Talk of which were targets seems to have been 'yes you can' and changed some years later to 'no you can't' probably as some targets are now in friendly nation states.
As for authentication, the process has changed a number of times. In the mid-60s it was rather less formal than BEagle relates.
In the vault I had a large board. On the board was a large number of cup hooks. In each was a bulldog clip and secured in each was a number of HMSO envelopes - RELRED, SCRAM, SCRAMCAN, POSREL. In each envelope was the appropriate code.
At one time the code was a word. For one exercise only the lead aircraft was given the code and all other aircraft were ordered to write down the codeword transmitted. We all dutifully wrote down the word APPLECART as relayed by ATC. At the debrief we were asked what we had heard and duly reported it. Subsequently we were told the actually transmitted codeword was APOTHECARY.
The exercise release codeword was easier NOURISHMENT but of course that was a lot word to broadcast in clear or by morse in a jamming enviroment. Later the codewords were changed to a simpler trigraph such as JULIET FOUR BRAVO. The same HMSO envelopes were used.
I have also come across stiff waterproof paxolin resin cards that had to be broken up (not easy) and waterproof foil envelopes - both very secure.