der: add EncodingRules enum

Adds an enum with `Ber` and `Der` (default) variants which can be used
to selectively allow a limited number of BER productions when decoding
certain BER-based security-oriented formats, e.g. CMS, PKCS#8.

Currently this doesn't actually do anything, however the goal is to
address #779, where we can't decode CMS generated by Apple tooling.

PR #810 is an example of how the rules could be relaxed to support
`IndefiniteLength`s.

der/src/encoding_rules.rs

@@ -0,0 +1,18 @@
+/// ASN.1 encoding rules.
+///
+/// This enum identifies the specific encoding rules which are applied at the time a given document
+/// is decoded from a byte/octet serialization.
+///
+/// In addition to the Distinguished Encoding Rules (DER), this crate also supports a strict subset
+/// of the Basic Encoding Rules (BER) which supports the minimum amount of additional productions
+/// beyond DER needed to interoperate with other implementations of cryptography-oriented formats
+/// which utilize BER, e.g. CMS, PKCS#8.
+#[derive(Clone, Copy, Debug, Default, Eq, PartialEq, PartialOrd, Ord)]
+pub enum EncodingRules {
+    /// Basic Encoding Rules.
+    Ber,
+
+    /// Distinguished Encoding Rules.
+    #[default]
+    Der,
+}

der/src/lib.rs

@@ -337,6 +337,7 @@ mod datetime;
 mod decode;
 mod encode;
 mod encode_ref;
+mod encoding_rules;
 mod error;
 mod header;
 mod length;
@@ -359,6 +360,7 @@ pub use crate::{
     decode::{Decode, DecodeOwned, DecodeValue},
     encode::{Encode, EncodeValue},
     encode_ref::{EncodeRef, EncodeValueRef},
+    encoding_rules::EncodingRules,
     error::{Error, ErrorKind, Result},
     header::Header,
     length::{IndefiniteLength, Length},

der/src/reader.rs

@@ -8,15 +8,18 @@ pub(crate) mod slice;
 pub(crate) use nested::NestedReader;
 
 use crate::{
-    asn1::ContextSpecific, Decode, DecodeValue, Encode, Error, ErrorKind, FixedTag, Header, Length,
-    Result, Tag, TagMode, TagNumber,
+    asn1::ContextSpecific, Decode, DecodeValue, Encode, EncodingRules, Error, ErrorKind, FixedTag,
+    Header, Length, Result, Tag, TagMode, TagNumber,
 };
 
 #[cfg(feature = "alloc")]
 use alloc::vec::Vec;
 
 /// Reader trait which reads DER-encoded input.
 pub trait Reader<'r>: Sized {
+    /// Get the [`EncodingRules`] which should be applied when decoding the input.
+    fn encoding_rules(&self) -> EncodingRules;
+
     /// Get the length of the input.
     fn input_len(&self) -> Length;
 

der/src/reader/nested.rs

@@ -1,6 +1,6 @@
 //! Reader type for consuming nested TLV records within a DER document.
 
-use crate::{reader::Reader, Error, ErrorKind, Header, Length, Result};
+use crate::{reader::Reader, EncodingRules, Error, ErrorKind, Header, Length, Result};
 
 /// Reader type used by [`Reader::read_nested`].
 pub struct NestedReader<'i, R> {
@@ -51,6 +51,10 @@ impl<'i, 'r, R: Reader<'r>> NestedReader<'i, R> {
 }
 
 impl<'i, 'r, R: Reader<'r>> Reader<'r> for NestedReader<'i, R> {
+    fn encoding_rules(&self) -> EncodingRules {
+        self.inner.encoding_rules()
+    }
+
     fn input_len(&self) -> Length {
         self.input_len
     }

der/src/reader/pem.rs

@@ -1,7 +1,7 @@
 //! Streaming PEM reader.
 
 use super::Reader;
-use crate::{Decode, Error, ErrorKind, Header, Length, Result};
+use crate::{Decode, EncodingRules, Error, ErrorKind, Header, Length, Result};
 use core::cell::RefCell;
 
 #[allow(clippy::arithmetic_side_effects)]
@@ -86,9 +86,7 @@ mod utils {
         fn as_slice(&self) -> &[u8] {
             &self.buf[self.pos..self.cap]
         }
-    }
 
-    impl<'i> BufReader<'i> {
         pub fn peek_byte(&self) -> Option<u8> {
             let s = self.as_slice();
             s.first().copied()
@@ -130,6 +128,9 @@ pub struct PemReader<'i> {
     /// Inner PEM decoder wrapped in a BufReader.
     reader: RefCell<utils::BufReader<'i>>,
 
+    /// Encoding rules to apply when decoding the input.
+    encoding_rules: EncodingRules,
+
     /// Input length (in bytes after Base64 decoding).
     input_len: Length,
 
@@ -148,6 +149,7 @@ impl<'i> PemReader<'i> {
 
         Ok(Self {
             reader: RefCell::new(reader),
+            encoding_rules: EncodingRules::default(),
             input_len,
             position: Length::ZERO,
         })
@@ -162,6 +164,10 @@ impl<'i> PemReader<'i> {
 
 #[cfg(feature = "pem")]
 impl<'i> Reader<'i> for PemReader<'i> {
+    fn encoding_rules(&self) -> EncodingRules {
+        self.encoding_rules
+    }
+
     fn input_len(&self) -> Length {
         self.input_len
     }

der/src/reader/slice.rs

@@ -1,13 +1,18 @@
 //! Slice reader.
 
-use crate::{BytesRef, Decode, Error, ErrorKind, Header, Length, Reader, Result, Tag};
+use crate::{
+    BytesRef, Decode, EncodingRules, Error, ErrorKind, Header, Length, Reader, Result, Tag,
+};
 
 /// [`Reader`] which consumes an input byte slice.
 #[derive(Clone, Debug)]
 pub struct SliceReader<'a> {
     /// Byte slice being decoded.
     bytes: BytesRef<'a>,
 
+    /// Encoding rules to apply when decoding the input.
+    encoding_rules: EncodingRules,
+
     /// Did the decoding operation fail?
     failed: bool,
 
@@ -20,6 +25,7 @@ impl<'a> SliceReader<'a> {
     pub fn new(bytes: &'a [u8]) -> Result<Self> {
         Ok(Self {
             bytes: BytesRef::new(bytes)?,
+            encoding_rules: EncodingRules::default(),
             failed: false,
             position: Length::ZERO,
         })
@@ -57,6 +63,10 @@ impl<'a> SliceReader<'a> {
 }
 
 impl<'a> Reader<'a> for SliceReader<'a> {
+    fn encoding_rules(&self) -> EncodingRules {
+        self.encoding_rules
+    }
+
     fn input_len(&self) -> Length {
         self.bytes.len()
     }