The xmlsec1 packages are only available on a few selected platforms where needed:

• 1.2.7-4ng is basically a rebuild of 1.2.7-4 on Fedora Core 4. This was needed since updates of libtool and libxml2 broke xmlsec1
• 1.2.11-5.3 is built for the OpenSuSE platforms 10.1, 10.2 and 10.3 since they do not have xmlsec1 as part of the distribution.