Issues with SP80056AConcatKDF in XML Encryption 1.1

We are looking at ECC encryption more closely.

1. How do we convert the AlgorithmID, PartyUInfo, PartyVInfo, 
SuppPubInfo, SubbPrivInfo to bit strings?  If we just concatenate the 
hexBinary values as is, then we won't be able to separate them. Should 
we use a fixed length bit strings or variable length strings with the 
length prefix as the NIST spec mentions?


2. These parameters can be quite complex. This is what the spec says

The |PartyUIInfo| attribute shall, when present, contain information 
identifying the sender of data encrypted (or authenticated) with the 
derived key. The encoding of the attribute value shall be as defined in 
[NIST SP800-56A <#ref-SP800-56A>], Section 5.8.1 (the Notes paragraph at 
the end of the section). At a minimum, this means that two substrings 
need to be present in the attribute value: One indicating the method 
used to identify the sender and one providing the identifying 
information. The initial substring shall be one octet (two hex digits) 
long and shall have the value "|00|" when the sender is identified with 
an X.509 certificate. Other values of the initial substring may be 
defined in later revisions of this specification. When identifying the 
sender with an X.509 certificate, the subsequent substring shall use the 
"variable-length" encoding defined in Section 5.8.1 of [NIST SP800-56A 
<#ref-SP800-56A>] and shall contain (the hex encoding of) the length of 
the certificate in big-endian representation immediately followed by the 
(hex encoding) of the DER-encoded certificate. The length field shall 
always be two octets (i.e. 4 hex digits) long.


Representing such a complex parameter as a single hexBinary attribute is 
very cryptic. Can't we define PartyUInfo in a more xml-ish way e.g. as 
an element which can have subelements representing each part?  Then we 
can define a uniform mechanism to convert each part to a bit string.

Pratik