Why are applied implicit coding Tagged SEQUENCE in the DVCS (RFC-3029) implementation of BouncyCastle project?

Question

Sorry, my English is bad, but I try formulate a question.

BouncyCastle project contains the implementation of the protocol DVCS (RFC-3029). The response(answer) to the DVCS request has the following structure

DVCSResponse ::= CHOICE {
    dvCertInfo      DVCSCertInfo ,
    dvErrorNote [0] DVCSErrorNotice 
}

DVCSCertInfo::= SEQUENCE {
    version             Integer DEFAULT 1 ,
    dvReqInfo           DVCSRequestInformation, 
    messageImprint      DigestInfo,
    serialNumber        Integer,
    responseTime        DVCSTime,
    dvStatus        [0] PKIStatusInfo OPTIONAL,
    policy          [1] PolicyInformation OPTIONAL,
    reqSignature        [2] SignerInfos OPTIONAL,
    certs           [3] SEQUENCE SIZE (1..MAX) OF TargetEtcChain OPTIONAL,
    extensions          Extensions OPTIONAL 
}

I am interested in the field dvStatus, here's the code from BouncyCastle in which the coding:

private static final int TAG_DV_STATUS = 0;
...
public DVCSCertInfo build()
{   
    ASN1EncodableVector v = new ASN1EncodableVector();
    ...     
    if (dvStatus != null)
    {
        // DERTaggedObject(bool explicitly, int tagNo, Org.BouncyCastle.Asn1.Asn1Encodable obj)
        v.add(new DERTaggedObject(false, TAG_DV_STATUS, dvStatus));  //Why FALSE?
    }
    ...     
    return DVCSCertInfo.getInstance(new DERSequence(v));
}

The field PkiStatusInfo in ASN1 as follows:

PkiStatusInfo ::= SEQUENCE {
    status        PKIStatus, 
    statusString  PkiFreeText     OPTIONAL,
    failInfo      PkiFailureInfo  OPTIONAL  
}

Dump of real dvcs responce:

 SEQUENCE :                 dvCertInfo  DVCSCertInfo 
    SEQUENCE :              dvReqInfo       DVCSRequestInformation
       ENUMERATED :             service         ServiceType
       INTEGER :                nonce           INTEGER 
       GENERALIZED TIME :       requestTime     DVCSTime 
    SEQUENCE :              messageImprint      DigestInfo
    INTEGER :               serialNumber        Integer
    GENERALIZED TIME :      responseTime        DVCSTime
    CONTEXT SPECIFIC (0) :  dvStatus        [0] PKIStatusInfo 
       INTEGER : 0              status  PKIStatus //Where SEQUENCE?

But I can not understand why it is encoded without Sequence.

Answer 1

If you look at RFC 3029 Appendix E, which contains the complete ASN.1 module, the third line includes "IMPLICIT TAGS". This indicates that BER or DER encodings of components with tags (such as [0]) should have the built-in UNIVERSAL tags replaced by the tag in the specification. This means that the context specific 0 is used instead of the UNIVERSAL 16 for the dvStatus component.

There are two excellent ASN.1 Books you can download free from http://www.oss.com/asn1/resources/books-whitepapers-pubs/asn1-books.html which will explain EXPLICIT vs. IMPLICIT tags clearly. You can also try your example in the free online ASN.1 compiler and encoder/decoder at http://asn1-playground.oss.com to see the effects of IMPLICIT vs. EXPLICIT tags on encodings.

Most new specifications created today use AUTOMATIC TAGS (implying implicit tagging). Many older specifications use IMPLICIT TAGS, while fewer, or more ancient specifications, tend to use EXPLICIT TAGS.

Answer 2

Words "IMPLICIT TAGS" not for imported types! X.208: "Note 5 – The value of “TagDefault” for the module definition affects only those types defined explicitly in the module. It does not affect the interpretation of imported types.". PKIStatusInfo - imported type from PKIXCMP (RFC 2510).

PKIXCMP {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-cmp(9)}
DEFINITIONS EXPLICIT TAGS ::=
  BEGIN
...
PKIStatusInfo ::= SEQUENCE {
      status        PKIStatus,
      statusString  PKIFreeText     OPTIONAL,
      failInfo      PKIFailureInfo  OPTIONAL}
...
END

PKIStatusInfo is defined EXPLICITLY!