How to name this key-oriented access-protection pattern?

Apparently this key-oriented access-protection pattern:

class SomeKey { 
    friend class Foo;
    SomeKey() {} 
    // possibly non-copyable too
};

class Bar {
public:
    void protectedMethod(SomeKey); // only friends of SomeKey have access
};

... doesn't have a known name yet, thus i'd like to find a good one for it so we can refer to it without breaking our tongues. Suggestions?

It should be:

• succinct
• convey the intent of access-protection
• ideally imply that no proxying is required (?)

I like, in decreasing preference:

• passkey friend idiom
• passkey-door friend idiom
• pass-door friend idiom
• key-door friend idiom
• partial-friend idiom
• restricted-friend idiom

_{I moved away from the key-lock/key-keyhole naming scheme to the pass naming scheme, which grew on me.}