question about implicitly tags in ASN.1

Question

From the X.509 RFC, in the "Implicitly Tagged Module" section:

CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint

DistributionPoint ::= SEQUENCE {
     distributionPoint       [0]     DistributionPointName OPTIONAL,
     reasons                 [1]     ReasonFlags OPTIONAL,
     cRLIssuer               [2]     GeneralNames OPTIONAL }

DistributionPointName ::= CHOICE {
     fullName                [0]     GeneralNames,
     nameRelativeToCRLIssuer [1]     RelativeDistinguishedName }

GeneralName ::= CHOICE {
     otherName                 [0]  AnotherName,
     rfc822Name                [1]  IA5String,
     dNSName                   [2]  IA5String,
     x400Address               [3]  ORAddress,
     directoryName             [4]  Name,
     ediPartyName              [5]  EDIPartyName,
     uniformResourceIdentifier [6]  IA5String,
     iPAddress                 [7]  OCTET STRING,
     registeredID              [8]  OBJECT IDENTIFIER }

How would x400Address be encoded? Like this?:

SEQUENCE {
  SEQUENCE {
    [0] {
      [3] {
        ...
        }
      }
    }
  }

The first [0] is from DistributionPoint (which is replacing DistributionPointName's [0] tag) and the [3] is from GeneralName.

If that's how it's supposed to work, though, then why does DistributionPoint even have context-specific tags?

Answer 1

You will get something like this:

SEQUENCE {
  SEQUENCE {
    [0] {
      [0] {
        [3] {
          ...
        }
      }
    }
  }
}

Explanations:

1. Even though the tags are supposed to be implicit (due to being in a module declared with IMPLICIT TAGS), implicit tags cannot be applied to a CHOICE (because otherwise the CHOICE would not work at all), so they are automatically converted to explicit tags in that situation. Here, the [0] tag on the distributionPoint field is in such a situation, so it wraps around, instead of replacing, the tag of the DistributionPointName field, which is also a [0].

2. Notice that fullName has type GeneralNames, not GeneralName. You do not quote GeneralNames in your message; it is a SEQUENCE OF GeneralName. The [0] implicit tag of the fullName field, being implicit, replaces that of the SEQUENCE.

For the first point, it is specified in X.680, clause 28.6 (from the 1994 version):

The tagging construction specifies explicit tagging if any of the following holds:

a) the "Tag EXPLICIT Type" alternative is used;

b) the "Tag Type" alternative is used and the value of "TagDefault" for the module is either "EXPLICIT TAGS" or is empty;

c) the "Tag Type" alternative is used and the value of "TagDefault" for the module is "IMPLICIT TAGS" or "AUTOMATIC TAGS", but the type defined by "Type" is a choice type, open type, or a "DummyReference" (see ITU-T Rec. X.683 | ISO/IEC 8824-4, clause 8.3).

The last clause (c) is what I explain above.