using the `...` ellipsis extension marker.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
 pub struct ContextSpecific<'a> {
-    /// Context-specific tag value sans the leading `0b10000000` class
+    /// Context-specific tag number sans the leading `0b10000000` class
     /// identifier bit and `0b100000` constructed flag.
-    pub(crate) tag: u8,
+    pub tag_number: TagNumber,
 
     /// Value of the field.
-    pub(crate) value: Any<'a>,
-}
-
-impl<'a> ContextSpecific<'a> {
-    /// Create a new context-specific field.
-    ///
-    /// The tag value includes only lower 6-bits of the context specific tag,
-    /// sans the leading `10` high bits identifying the context-specific tag
-    /// class as well as the constructed flag.
-    pub fn new(tag: u8, value: Any<'a>) -> Result<Self> {
-        // Ensure we consider the context-specific tag valid
-        Tag::context_specific(tag)?;
-
-        Ok(Self { tag, value })
-    }
-
-    /// Get the context-specific tag for this field.
-    ///
-    /// The tag value includes only lower 6-bits of the context specific tag,
-    /// sans the leading `10` high bits identifying the context-specific tag
-    /// class as well as the constructed flag.
-    pub fn tag(self) -> u8 {
-        self.tag
-    }
-
-    /// Get the value of this context-specific tag.
-    pub fn value(self) -> Any<'a> {
-        self.value
-    }
+    pub value: Any<'a>,
 }
 
 impl<'a> Choice<'a> for ContextSpecific<'a> {
     fn can_decode(tag: Tag) -> bool {
-        tag.is_context_specific()
+        matches!(tag, Tag::ContextSpecific(_))
     }
 }
 
@@ -66,7 +37,7 @@ impl<'a> Encodable for ContextSpecific<'a> {
     }
 
     fn encode(&self, encoder: &mut Encoder<'_>) -> Result<()> {
-        let tag = Tag::context_specific(self.tag)?;
+        let tag = Tag::ContextSpecific(self.tag_number);
         Header::new(tag, self.value.encoded_len()?)?.encode(encoder)?;
         self.value.encode(encoder)
     }
@@ -82,21 +53,17 @@ impl<'a> TryFrom<Any<'a>> for ContextSpecific<'a> {
     type Error = Error;
 
     fn try_from(any: Any<'a>) -> Result<ContextSpecific<'a>> {
-        let tag = if any.tag().is_context_specific() {
-            (any.tag() as u8)
-                .checked_sub(0xA0)
-                .ok_or(ErrorKind::Overflow)?
-        } else {
-            return Err(ErrorKind::UnexpectedTag {
+        match any.tag() {
+            Tag::ContextSpecific(tag_number) => Ok(Self {
+                tag_number,
+                value: Any::from_der(any.as_bytes())?,
+            }),
+            actual => Err(ErrorKind::UnexpectedTag {
                 expected: None,
-                actual: any.tag(),
+                actual,
             }
-            .into());
-        };
-
-        let value = Any::from_der(any.as_bytes())?;
-
-        Self::new(tag, value)
+            .into()),
+        }
     }
 }
 
@@ -113,11 +80,9 @@ mod tests {
     #[test]
     fn round_trip() {
         let field = ContextSpecific::from_der(EXAMPLE_BYTES).unwrap();
-        assert_eq!(field.tag(), 1);
-
-        let value = field.value();
-        assert_eq!(value.tag(), Tag::BitString);
-        assert_eq!(value.as_bytes(), &EXAMPLE_BYTES[5..]);
+        assert_eq!(field.tag_number.value(), 1);
+        assert_eq!(field.value.tag(), Tag::BitString);
+        assert_eq!(field.value.as_bytes(), &EXAMPLE_BYTES[5..]);
 
         let mut buf = [0u8; 128];
         let encoded = field.encode_to_slice(&mut buf).unwrap();

der/src/lib.rs

@@ -5,7 +5,8 @@
 //!
 //! This crate provides a `no_std`-friendly implementation of a subset of ASN.1
 //! DER necessary for decoding/encoding various cryptography-related formats
-//! implemented as part of the [RustCrypto] project, e.g. the [`pkcs8`] crate.
+//! implemented as part of the [RustCrypto] project, e.g. the [`pkcs5`] and
+//! [`pkcs8`] crates.
 //!
 //! The core implementation avoids any heap usage (with convenience methods
 //! that allocate gated under the off-by-default `alloc` feature).
@@ -315,6 +316,7 @@
 //!
 //! [X.690]: https://www.itu.int/rec/T-REC-X.690/
 //! [RustCrypto]: https://github.com/rustcrypto
+//! [`pkcs5`]: https://docs.rs/pkcs5/
 //! [`pkcs8`]: https://docs.rs/pkcs8/
 //! [RustCrypto/utils#370]: https://github.com/RustCrypto/utils/issues/370
 //! [RFC 5280 Section 4.1.1.2]: https://tools.ietf.org/html/rfc5280#section-4.1.1.2
@@ -376,7 +378,7 @@ pub use crate::{
     header::Header,
     length::Length,
     message::Message,
-    tag::{Class, Tag, Tagged},
+    tag::{Class, Tag, TagNumber, Tagged},
 };
 
 pub(crate) use crate::byte_slice::ByteSlice;