der: add ClassTaggedExplicit wrapper

Generic wrapper around `APPLICATION`, `CONTEXT-SPECIFIC` and `PRIVATE` tags.
Such struct simplifies decoding, as previously we had to assert that
decoded tag number is the expected one.

Author: dishmaker

der/src/asn1.rs

@@ -11,6 +11,7 @@ pub(crate) mod bit_string;
 mod bmp_string;
 mod boolean;
 mod choice;
+mod class_tagged;
 mod context_specific;
 mod general_string;
 mod generalized_time;
@@ -35,18 +36,18 @@ mod videotex_string;
 
 pub use self::{
     any::AnyRef,
-    application::{Application, ApplicationRef},
+    application::{Application, ApplicationExplicit, ApplicationRef},
     bit_string::{BitStringIter, BitStringRef},
     choice::Choice,
-    context_specific::{ContextSpecific, ContextSpecificRef},
+    context_specific::{ContextSpecific, ContextSpecificExplicit, ContextSpecificRef},
     general_string::GeneralStringRef,
     generalized_time::GeneralizedTime,
     ia5_string::Ia5StringRef,
     integer::{int::IntRef, uint::UintRef},
     null::Null,
     octet_string::OctetStringRef,
     printable_string::PrintableStringRef,
-    private::{Private, PrivateRef},
+    private::{Private, PrivateExplicit, PrivateRef},
     sequence::{Sequence, SequenceRef},
     sequence_of::{SequenceOf, SequenceOfIter},
     set_of::{SetOf, SetOfIter},

der/src/asn1/application.rs

@@ -2,8 +2,9 @@
 
 use crate::{
     Choice, Class, Decode, DecodeValue, DerOrd, Encode, EncodeValue, EncodeValueRef, Error, Header,
-    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer, asn1::AnyRef,
-    tag::IsConstructed,
+    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer,
+    asn1::{AnyRef, class_tagged::ClassTaggedExplicit},
+    tag::{IsConstructed, class::CLASS_APPLICATION},
 };
 use core::cmp::Ordering;
 
@@ -12,3 +13,7 @@ use crate::ErrorKind;
 
 impl_custom_class!(Application, Application, "APPLICATION", "0b01000000");
 impl_custom_class_ref!(ApplicationRef, Application, "APPLICATION", "0b01000000");
+
+/// Application class, EXPLICIT
+pub type ApplicationExplicit<const NUMBER: u32, T> =
+    ClassTaggedExplicit<NUMBER, T, CLASS_APPLICATION>;

der/src/asn1/class_tagged.rs

@@ -0,0 +1,68 @@
+use crate::{
+    Class, Decode, DecodeValue, Encode, EncodeValue, Error, FixedTag, Header, Length, Reader, Tag,
+    TagMode, TagNumber, Writer,
+};
+
+/// `APPLICATION`, `CONTEXT-SPECIFIC` or `PRIVATE` reference, with const `EXPLICIT` encoding.
+///
+///
+/// This type encodes a field which is specific to a particular context
+/// and is identified by a [`TagNumber`].
+///
+/// Inner value might implement [`Encode`], [`Decode`] or both.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+pub struct ClassTaggedExplicit<const NUMBER: u32, T, const CLASS_BITS: u8> {
+    /// Inner value might implement [`Encode`], [`Decode`] or both.
+    pub value: T,
+}
+
+impl<const NUMBER: u32, T, const CLASS_BITS: u8> ClassTaggedExplicit<NUMBER, T, CLASS_BITS> {
+    /// Returns const [`TagNumber`], associated with this `EXPLICIT` tag wrapper.
+    pub const fn tag_number() -> TagNumber {
+        TagNumber(NUMBER)
+    }
+
+    /// Returns const [`TagMode::Explicit`], associated with this `EXPLICIT` tag wrapper.
+    pub const fn tag_mode() -> TagMode {
+        TagMode::Explicit
+    }
+}
+
+impl<const NUMBER: u32, T, const CLASS_BITS: u8> EncodeValue
+    for ClassTaggedExplicit<NUMBER, T, CLASS_BITS>
+where
+    T: Encode,
+{
+    fn value_len(&self) -> Result<Length, Error> {
+        self.value.encoded_len()
+    }
+
+    fn encode_value(&self, writer: &mut impl Writer) -> Result<(), Error> {
+        // Encode EXPLICIT value (with tag and length)
+        self.value.encode(writer)
+    }
+}
+
+impl<'a, const NUMBER: u32, T, const CLASS_BITS: u8> DecodeValue<'a>
+    for ClassTaggedExplicit<NUMBER, T, CLASS_BITS>
+where
+    T: Decode<'a>,
+{
+    type Error = T::Error;
+
+    fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> Result<Self, Self::Error> {
+        // encoding shall be constructed
+        if !header.tag().is_constructed() {
+            return Err(reader.error(header.tag().non_canonical_error()).into());
+        }
+        Ok(Self {
+            value: T::decode(reader)?,
+        })
+    }
+}
+
+impl<const NUMBER: u32, T, const CLASS_BITS: u8> FixedTag
+    for ClassTaggedExplicit<NUMBER, T, CLASS_BITS>
+{
+    const TAG: Tag = Tag::new_non_universal(Class::from_bits(CLASS_BITS), TagNumber(NUMBER), true);
+}

der/src/asn1/context_specific.rs

@@ -2,8 +2,9 @@
 
 use crate::{
     Choice, Class, Decode, DecodeValue, DerOrd, Encode, EncodeValue, EncodeValueRef, Error, Header,
-    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer, asn1::AnyRef,
-    tag::IsConstructed,
+    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer,
+    asn1::{AnyRef, class_tagged::ClassTaggedExplicit},
+    tag::{IsConstructed, class::CLASS_CONTEXT_SPECIFIC},
 };
 use core::cmp::Ordering;
 
@@ -23,13 +24,17 @@ impl_custom_class_ref!(
     "0b10000000"
 );
 
+/// ContextSpecific class, EXPLICIT
+pub type ContextSpecificExplicit<const NUMBER: u32, T> =
+    ClassTaggedExplicit<NUMBER, T, CLASS_CONTEXT_SPECIFIC>;
+
 #[cfg(test)]
 #[allow(clippy::unwrap_used)]
 mod tests {
     use super::ContextSpecific;
     use crate::{
         Decode, Encode, SliceReader, TagMode, TagNumber,
-        asn1::{BitStringRef, ContextSpecificRef, SetOf, Utf8StringRef},
+        asn1::{BitStringRef, ContextSpecificExplicit, ContextSpecificRef, SetOf, Utf8StringRef},
     };
     use hex_literal::hex;
 
@@ -194,4 +199,34 @@ mod tests {
         assert_eq!(field.value.get(0).cloned(), Some(hello));
         assert_eq!(field.value.get(1).cloned(), Some(world));
     }
+
+    #[test]
+    fn round_trip_explicit() {
+        let field =
+            ContextSpecificExplicit::<1, BitStringRef<'_>>::from_der(EXAMPLE_BYTES).unwrap();
+        assert_eq!(
+            field.value,
+            BitStringRef::from_bytes(&EXAMPLE_BYTES[5..]).unwrap()
+        );
+        assert_eq!(
+            ContextSpecificExplicit::<1, BitStringRef<'_>>::tag_mode(),
+            TagMode::Explicit
+        );
+        assert_eq!(
+            ContextSpecificExplicit::<1, BitStringRef<'_>>::tag_number(),
+            TagNumber(1)
+        );
+
+        let mut buf = [0u8; 128];
+        let encoded = field.encode_to_slice(&mut buf).unwrap();
+        assert_eq!(encoded, EXAMPLE_BYTES);
+
+        // should not decode as tag CONTEXT-SPECIFIC [2]
+        assert!(ContextSpecificExplicit::<2, BitStringRef<'_>>::from_der(EXAMPLE_BYTES).is_err());
+
+        // should be different than CONTEXT-SPECIFIC [1]
+        let invalid_field = ContextSpecificExplicit::<2, BitStringRef<'_>> { value: field.value };
+        let invalid_encoded = invalid_field.encode_to_slice(&mut buf).unwrap();
+        assert_ne!(invalid_encoded, EXAMPLE_BYTES);
+    }
 }

der/src/asn1/private.rs

@@ -2,8 +2,10 @@
 
 use crate::{
     Choice, Class, Decode, DecodeValue, DerOrd, Encode, EncodeValue, EncodeValueRef, Error, Header,
-    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer, asn1::AnyRef,
+    Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer,
+    asn1::{AnyRef, class_tagged::ClassTaggedExplicit},
     tag::IsConstructed,
+    tag::class::CLASS_PRIVATE,
 };
 use core::cmp::Ordering;
 
@@ -12,3 +14,6 @@ use crate::ErrorKind;
 
 impl_custom_class!(Private, Private, "PRIVATE", "0b11000000");
 impl_custom_class_ref!(PrivateRef, Private, "PRIVATE", "0b11000000");
+
+/// Private class, EXPLICIT
+pub type PrivateExplicit<const NUMBER: u32, T> = ClassTaggedExplicit<NUMBER, T, CLASS_PRIVATE>;

der/src/tag.rs

@@ -1,7 +1,7 @@
 //! ASN.1 tags.
 #![cfg_attr(feature = "arbitrary", allow(clippy::arithmetic_side_effects))]
 
-mod class;
+pub(crate) mod class;
 mod mode;
 mod number;
 
@@ -165,6 +165,31 @@ impl Tag {
     /// rules implemented by this crate.
     pub(crate) const MAX_SIZE: usize = 6;
 
+    /// Creates a [`Tag`] of non-`UNIVERSAL` class.
+    /// Allowed classes are:
+    /// - `APPLICATION` [`Class::Application`],
+    /// - `CONTEXT-SPECIFIC` [`Class::ContextSpecific`],
+    /// - `PRIVATE` [`Class::Private`],
+    ///
+    /// Returns [`Tag::Null`] otherwise.
+    pub const fn new_non_universal(class: Class, number: TagNumber, constructed: bool) -> Tag {
+        match class {
+            Class::Application => Tag::Application {
+                constructed,
+                number,
+            },
+            Class::ContextSpecific => Tag::ContextSpecific {
+                constructed,
+                number,
+            },
+            Class::Private => Tag::Private {
+                constructed,
+                number,
+            },
+            Class::Universal => Tag::Null,
+        }
+    }
+
     /// Decode a [`Tag`] in addition to returning the value of the constructed bit.
     pub(crate) fn decode_with_constructed_bit<'a>(
         reader: &mut impl Reader<'a>,