How to read PKCS ASN.1 specs

Question

Eg https://datatracker.ietf.org/doc/html/rfc5208#appendix-A

There is no top level structure defined. Just a bunch of internal bits that appear in some order. But where is the outer Sequence?

Also, where does one actually find the imported defenitions, eg of Attribute?

Answer 1

There is top level structure, it is PrivateKeyInfo :

PrivateKeyInfo ::= SEQUENCE {
  version Version,
  privateKeyAlgorithm AlgorithmIdentifier {{PrivateKeyAlgorithms}},
  privateKey PrivateKey,
  attributes [0] Attributes OPTIONAL }

Also, where does one actually find the imported defenitions, eg of Attribute?

from ASN.1 module:

Attribute
         FROM InformationFramework informationFramework

Look for imports: IMPORTS

informationFramework
         FROM UsefulDefinitions {joint-iso-itu-t(2) ds(5) module(1)
                                 usefulDefinitions(0) 3}

this stands for OID 1.2.5.1.0.3 InformationFramework is defined in X.501: https://www.itu.int/ITU-T/formal-language/itu-t/x/x501/2019/InformationFramework.html