Decoding explicitly-tagged sequences?

[James-E-A]

Explicitly tagged ("doubly-constructed"?) sequences do seem to be a thing:

ContentInfo ::= SEQUENCE {
  contentType ContentType,
  content [0] EXPLICIT ANY DEFINED BY contentType }

EnvelopedData ::= SEQUENCE { -- this is content when contentType is 1.2.840.113549.1.7.3
  version CMSVersion,
  originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
  recipientInfos RecipientInfos,
  encryptedContentInfo EncryptedContentInfo,
  unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

However, the current codebase chokes on them:

from Crypto.Util.asn1 import *

import email.parser
import subprocess

cmd = ['openssl', 'cms', '-encrypt', '-pwri_password', 'lol123']
plaintext = 'Plaintext Message'.encode()
m, err = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE).communicate(plaintext)
assert err is None, err.decode()
m_der = email.parser.BytesParser().parsebytes(m).get_payload(decode=True)
print(m_der.hex())
m_obj = DerSequence().decode(m_der)

taggedEnvelopedData_der = m_obj[1]

envelopedData_obj = None
try:
    envelopedData_obj = DerSequence().decode(taggedEnvelopedData_der)
except ValueError:
    print("Couldn't parse it as a Sequence!")
    try:
        envelopedData_obj = DerOctetString().decode(taggedEnvelopedData_der)
    except ValueError:
        print("Couldn't parse it as an Octet String!")

if envelopedData_obj is None:
    print("Falling back on instantiating a DerObject, then taking the payload.")
    print("The tag octet we can't seem to cope with is 0x%02x." % taggedEnvelopedData_der[0])
    _envelopedData_obj = DerObject().decode(taggedEnvelopedData_der)
    envelopedData_obj = DerSequence().decode(_envelopedData_obj.payload)

taggedEnvelopedData_obj = DerSequence(envelopedData_obj, explicit=0)  # This line is what's currently erroring out
assert taggedEnvelopedData_obj.encode() == taggedEnvelopedData_der, "If you're seeing this message, something unexpected has gone wrong"

This might prove an issue for CMS support, which is on the future roadmap / wishlist.