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der: docs + ErrorKind::Noncanonical in EXPLICIT when primitive #1818

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baloo merged 1 commit into from
May 6, 2025
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der: docs + ErrorKind::Noncanonical in EXPLICIT when primitive #1818

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30 changes: 16 additions & 14 deletions der/src/asn1/context_specific.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,18 +28,16 @@ impl<T> ContextSpecific<T> {
/// Attempt to decode an `EXPLICIT` ASN.1 `CONTEXT-SPECIFIC` field with the
/// provided [`TagNumber`].
///
/// This method has the following behavior which is designed to simplify
/// handling of extension fields, which are denoted in an ASN.1 schema
/// using the `...` ellipsis extension marker:
/// This method has the following behavior which decodes tag numbers one by one
/// in extension fields, which are denoted in an ASN.1 schema using
/// the `...` ellipsis extension marker:
///
/// - Skips over [`ContextSpecific`] fields with a tag number lower than
/// the current one, consuming and ignoring them.
/// - Returns `Ok(None)` if a [`ContextSpecific`] field with a higher tag
/// number is encountered. These fields are not consumed in this case,
/// allowing a field with a lower tag number to be omitted, then the
/// higher numbered field consumed as a follow-up.
/// - Returns `Ok(None)` if anything other than a [`ContextSpecific`] field
/// is encountered.
/// - Returns `Ok(None)` if a [`ContextSpecific`] field with a different tag
/// number is encountered. These fields are not consumed in this case.
/// - Returns `Err(ErrorKind::Noncanonical)` if constructed bit is primitive.
/// - Returns `Ok(Some(..))` if tag number matches.
pub fn decode_explicit<'a, R: Reader<'a>>(
reader: &mut R,
tag_number: TagNumber,
Expand All @@ -59,6 +57,10 @@ impl<T> ContextSpecific<T> {
/// This method otherwise behaves the same as `decode_explicit`,
/// but should be used in cases where the particular fields are `IMPLICIT`
/// as opposed to `EXPLICIT`.
///
/// Differences from `EXPLICIT`:
/// - Returns `Err(ErrorKind::Noncanonical)` if constructed bit
/// does not match constructed bit of the base encoding.
pub fn decode_implicit<'a, R: Reader<'a>>(
reader: &mut R,
tag_number: TagNumber,
Expand Down Expand Up @@ -111,12 +113,12 @@ where
// Decode EXPLICIT header
let header = Header::decode(reader)?;

// encoding shall be constructed
if !header.tag.is_constructed() {
return Err(header.tag.non_canonical_error().into());
}
match header.tag {
Tag::ContextSpecific {
number,
// encoding shall be constructed
constructed: true,
} => Ok(Self {
Tag::ContextSpecific { number, .. } => Ok(Self {
tag_number: number,
tag_mode: TagMode::default(),
value: reader.read_nested(header.length, |reader| {
Expand Down