🚀 go-pugleaf

In article <1drej9INN8t8@svcase.sp.paramax.com> scase@email.sp.paramax.com (Steve Case) writes:
>I am trying to understand the various ASN.1 encoding and I cannot seem
>to understand how the indefinite form for the length octets can possibly
>work!  To make things simple, let me give you a specific example and
>hopefully someone can clarify the situation for me.
>
>Let's assume we are encoding a bitstring value.  The particular bitstring
>to be encoded is '0900000009'H.  If I use the indefinite length format, 
>wouldn't this be encoded as follows:
>
>   BitString Length Contents
>   23        80     000900000009
>
>   EOC       Length
>   00        00
>
>
>My question, then, is when decoding this octet stream, how do you know
>that the bitstring is '0900000009'H and not '09'H or '090'H or '0900'H
>or '09000'H?  All three of these seem to be possible decodings?
>
 
First of all a correction to your 'correction posting' (second posting that
I did not include) -

No, The Bit String Identifier should be '23' (instead of '03') as mentioned in 
your posting (the first reaction is the correct reaction) :-), as opposed to 
the correction ('03') mentioned in your second posting. 

The reason is, although, 
a Bit string can be encoded in Primitive Form (identifier '03') OR 
Constructor Form (identifier '23'),
you can encode only in Constructor Form using EOC. 

If it is Primitive Form, it will be encoded as

Bit String	Length	Contents
03		06	000900000009

If it is Constructor Form, you can encode it in many ways
Two of the ways are - 

Bit String	Length 	Contents
23  		80	
			Bit String	Length	Contents
			03		06	000900000009
			EOC		Length
			00		00
OR

Bit String	Length 	Contents
23  		80	
			Bit String	Length	Contents
			03		04	00090000
			Bit String	Length	Contents
			03		03	000009
			EOC		Length
			00 		00

I think, you mixed up the encodings of the Primitive Form and Constructor
Form for the Bit String in your example - and hence the confusion.

Hope this helps.

- Ramakrishna			E-mail: rkc@maestro.bellcore.com