>keystone
Sounds like the keystone could be a word or phrase that hussein mentioned in the aids vid.
About Q's use of "decider" vs the word decipher, is there any known cryptography that would translate the letter D to PH?
Here is another item I found, in case related:
Cryptoprocessors input program instructions in encrypted form, decrypt the instructions to plain instructions which are then executed…
The purpose of a secure cryptoprocessor is to act as the
keystone
of a security subsystem, eliminating the need to protect the rest of the subsystem…
wikipedia. org/wiki/Secure_cryptoprocessor