cms: tag TimeStampedData / ERS types implicitly, as RFC 5544 and RFC 4998 define (#2465)

Both ASN.1 modules are DEFINITIONS IMPLICIT TAGS, but the context-tagged
fields of Evidence, EvidenceRecord and ArchiveTimeStamp used the derive's
default, EXPLICIT. So cms could not decode an RFC-encoded evidence
record: e.g. EvidenceRecord.encryptionInfo [1] fails with "expected
SEQUENCE, got OBJECT IDENTIFIER", and encoding produced bytes other
implementations don't read.

Mark the eight fields tag_mode = "IMPLICIT" (constructed for the CHOICE
alternatives, as in revocation.rs). New tests decode and re-encode an
EvidenceRecord produced by pyasn1 from the RFC 4998 module, standalone
and as Evidence::ErsEvidence.

Author: yuxi-liu-wired

cms/src/timestamped_data.rs

@@ -78,11 +78,11 @@ pub struct MetaData {
 #[derive(Clone, Debug, Eq, PartialEq, Choice)]
 #[allow(missing_docs)]
 pub enum Evidence {
-    #[asn1(context_specific = "0")]
+    #[asn1(context_specific = "0", tag_mode = "IMPLICIT", constructed = "true")]
     TstEvidence(TimeStampTokenEvidence),
-    #[asn1(context_specific = "1")]
+    #[asn1(context_specific = "1", tag_mode = "IMPLICIT", constructed = "true")]
     ErsEvidence(EvidenceRecord),
-    #[asn1(context_specific = "2")]
+    #[asn1(context_specific = "2", tag_mode = "IMPLICIT", constructed = "true")]
     OtherEvidence(OtherEvidence),
 }
 
@@ -124,9 +124,9 @@ pub type TimeStampToken = ContentInfo;
 pub struct EvidenceRecord {
     pub version: TsdVersion,
     pub digest_algorithm: Vec<AlgorithmIdentifierOwned>,
-    #[asn1(context_specific = "0", optional = "true")]
+    #[asn1(context_specific = "0", optional = "true", tag_mode = "IMPLICIT")]
     pub crypto_infos: Option<CryptoInfos>,
-    #[asn1(context_specific = "1", optional = "true")]
+    #[asn1(context_specific = "1", optional = "true", tag_mode = "IMPLICIT")]
     pub encryption_info: Option<EncryptionInfo>,
     pub archive_timestamp_sequence: ArchiveTimeStampSequence,
 }
@@ -171,11 +171,11 @@ pub type ArchiveTimeStampChain = Vec<ArchiveTimeStamp>;
 #[derive(Clone, Debug, Eq, PartialEq, Sequence)]
 #[allow(missing_docs)]
 pub struct ArchiveTimeStamp {
-    #[asn1(context_specific = "0", optional = "true")]
+    #[asn1(context_specific = "0", optional = "true", tag_mode = "IMPLICIT")]
     digest_algorithm: Option<AlgorithmIdentifierOwned>,
-    #[asn1(context_specific = "1", optional = "true")]
+    #[asn1(context_specific = "1", optional = "true", tag_mode = "IMPLICIT")]
     attributes: Option<Attributes>,
-    #[asn1(context_specific = "2", optional = "true")]
+    #[asn1(context_specific = "2", optional = "true", tag_mode = "IMPLICIT")]
     reduced_hashtree: Option<Vec<PartialHashtree>>,
     time_stamp: ContentInfo,
 }

cms/tests/timestamped_data.rs

@@ -0,0 +1,36 @@
+//! TimeStampedData / EvidenceRecord tests.
+//!
+//! RFC 4998 (ERS) and RFC 5544 (TimeStampedData) define their ASN.1 modules with
+//! `DEFINITIONS IMPLICIT TAGS`, so their context-specific tags replace the inner type's tag.
+
+use cms::timestamped_data::{Evidence, EvidenceRecord};
+use der::{Decode, Encode};
+use hex_literal::hex;
+
+/// An `EvidenceRecord` encoded by pyasn1 from the RFC 4998 ASN.1 module, with
+/// `encryptionInfo [1]` and one `ArchiveTimeStamp` carrying `digestAlgorithm [0]` and
+/// `reducedHashtree [2]`.
+const EVIDENCE_RECORD: &[u8] = &hex!(
+    "3064020101300d300b0609608648016503040201a10606022a030500304830463044a00b0609608648016503040201"
+    "a224302204201111111111111111111111111111111111111111111111111111111111111111"
+    "300f06092a864886f70d010702a0020500"
+);
+
+#[test]
+fn evidence_record_implicit_tags_round_trip() {
+    let record = EvidenceRecord::from_der(EVIDENCE_RECORD).unwrap();
+    assert!(record.encryption_info.is_some());
+    assert_eq!(record.archive_timestamp_sequence.len(), 1);
+    assert_eq!(record.archive_timestamp_sequence[0].len(), 1);
+    assert_eq!(record.to_der().unwrap(), EVIDENCE_RECORD);
+}
+
+#[test]
+fn evidence_choice_implicit_tags_round_trip() {
+    // ersEvidence [1] EvidenceRecord: the SEQUENCE tag (0x30) is replaced by [1] (0xa1).
+    let mut ers = EVIDENCE_RECORD.to_vec();
+    ers[0] = 0xa1;
+    let evidence = Evidence::from_der(&ers).unwrap();
+    assert!(matches!(evidence, Evidence::ErsEvidence(_)));
+    assert_eq!(evidence.to_der().unwrap(), ers);
+}