Cisco AnyConnect Secure Mobility Client on OS X Yosemite - CSD library signature verification failed
I had the same problem as the OP with Cisco AnyConnect version 2.3.2016: installing OS X 10.10 Yosemite broke it. Scrounging up a copy of 3.1.05187 WORKS (kind of) on my Mac.

I said "scrounge" because I couldn't download the correct version from Cisco's web site, because I don't have a login there associated with our license, and I couldn't get our local tech folks to understand why I couldn't just install the "MSI" file on my Mac.

Anyway, searching the web for "anyconnect-macosx-i386-3.1.05187-k9.dmg" unearthed a copy from a university web site (not something I like to do, particularly with security software), and behold, it installed and runs!

One wrinkle: unlike the older version, the newer one didn't route DNS lookups through the VPN. When Cisco establishes a VPN connection, it seems they edit "/etc/resolv.conf" to add the VPN's DNS servers. That doesn't work, and there's a warning to that effect in the Apple-generated "resolv.conf" file. 

My work-around is to consult what Cisco added and manually add those servers through the Mac OS X Network Preference Pane.