What I would like to do is to extract the payload from the PER encoded
PDU then encrypt it and replace the payload in the PDU with the
encrypted "payload".
But once you encrypt the payload, it is no longer a SEQUENCE. That is, the
length and value fields of dataA and dataB are scrambled. Perhaps you
would prefer either
a) encrypt dataA and dataB before inserting them into the payload
SEQUENCE; or
b) not specifying payload as a SEQUENCE, but instead an OCTET STRING.
In the case of (b), you could encode the entire payload SEQUENCE, then
insert it as an OCTET STRING into pdu.
This way of describing data with different views
requires something like a "C" UNION functionality. Is this available in
ASN.1?
Yes, it's called a CHOICE, but I don't see how it applies.
How is this normally treated in a ASN1 specification?
The padding is needed since I currently operate on the PER encoded data
when I replace the payload the size of it must fit with the block size
of encryption algorithm.
The encryption is outside of the ASN.1 definition. If you were to encrypt
something, then it's encrypted, but only you know! By way of example, if
I have
Payload SEQUENCE {
dataA INTEGER,
dataB INTEGER
}
where dataA is 15 and dataB is 91, I code (as I described in (a)) encode
dataA and dataB separately. Let's say the algorithm requires 32 bits as
input. I would feed the algorithm 15 right-adjusted in a 32-bit field and
again 91 right-adjusted. Let's say the encrypted values are 410578725 and
18826982128433. I would then take those two values and, simply treating
them as integers, populate the Payload struct. Would this not work?
In ASN.1 it might be specified like this
Payload ::= SEQUENCE {
dataA INTEGER (CONSTRAINED BY { -- encryption -- }),
dataB INTEGER (CONSTRAINED BY { -- encryption -- })
}
which says that dataA and dataB are handled by a user-defined constraint.
In the case of the OSS ASN.1 Tools, this is implemented with a user
supplied function; other vendors might do it differently. If you are
encoding it by hand, you can do it however you wish.
=====================================================================
Conrad Sigona Voice Mail : 1-732-302-9669 x400
OSS Nokalva Fax : 1-614-388-4156
[EMAIL PROTECTED] My direct line : 1-315-845-1773
_______________________________________________
Asn1 mailing list
[email protected]
http://lists.asn1.org/mailman/listinfo/asn1
