/usr/bin/codesign from Mac 10.9
From OS X 10.9.1: /usr/bin/codesign

Alternatively, grab it from your Recovery HD:

diskutil mount Recovery\ HD
hdiutil mount /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg
sudo cp /Volumes/OS\ X\ Base\ System/usr/bin/codesign /usr/bin/codesign


I recommend you do a permissions repair after copying.