To speculate the B61 is a dumb bomb. And most photo's of the casing show no plug/contact point for electronics. I'm assuming at take off. When the neutron reflector's distance is set, and the barometric pressure for denotation height is calibrated.
These are just proximity fuses. I can find no reference to actual communication with in-flight artillery shells. Furthermore Nuclear Artillery shells were armed when loaded. They were one of the few systems outside of the two-man rule. As a single inferior officer would arm the shell when loading it.
https://en.wikipedia.org/wiki/Permissive_Action_Link#Develop...
Edit: Unless you mean that bike locks that the UK used to secure nukes, these days we just trust our submarine crews not to do anything rash.
>To arm the weapons you just open a panel held by two captive screws - like a battery cover on a radio - using a thumbnail or a coin.
>Inside are the arming switch and a series of dials which you can turn with an Allen key to select high yield or low yield, air burst or groundburst and other parameters.
>The Bomb is actually armed by inserting a bicycle lock key into the arming switch and turning it through 90 degrees. There is no code which needs to be entered or dual key system to prevent a rogue individual from arming the Bomb.
https://web.archive.org/web/20120311235156/http://news.bbc.c...
http://www.liveleak.com/view?i=4ad_1227686113&comments=1
I like to believe it's the same bike locks that could be defeated with a ballpoint pen:
