crmf: tag non-CHOICE fields implicitly, as RFC 4211 defines - #2466
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RFC 4211's ASN.1 module is DEFINITIONS IMPLICIT TAGS. A context tag is explicit only when the tagged type is a CHOICE (X.680 31.2.7), which crmf already does for Name, Time, GeneralName and POPOPrivKey. Eight other tags were EXPLICIT although their types are not CHOICEs: - POPOPrivKey: thisMessage [0] and dhMAC [2] BIT STRING, subsequentMessage [1] INTEGER, agreeMAC [3] PKMACValue, encryptedKey [4] EnvelopedData - PKIArchiveOptions: keyGenParameters [1] OCTET STRING, archiveRemGenPrivKey [2] BOOLEAN - EncryptedKey: envelopedData [0] EnvelopedData So crmf/cmpv2 could not decode, e.g., an ir from OpenSSL's `openssl cmp -popo 2` (key-encipherment POP `a2 03 81 01 00`: "got CONTEXT-SPECIFIC [1] (primitive)"), and encoded these fields in a form OpenSSL rejects. OpenSSL's crmf_asn.c uses IMPLICIT for all of them. New tests decode and re-encode each alternative from pyasn1 encodings of the RFC module, and OpenSSL's key-encipherment POP. OpenSSL 3.5 ir messages with POP RAVERIFIED, SIGNATURE and KEYENC (RSA and EC) now all decode via cmpv2 and re-encode byte for byte.
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RFC 4211's ASN.1 module is
DEFINITIONS IMPLICIT TAGS. A context tag is explicit only when the tagged type is a CHOICE (X.680 31.2.7), and crmf already does that forName,Time,GeneralNameandPOPOPrivKey. Eight other tags are declaredEXPLICITalthough their types aren't CHOICEs:POPOPrivKeythisMessage [0],dhMAC [2]POPOPrivKeysubsequentMessage [1]POPOPrivKeyagreeMAC [3]POPOPrivKeyencryptedKey [4]PKIArchiveOptionskeyGenParameters [1]PKIArchiveOptionsarchiveRemGenPrivKey [2]EncryptedKeyenvelopedData [0]OpenSSL's
crypto/crmf/crmf_asn.cusesASN1_IMPfor every one of these that it implements. In practice, crmf/cmpv2 can't decode a CMPirfromopenssl cmp -popo 2(key-encipherment proof of possession), whose POP isa2 03 81 01 00:Encoding has the same problem: crmf writes these fields in a form OpenSSL rejects. The existing tests only round-trip crmf's own output, so they didn't notice.
The fix changes the eight tags to
IMPLICIT(primitive for the BIT STRING, INTEGER, OCTET STRING and BOOLEAN alternatives).Tests:
tests/implicit_tags.rsdecodes and re-encodes, byte for byte, eachPOPOPrivKeyandPKIArchiveOptionsalternative as encoded by pyasn1 from the RFC 4211 module, plus the key-encipherment POP from OpenSSL. All three tests fail on master. I also checked that OpenSSL 3.5irmessages with POP RAVERIFIED, SIGNATURE and KEYENC (RSA and EC keys) all decode throughcmpv2::message::PkiMessageand re-encode identically; two of the four fail on master. Thecrmf.ymlmatrix passed locally (powerset tests on stable and 1.85, thumbv7em powerset build, fmt, clippy), along with thecmpv2all-features tests.This PR was produced by AI agents (Claude) and reviewed by me before submission.