Is there a more colloquial term for a "confidant", or someone who has been entrusted with sensitive information to be disclosed only under certain conditions?
This is related to my previous question on trusted proxies.
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In Harry Potter, they call that person a "Secret Keeper", although that also involves magic and whatnot.
In computer terms, a group of people who are entrusted with secret information (such as SSH keys) are indicated to be a part of (or within) a circle of trust.
In High School, girls call those people BFFs.
I don't think there is a more colloquial term that fully encompasses the meaning of confidant. I can't think of a more formal one, either.
Confidant and confidante are not particularly formal terms, even if they sound it. These are not uncommon in everyday parlance.
If you truly require something more colloquial, you'll probably have to settle for a longer phrase. Even then, many of the terms I might use to construct such a phrase seem no less formal than confidant:
A right-hand man is colloquially used to refer to someone whom you can trust with your secrets.