So you contend the PIN exchange has no purpose?
Unfortunately the answer is "Yes" from the moment I can reach the PIN generator with the password. It only adds complication, but does not truthfully give an extra level of security.
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Don't quote me on this, but I'm assuming that access to Keychain entries are authorized on a per thread/application ...
Indeed, but my point is to look at it from a how it happened perspective;
1. user logs into computer (most often with passoword)
2. user starts Yosemite install which requires Admin user/password (which gives access to keychain)
3. Install occurs
4. Same user logs back into his own account
5. User is asked for iCloud password ... !!
Here there are 3 things.
A. User may just want to recover a file but is forced to enter a password for services he does not even want
B. Its the same user, who has Admin access.
C. If the user logged into the account, then either give him the keychain iCloud password, or ask for it "after giving him access to his local MAC account files"
Really makes me think that the "more I have to behave like a trained monkey" the less there is security. Here slamming in passwords, PINs, etc. without consideration of what is the "real key" is pointless from a security perspective.