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human
[MCU] Would whispering "Hail Hydra" into somebody's ear actually identify undercover HYDRA agents?

Question was inspired by the Winter Soldier HISHE.
gpt
Possibly, but doubtful. 

I would *assume* HYDRA agents are going to be especially careful about their identity when faced with an unknown "HYDRA agent" who happens to whisper "Hail HYDRA" in their ear. Sure it's their secret phrase, but what if an enemy faction learns it? 

Likely they will act confused unless they (or their HYDRA affiliated guards) can assume without significant doubt that the "HYDRA agent" is actually legitimate. Moreso, they usually won't talk about HYDRA affairs unless in a secure location *or* when in a prearranged meeting with known HYDRA affiliates. So you walking up to a suspected HYDRA goon on the streets and whispering "Hail HYDRA" isn't going work out. The target will act like you've just propositioned them for sex with an Oak tree and a towel rack, and then secretly get HYDRA to monitor/capture you for intel. They might even change their code phrase if they think SHIELD/etc is onto them. 

The other downside is if you get the wrong guy. Say a Senator is acting corrupt and shifty, and you approach him with "Hail HYDRA". He could be one of the few working for HYDRA, or more likely he's actively involved against HYDRA and takes your whispered comment as an admission of involvement with HYDRA. You then get captured by government officials and questioned (and or tortured) for your involvement in Hydra, and any attempts to explain yourself get treated as "advanced counter-interrogation tactics". Only after significant investigation and painful torture can they be sure your story is legitimate and you have no real HYDRA intel to offer, so it's a rough ordeal. 

So yes, it would work if you almost certainly knew your target was in HYDRA, if you managed to pass yourself off as part of HYDRA *and* if you targeted one or two high profile or only a few low profile members. If word starts getting around that HYDRA members were getting busted because the code phrase was cracked, they're going to change it (and likely target you for messing with their plans). Otherwise you're going to get targeted by both HYDRA (for attempting to infiltrate and uncover their ranks) and law enforcement/SHIELD (for passing yourself off as HYDRA).