der: Generic custom-class long > 30 tags PRIVATE, CONTEXT-SPECIFIC, APPLICATION

[dishmaker]

• Continuation of:
  PR support private tags and tag numbers >30 that are stored in long form #1416
  Issue der: Encoding higher tag number #1381
  Issue der: implementing other tag classes than Context-Specific #877

In this PR:

• Allow the use of tag numbers >30. In the der representation they are stored in the long form as multiple bytes.

• Allow APPLCATION and PRIVATE tags to be annotated like context specific ones in the derive macros

• Merge application, private, context-specific tags into one generic codebase CustomClass.

• Split EXPLICIT and IMPLICIT encoding into CustomClassExplicit/Implicit

• Add generic TAG: u16 to eg. ContextSpecificExplicit wrapper.

• Fix the need of implementing both Decode and DecodeValue on CONTEXT-SPECIFIC tags.

  • EXPLICIT tags only need to implement Decode<'a>.
  • IMPLICIT tags only need to implement DecodeValue<'a> and Tagged.
  • todo: maybe drop the requirement of Tagged too - it is needed for checking constructed bit.

• Add read_nested for IMPLICIT decoder (I had to fix x509-cert then tests, negative ones)

  In implicit Decode there was no read_nested before:

  formats/der/src/asn1/context_specific.rs

  Lines 66 to 67 in c501837

  	let header = Header::decode(reader)?;
  	let value = T::decode_value(reader, header)?;

Main breaking feature is here:

formats/der/src/asn1/context_specific.rs

Lines 9 to 23 in 756ef5f

	/// Context-specific class, EXPLICIT
	pub type ContextSpecificExplicit<const TAG: u16, T> =
	CustomClassExplicit<TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, IMPLICIT
	pub type ContextSpecificImplicit<const TAG: u16, T> =
	CustomClassImplicit<TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, reference, EXPLICIT
	pub type ContextSpecificExplicitRef<'a, const TAG: u16, T> =
	CustomClassExplicitRef<'a, TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, reference, IMPLICIT
	pub type ContextSpecificImplicitRef<'a, const TAG: u16, T> =
	CustomClassImplicitRef<'a, TAG, T, CLASS_CONTEXT_SPECIFIC>;

[dishmaker]

Sorry for not providing documentation on ContextSpecificExplicit and AnyCustomClassExplicit etc...
I'll try maybe next week.

[brandonweeks]

I've tested this PR with a prototype for parsing the Android key attestation X.509 extension that exclusively has tags >31. No issues discovered.

https://developer.android.com/privacy-and-security/security-key-attestation#key_attestation_ext_schema

[dishmaker]

Currently, CONTEXT-SPECIFIC IMPLICIT forces the user to implement FixedTag (which implies Tagged). But the tag isn't actually used anywhere.

formats/der/src/asn1/custom_class.rs

Line 184 in 116ce15

let constructed = <T as FixedTag>::TAG.is_constructed();

I think it might be useful for some types to implement IsConstructed, but not FixedTag
(for example custom CHOICE would not be FixedTag).

But if FixedTag is available, then it will be auto-implemented.
Something like:

impl<T: FixedTag> IsConstructed for T {
    /// ASN.1 constructed bit, used for CONTEXT-SPECIFIC IMPLICIT encoding
    const CONSTRUCTED: bool = T::TAG.is_constructed();
}