iaik.asn1

Class TaggedType

java.lang.Object

iaik.asn1.ASN1Object

iaik.asn1.ConstructedType

iaik.asn1.TaggedType

Direct Known Subclasses:

APPLICATION, CON_SPEC

public abstract class TaggedType
extends ConstructedType

This class is the super-class of all tagged ASN.1 types.

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.

A tagged ASN.1 type is any ANS.1 type with a tag from the Application, Context-specific, or Private tag class. An ASN.1 type with a tag from the Universal class is either a simple or constructed type. This class will be the super class for classes that represent the Application tagged ASN.1 type, the Context-specific tagged type, and the Private tagged ASN.1 type.

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:

7.2 SP1

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

Method Summary

Methods

Modifier and Type	Method and Description
protected void	decode(int length, java.io.InputStream is) Decodes the next available data from the InputStream.
protected void	encode(java.io.OutputStream os) DER encodes this TaggedType ASN1Object and writes the result to the given output stream.
void	forceImplicitlyTagged(ASN type) Forces a tagged ASN.1 object to be implicitly tagged.
java.lang.Object	getValue() Returns the value of this TaggedType as an ASN1Object.
boolean	isImplicitlyTagged() Returns true if this tagged ASN1Object is implicitly tagged.
java.lang.String	toString() Returns a string that represents the contents of this tagged ASN1Object.

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

Method Detail

getValue

public java.lang.Object getValue()

Returns the value of this TaggedType as an ASN1Object.

Overrides:

getValue in class ConstructedType

Returns:

the value of this TaggedType as an ASN1Object

isImplicitlyTagged

public boolean isImplicitlyTagged()

Returns true if this tagged ASN1Object is implicitly tagged.

Returns:

true if implicitly tagged, false if explicitly tagged

forceImplicitlyTagged

public void forceImplicitlyTagged(ASN type)
                           throws CodingException

Forces a tagged ASN.1 object to be implicitly tagged.

There is no way for the DER decoder to detect if a tagged ASN.1 object is

• explicitly tagged or
• implicitly tagged containing one constructed element.

Example (using a Context-specific tagged type):

 problem [0] EXPLICIT InnerObject;
 InnerObject ::= INTEGER;
 
 problem [0] IMPLICIT InnerObject;
 InnerObject ::= SEQUENCE {
   test INTEGER;
 }
 

Both examples lead to the same ASN.1 DER encoding:

 A0 03 02 01 01   if test = 1
 

Only the application which parses the ASN1Object knows if an implicitly or an explicitly tagged type defined. The solution for this problem looks like:

 ASN1Object problem = ...;

 problem.forceImplicitlyTagged(ASN.SEQUENCE);
 

Constructed tagged ASN.1 objects are always decoded as explicitly tagged per default.

Parameters:

type - the type of the lost implicitly tagged type

Throws:

CodingException - if no instance of the type can be created

encode

protected void encode(java.io.OutputStream os)
               throws java.io.IOException

DER encodes this TaggedType ASN1Object and writes the result to the given output stream. This is a protected method and will not be used by an application for DER encoding an TaggedType object. An application will call one of the encode methods of the DerCoder class for performing the encoding. The encode method of this TaggedType class will be called internally from the DerCoder.

Overrides:

encode in class ConstructedType

Parameters:

os - the output stream to which to write the data

Throws:

java.io.IOException - if an error occurs while writing to the stream

decode

protected void decode(int length,
          java.io.InputStream is)
               throws java.io.IOException,
                      CodingException

Decodes the next available data from the InputStream.

The supplied input stream carries DER encoded data. The next length bytes represent the encoding of this tagged ASN.1 object to be decoded. If length = 0, the supplied tagged ASN.1 type has been encoded by means of the indefinite length encoding method, and so the encoding has to be parsed for two consecutive EOC octets of all zeros. Indefinite length encoding only is appropriate (and therefore allowed) for constructed ASN.1 types.

This is a protected method and will not be used by an application for DER decoding an encoded TaggedType. An application will call one of the decode methods of the DerCoder class for performing the decoding. The DerCoder then determines the number of bytes (length) obtained by the particular TaggedType and internally calls the decode method of that TaggedType instance.

Overrides:

decode in class ConstructedType

Parameters:

length - the length, i.e. number of the bytes representing the ASN1Object to be decoded

is - the input stream from which the DER encoded data is read in

Throws:

java.io.IOException - if there is a problem with the InputStream

CodingException - if the bytes from is cannot be decoded

toString

public java.lang.String toString()

Returns a string that represents the contents of this tagged ASN1Object.

Overrides:

toString in class ASN1Object

Returns:

the string representation

See Also:

ASN1Object.toString()