I read the following PER encoding example from the book "ASN.1 complete".
Choice-example ::= CHOICE
{ normal NULL,
high NULL,
... !2 -- see para 14.6 for exception handling --
medium NULL }
first-choice Choice-example ::= normal:NULL
second-choice Choice-example ::= medium:NULL
first-choice: E: 0 I: 0 C: (a total of two bits)
second-choice: E: 1 (extensions bit set)
I: 000000 (index as a normally small whole number)
L: P00000001 (general length "wrapper")
C: 00000000 (padded encoding of NULL)
Should the length and contect of the second-choice be
L: P00000000
C:
, since there is no need to encode NULL?
Dear Mr. Chin:
In a way, you're right, but you have misinterpreted what the C field
contains. Indeed NULL is not encoded, but this leaves the extension an
empty bit string. An extension is actually an open type and an open type
is itself treated as a full encoding unto itself. So, just as if we were
to encode a solitary PDU containing only
null NULL ::= NULL
as 00000000 since there cannot be a PER encoding without bits, so also an
extension containing, in effect, that same null would be encoded as
00000000.
Please let me know if, rather than make it clearer, I only managed to make
it cloudier.
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