iaik.asn1

Class CON_SPEC

java.lang.Object

iaik.asn1.ASN1Object

iaik.asn1.ConstructedType

iaik.asn1.TaggedType

iaik.asn1.CON_SPEC

public class CON_SPEC
extends TaggedType

This class implements the tagged ASN.1 type "CONTEXT SPECIFIC".

ASN.1 types are unequivocally identified by their tags. Every ASN.1 type - except for ANY and CHOICE types - is allocated a particular tag specifying the tag class to which this type belongs and the tag number which separates the current ASN.1 type from the other types belonging to the same tag class. ASN.1 defines four tag classes:

• The Universal Class, which specifies types defined by the ASN.1 standard and having universal validity for any application.
• The Application Class for types defined within a particular application standard.
• The Context-specific Class, which is used when there is need for unequivocally identifying types within a given context (for instance when using CHOICE types or for separating component types with equal underlying tags within a structered type).
• The Private Class for user defined, enterprise-specific types.

For BER (respectively DER) encoding a tagged type which is derived from some underlying type, either EXPLICIT or IMPLICIT tagging may be used. The tag of the underlying type explicitly has to be encoded when EXPLICIT tagging is used. IMPLICIT tagging avoids the extra encoding of the tag of the underlying type and may be used when the actual tag can be derived from the context.

Explicit tagging always enforces constructed encoding, and the contents consists of the full encoding of the underlying base type. When implicit tagging is required, the kind of encoding (primitive or constructed) depends on the base type: if the base type is a simple type, the tagged type is primitive encoded, but if the base type is constructed the tagged type also has to be encoded constructed.

Since explicit tagging is the default value and requires constructed encoding, this CON_SPEC class inherits from the ConstructedType class. Implicit tagging may be enforced by setting the implicitlyTagged parameter to true when creating a new CON_SPEC object by means of the CON_SPEC(int conSpecTag, ASN1Object obj, boolean implicitlyTagged) constructor. The following example creates an ASN.1 SEQUENCE object consisting of two INTEGER components, and one OCTET_STRING component, which is tagged context specifiic implicit with tag number 0:

 INTEGER i1 = new INTEGER(1);
 OCTET_STRING oct = new OCTET_STRING(new byte[] {(byte)0xAB, (byte)0xCD});
 INTEGER i2 = new INTEGER(2);
 SEQUENCE seq = new SEQUENCE();
 seq.addComponent(i1);
 seq.addComponent(new CON_SPEC(0, oct, true));
 seq.addComponent(i2);
 

The example above will give the following DER encoding:

 30:0A:02:01:01:80:02:AB:CD:02:01:02
 

For comparison, if the cotext specific tagged OCTET_STRING component of the SEQUENCE above is tagged explicitly, it will be encoded constructed and OCTET_STRING tag (0x04) and length are included in the encoding:

 30:0C:02:01:01:A0:04:04:02:AB:CD:02:01:02
 

Attention has to be paid when implicitly tagging a structured component, e.g. a SEQUENCE component. Since the SEQUENCE tag (0x30) is not included into the encoding, particular care has to be taken when decoding and parsing such a structure. Consider, for instance, two ASN.1 SEQUENCE structures, both containing a context specific tagged component with the same tag number 0. The context specific tagged component of the first SEQUENCE shall be explicitly tagged and shall represent a simple OCTET_STRING component. The context specific tagged component of the second SEQUENCE object shall be implicitly tagged and shall represent a SEQUENCE component containing one OCTET_STRING component with the same value as the OCTET_STRING component of the first SEQUENCE object:

First SEQUENCE:

 SEQUENCE[C] = 3 elements
  INTEGER = 1
  CONTEXTSPECIFIC[C] = [0] EXPLICIT
   OCTET STRING = 2 bytes: AB:CD
  INTEGER = 2
 

Second SEQUENCE:

 SEQUENCE[C] = 3 elements
  INTEGER = 1
  CONTEXTSPECIFIC[C] = [0] IMPLICIT
   SEQUENCE[C] = 1 elements
    OCTET STRING = 2 bytes: AB:CD
  INTEGER = 2
 

The encoding will be the same, since the context specific tagged component of the first SEQUENCE object is explicitly tagged and therefore constructed encoded giving a tag of 0xA0 (10100000: tag class: context specific (10), encoding type: constrcuted (1), tag number: 0 (00000). The context specific tagged component of the second SEQUENCE object is implicitly tagged and therefore the encoding type (primitive or constructed) is derived from the base type, which is a SEQUENCE and therefore gives the same context specific tag as in the first example: 0xA0. Since implicit tagging does not encode the tag of the base type (SEQUENCE 0x30), the encoding result is the same for both SEQUENCE objects:

 30:0C:02:01:01:A0:04:04:02:AB:CD:02:01:02
 

However the application that parses the received SEQUENCE object knows that implictly tagging has been used and therefore can recognize the right encoding by using the forceImplicitlyTagged method:

 SEQUENCE obtained_sequence = (SEQUENCE)DerCoder.decode(encoding);
 CON_SPEC cs = (CON_SPEC)obtained_sequnce.getComponentAt(1);
 cs.forceImplicitlyTagged(ASN.SEQUENCE);
 SEQUENCE seq_component = (SEQUENCE)cs.getValue();
 OCTET_STRING oct = (OCTET_STRING)seq_component.getComponentAt(0);
 

See Also:

ASN1Object, ASN, DerCoder

Field Summary

Fields inherited from class iaik.asn1.ConstructedType

content_count, content_data

Fields inherited from class iaik.asn1.ASN1Object

asnType, constructed, encode_listener, indefinite_length, isStringType, stream_mode

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	CON_SPEC() Creates an empty CON_SPEC object.
	CON_SPEC(int conSpecTag, ASN1Object obj) Creates a new explicitly tagged context specific tagged ASN.1 type.
	CON_SPEC(int conSpecTag, ASN1Object obj, boolean implicitlyTagged) Creates a new context specific tagged ASN.1 type.

Method Summary

Modifier and Type	Method and Description
static OCTET_STRING	parseImplicitContextSpecificTaggedOctetString(ASN1Object obj, int expectedTag, java.lang.String objName) Ensures that the ASN.1 object is an [CONTEXT SPECIFIC] IMPLICIT OCTET STRING with the indicated value.

Methods inherited from class iaik.asn1.TaggedType

decode, encode, forceImplicitlyTagged, getValue, isImplicitlyTagged, toString

Methods inherited from class iaik.asn1.ConstructedType

addComponent, addComponent, addEncodeListener, countComponents, getComponentAt, getComponents, removeComponent, removeComponent, setComponent, setValue

Methods inherited from class iaik.asn1.ASN1Object

addEncodeListener, encodeObject, getAsnType, indefiniteLength, isA, isConstructed, isStringType, setAutomaticRepair, setAutomaticRepairRecursive, setIndefiniteLength, setIndefiniteRecursive

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

CON_SPEC

protected CON_SPEC()

Creates an empty CON_SPEC object.

CON_SPEC

public CON_SPEC(int conSpecTag,
                ASN1Object obj)

Creates a new explicitly tagged context specific tagged ASN.1 type. Use the #CON_SPEC(int, ASN1Object, boolean) constructor for enforcing implicit tagging.

Parameters:

conSpecTag - the context specific tag number

obj - the underlying ASN1Object

CON_SPEC

public CON_SPEC(int conSpecTag,
                ASN1Object obj,
                boolean implicitlyTagged)

Creates a new context specific tagged ASN.1 type. The calling application decides if implicit or explicit tagging is to be used.

Parameters:

conSpecTag - the context specific tag number

obj - the underlying ASN1Object

implicitlyTagged - true if creating an implicitly tagged object, false if creating an explicitly tagged object

Method Detail

parseImplicitContextSpecificTaggedOctetString

public static OCTET_STRING parseImplicitContextSpecificTaggedOctetString(ASN1Object obj,
                                                                         int expectedTag,
                                                                         java.lang.String objName)
                                                                  throws CodingException

Ensures that the ASN.1 object is an [CONTEXT SPECIFIC] IMPLICIT OCTET STRING with the indicated value.

Parameters:

obj - an ASN.1 object

expectedTag - the tag value that is expected

objName - a string name for this ASN.1 object (used in excpetion messages)

Returns:

the OCTET STRING that this [CONTEXT SPECIFIC] IMPLICIT tagged ASN.1 object represents

Throws:

CodingException - if the ASN.1 object is not an [CONTEXT SPECIFIC] IMPLICIT OCTET STRING with the indicated value