matching large attributes

["Kevin J. McCarthy" <kevin@kogz.com>]

I need to be able to store (somehow) a 128-bit fixed length security key
as an LDAP attribute and search very efficiently to locate objects
containing this attribute (equality only; no substrings or other
comparisions are necessary).
 
What syntax and matching rules should I specify? It looks like the
attribute syntax could be binary (1.3.6.1.4.1.1466.115.121.1.5) but I
see no binary matching syntax.
 
I can always convert it into text if I have to, but efficiency will be
very important here and I think maintaining and index on a ~32 character
string would not be the most efficient way to do this. I did not see
anything in RFC 2252 pertaining to matching binary data.
 
Thanks,
Kevin