you will need openssl.
Obviously this is not a Cisco/TAC tool so it is somehow expected that some engineers from the TAC do not know it.
Also, .cer is not a format.
So, you will have to first identify the origin of the file.
Known format are pks12, der, pem.
Was this file created on a windows machine ?
If so, this is most probably pkcs12 or pkcs7.
If this pkcs12, you can use 'openssl pkcs12 ?' to get a list of options.
Searching with google will also give you the exact command.
http://www.gagravarr.org/writing/openssl-certs/general.shtml
Regards,
Gilles.
