The best page on secure, robust repos is Wheeler’s:

http://www.dwheeler.com/essays/scm-security.html

Most VCS’s had basically no security. I first saw Aegis on that page since they thought ahead enough to add quite a bit:

http://aegis.sourceforge.net/propaganda/security.html

At this point, Im not sure if the better idea is to get Aegis into marketable shape or port some of its advantages into DVCS competition.