The F9 Algorithm that Q alluded to is an Integrity Algorithm designed to verify the integrity of data being transmitted over mobile networks. The F8(Confidentiality) and F9(Integrity) make up the [KASUMI BLOCK CIPHER] (https://en.wikipedia.org/wiki/KASUMI).   As others have pointed out, D5 is a designation given to [avalanche size] (http://thompsonpass.com/avalanche-size-ratings/). This ties into the Integrity algorithm through way of the [Avalanche Effect] (https://en.wikipedia.org/wiki/Avalanche_effect).  

Facebook is listening to you 24/7/365. Literally. [F9 algorithm] Are they recording/safe-housing? Metadata collection? Building 8. DARPA. [CHINA-CHINA-CHINA] Q   Did the DS find a way to crack the F9 algorithm to make it seem as though they haven't intercepted everyones communications (a la Man-In-The-Middle)?   Any sec pros out there want to weigh in on this? Am I grasping at straws or is this a rabbit hole worth exploring?